## aperture or focal length most relevant to good/bad seeing ?

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• i have often read comments that bad seeing generally make large scopes unsuitable for imaging (or visual). i understand that (and bad seeing in general is
Message 1 of 21 , Aug 14, 2013
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i have often read comments that bad seeing generally make large scopes unsuitable for imaging (or visual). i understand that (and bad seeing in general is "bad" :). more specifically are there guidelines (common wisdom) on how much focal length or aperture is usable under specific seeing conditions.

suppose my seeing is generally "good" (1.5-2.0 asec) -- is there a max scope size suggested for those conditions and is it generally limited by aperture or focal length?? big, big aperture and short focal length or focal length limited primarily?? or if my seeing is never better than 3.5 asec, then don't bother getting a scope bigger than "X" (focal length or aperture?)

or does it become irrelevant if you always "match" the camera/scope image scale to the seeing (i.e., 1/2 to 1/3 of the seeing)

thanks

Lee
• ... Seeing affects the ability to obtain resolution below a certain threshold. Seeing is often quantified by FWHM, which roughly means that a sufficiently
Message 2 of 21 , Aug 15, 2013
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--- "lmbuck2000" <lmbuck2000@...> wrote:
> large scopes unsuitable for imaging (or visual)...
> ...how much focal length or aperture is usable under
> specific seeing conditions.

Seeing affects the ability to obtain resolution below a certain threshold. Seeing is often quantified by FWHM, which roughly means that a sufficiently resolving optic is limited by the seeing to produce a Gaussian spot with that angular FWHM.

An optic might be diffraction limited to an equivalent of <1" (arcsec) but if the seeing = 2.5" then the image is limited to 2.5" by the seeing. Resolution convolution is essentially quadratic:

image FWHM = sqrt(optic^2 + camera^2 + tracking^2 + seeing^2)

where:
optic = FWHM of diffraction, defocus, misscollimation, etc.
camera = FHWM of MTF of sampling (an esoteric quality)
tracking = FWHM tracking errors approximates RMS of fast guide star

If the scope and tacking FWHM are good then seeing is usually the limiting factor. But small scopes, mediocre tracking and undersampling can easily tip that equation to produce "scope/camera limited" resolution.

Small apertures and short FL combined with undersampling by the camera conspire to produce fat FWHM that often exceeds the seeing. It is somewhat rare for a small short refractor to produce a spot with FWHM < 2-3" and even if it does few users sample adequately to take advantage of that (undersampling blurs the resolution). So such scopes are largely immune to seeing and produce similar images regardless of the seeing.

Large apertures have intrinsically superior resolution (unless something is wrong) are on sturdier mounts that improve tracking and have longer FL that prevents undersampling. Such scopes produce tight FWHM and so are much more susceptible to seeing limitations.

On a related issue there is a myth that occasionally crops up to the effect that a high quality small aperture performs better than a large aperture in poor seeing. This "Questar" effect (they used to tout this notion in advertisements) is said to be due to the physical cell size of the seeing turbulence. Scientific seeing is quantified by "R-zero" that characterizes the RMS of seeing "cells" and a typical value for mediocre seeing is 4" (inches) - so a 3.5" Questar is thought to be largely unaffected by most seeing; the image jumps around but has a measure of integrity, whereas a larger aperture convolves multiple cells so the image doesn't jump around so much but is blurrier. However, that notion has been repeatedly disproved both theoretically and practically.

Stan
• thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have
Message 3 of 21 , Aug 16, 2013
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thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have under "typical" seeing conditions.

Lee

--- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@...> wrote:
>
• Having worked with 16 , 20 , and 40 telescope at this point, I would describe seeing not as a problem or a difficulty, but as appropriate or not. For example,
Message 4 of 21 , Aug 16, 2013
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Having worked with 16", 20", and 40" telescope at this point, I would describe seeing not as a problem or a difficulty, but as appropriate or not.

For example, if I want fine detail, then I'm going to wait until I have really good seeing. If I want to record an extremely dim object, I also need best possible seeing.

But even with modest seeing, I can take satisfying images of many targets.

You have to take the seeing you get. Everyone can decide for themselves when the seeing is "too bad for imaging."

For example, I get really excited when the seeing drops below 1", because the one-meter telescope can take advantage of that.

So "bad seeing" on that scope is anything over about an arcsecond. But if we didn't use the scope when the seeing was 1.2" or 1.5" or even 2", we wouldn't use it very much. :-)

So I get excited when the seeing approaches the capability of a given telescope, but that's not the only time I use it!

As I will be describing in a talk at the Australian AIC next week, there are also things you can do to control the net seeing effects at the focal plane. (Will also come out in book form later this year.)

So that's the way I see it: just part of the job. You do what you can in terms of locating your observatory or observing location; you build your observatory to minimize seeing effects, you design (or modify) the telescope to minimize its contribution to seeing, you deal with temperature differences in the observatory, telescope, and/or optics, and so on.

In my experience, you can gain from 0.5-1.5" of improved seeing by knowing what to do with the above variables. (More, if you can get a site at 10,000 feet, of course!) But at a given location, with the right skills, you can definitely improve your best FWHM.

nature's part in the seeing can't be modified, but there are seeing forecasts to help you plan. :-)

So a bigger aperture may not give you a smaller FWHM if your site can't deliver one. But it is a bigger aperture, so even if you can't get much additional detail, you are getting more light, and you can image more efficiently at the image scale that your site and equipment allow.

On Aug 16, 2013, at 9:56 AM, lmbuck2000 <lmbuck2000@...> wrote:

>
>
> thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have under "typical" seeing conditions.
>
> Lee
>
> --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@...> wrote:
> >
>
>

[Non-text portions of this message have been removed]
• really helpful/encouraging explanation, ron. thank you. definitely will be looking forward to your new book. Lee
Message 5 of 21 , Aug 17, 2013
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really helpful/encouraging explanation, ron. thank you. definitely will be looking forward to your new book.

Lee

--- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
>
• Dumb question for either you, Ron, or Stan, or maybe both: as a practical matter, how do you measure seeing so as to arrive at the 1 arc sec value or whatever?
Message 6 of 21 , Aug 19, 2013
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Dumb question for either you, Ron, or Stan, or maybe both: as a practical matter, how do you measure seeing so as to arrive at the 1 arc sec value or whatever?

--- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
>
> Having worked with 16", 20", and 40" telescope at this point, I would describe seeing not as a problem or a difficulty, but as appropriate or not.
>
> For example, if I want fine detail, then I'm going to wait until I have really good seeing. If I want to record an extremely dim object, I also need best possible seeing.
>
> But even with modest seeing, I can take satisfying images of many targets.
>
> You have to take the seeing you get. Everyone can decide for themselves when the seeing is "too bad for imaging."
>
> For example, I get really excited when the seeing drops below 1", because the one-meter telescope can take advantage of that.
>
> So "bad seeing" on that scope is anything over about an arcsecond. But if we didn't use the scope when the seeing was 1.2" or 1.5" or even 2", we wouldn't use it very much. :-)
>
> So I get excited when the seeing approaches the capability of a given telescope, but that's not the only time I use it!
>
> As I will be describing in a talk at the Australian AIC next week, there are also things you can do to control the net seeing effects at the focal plane. (Will also come out in book form later this year.)
>
> So that's the way I see it: just part of the job. You do what you can in terms of locating your observatory or observing location; you build your observatory to minimize seeing effects, you design (or modify) the telescope to minimize its contribution to seeing, you deal with temperature differences in the observatory, telescope, and/or optics, and so on.
>
> In my experience, you can gain from 0.5-1.5" of improved seeing by knowing what to do with the above variables. (More, if you can get a site at 10,000 feet, of course!) But at a given location, with the right skills, you can definitely improve your best FWHM.
>
> nature's part in the seeing can't be modified, but there are seeing forecasts to help you plan. :-)
>
> So a bigger aperture may not give you a smaller FWHM if your site can't deliver one. But it is a bigger aperture, so even if you can't get much additional detail, you are getting more light, and you can image more efficiently at the image scale that your site and equipment allow.
>
>
>
>
> On Aug 16, 2013, at 9:56 AM, lmbuck2000 <lmbuck2000@...> wrote:
>
> >
> >
> > thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have under "typical" seeing conditions.
> >
> > Lee
> >
> > --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@> wrote:
> > >
> >
> >
>
>
>
> [Non-text portions of this message have been removed]
>
• The most accurate method is to measure the high-speed scintillations. In simplest terms, one samples a star about 100x per second, and you determine the amount
Message 7 of 21 , Aug 19, 2013
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The most accurate method is to measure the high-speed scintillations. In simplest terms, one samples a star about 100x per second, and you determine the amount its centroid moves. The actual calculation is somewhat complex because the star is not going to move the same amount every time. :-) You need to develop statistics about the star's wanderings. The most common way this is expressed as the FWHM of the star's centroids.

This is not the same as taking an image of a star for one second and measuring the FWHM, since a seeing monitor attempts to measure purely atmospheric seeing. Your telescope has to deal with additional problems, such as dome effects, tube currents, etc.

Also, the height above ground where the seeing measurement is taken is critical, since most seeing effects are the result of mixing and temperature differences based on the ground and stuff attached to it, such as trees and buildings.

A seeing monitor typically uses either a double aperture (the light from one star goes through two apertures on the same telescope, and the differences between the air masses they pass thru result in a seeing calculation) or a single aperture (using TDI, Time Delay Integration, which is reading out the chip as light continues to hit it, thus spreading the star's light out into a series of lines of varying width) to measure seeing metrics.

Both of those methods develop much more accurate seeing information than an integrated exposure because of the series of short exposures, each of which contains information about a specific moment in the process.

These individual measurements are typically combined, use different statistical methods, over a period of time (say, a minute or two) in order to characterize the seeing in a useful way for 'long' exposures (which for professional telescope are often simply a few minutes duration at most).

A good reason for measuring purely atmospheric seeing is that it is the uncontrollable part of seeing. As I will be discussing at length at both AAIC and AIC, other aspects of seeing are controllable - the contribution of your location, your observatory, and your telescope can all be mitigated by taking the right steps, both passive (height above ground; open truss, etc.) and active (fans or air blades near your optical surfaces).

On Aug 19, 2013, at 8:56 AM, CurtisC <calypte@...> wrote:

> Dumb question for either you, Ron, or Stan, or maybe both: as a practical matter, how do you measure seeing so as to arrive at the 1 arc sec value or whatever?
>
> --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
> >
> > Having worked with 16", 20", and 40" telescope at this point, I would describe seeing not as a problem or a difficulty, but as appropriate or not.
> >
> > For example, if I want fine detail, then I'm going to wait until I have really good seeing. If I want to record an extremely dim object, I also need best possible seeing.
> >
> > But even with modest seeing, I can take satisfying images of many targets.
> >
> > You have to take the seeing you get. Everyone can decide for themselves when the seeing is "too bad for imaging."
> >
> > For example, I get really excited when the seeing drops below 1", because the one-meter telescope can take advantage of that.
> >
> > So "bad seeing" on that scope is anything over about an arcsecond. But if we didn't use the scope when the seeing was 1.2" or 1.5" or even 2", we wouldn't use it very much. :-)
> >
> > So I get excited when the seeing approaches the capability of a given telescope, but that's not the only time I use it!
> >
> > As I will be describing in a talk at the Australian AIC next week, there are also things you can do to control the net seeing effects at the focal plane. (Will also come out in book form later this year.)
> >
> > So that's the way I see it: just part of the job. You do what you can in terms of locating your observatory or observing location; you build your observatory to minimize seeing effects, you design (or modify) the telescope to minimize its contribution to seeing, you deal with temperature differences in the observatory, telescope, and/or optics, and so on.
> >
> > In my experience, you can gain from 0.5-1.5" of improved seeing by knowing what to do with the above variables. (More, if you can get a site at 10,000 feet, of course!) But at a given location, with the right skills, you can definitely improve your best FWHM.
> >
> > nature's part in the seeing can't be modified, but there are seeing forecasts to help you plan. :-)
> >
> > So a bigger aperture may not give you a smaller FWHM if your site can't deliver one. But it is a bigger aperture, so even if you can't get much additional detail, you are getting more light, and you can image more efficiently at the image scale that your site and equipment allow.
> >
> >
> >
> >
> > On Aug 16, 2013, at 9:56 AM, lmbuck2000 <lmbuck2000@...> wrote:
> >
> > >
> > >
> > > thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have under "typical" seeing conditions.
> > >
> > > Lee
> > >
> > > --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@> wrote:
> > > >
> > >
> > >
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
>

[Non-text portions of this message have been removed]
• At Palomar Observatory they have an SBIG Seeing Monitor in the old 18-inch Schmidt dome. It monitors Polaris. The graph it produces is interesting, but it s
Message 8 of 21 , Aug 19, 2013
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At Palomar Observatory they have an SBIG Seeing Monitor in the old 18-inch Schmidt dome. It monitors Polaris. The graph it produces is interesting, but it's not the same value as reported in the Night Assistant's report, which is somehow measured within the telescope (Hale 200-inch) and associated cameras. I once asked the Lead Night Assistant how they measure seeing, and she even showed me a readout on a screen, but the explanation sailed right past me. The Night Assistant's seeing value (which appears to be an average) is usually better than that reported by the Seeing Monitor. For example, last night (Aug 18-19, 2013) the Night Assistant reported seeing of 1.2, whereas the Seeing Monitor reported an average of 1.71. Seeing at the telescope is related to the objects being observed, which are typically higher in the sky than Polaris.

--- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
>
> The most accurate method is to measure the high-speed scintillations. In simplest terms, one samples a star about 100x per second, and you determine the amount its centroid moves. The actual calculation is somewhat complex because the star is not going to move the same amount every time. :-) You need to develop statistics about the star's wanderings. The most common way this is expressed as the FWHM of the star's centroids.
>
> This is not the same as taking an image of a star for one second and measuring the FWHM, since a seeing monitor attempts to measure purely atmospheric seeing. Your telescope has to deal with additional problems, such as dome effects, tube currents, etc.
>
> Also, the height above ground where the seeing measurement is taken is critical, since most seeing effects are the result of mixing and temperature differences based on the ground and stuff attached to it, such as trees and buildings.
>
> A seeing monitor typically uses either a double aperture (the light from one star goes through two apertures on the same telescope, and the differences between the air masses they pass thru result in a seeing calculation) or a single aperture (using TDI, Time Delay Integration, which is reading out the chip as light continues to hit it, thus spreading the star's light out into a series of lines of varying width) to measure seeing metrics.
>
> Both of those methods develop much more accurate seeing information than an integrated exposure because of the series of short exposures, each of which contains information about a specific moment in the process.
>
> These individual measurements are typically combined, use different statistical methods, over a period of time (say, a minute or two) in order to characterize the seeing in a useful way for 'long' exposures (which for professional telescope are often simply a few minutes duration at most).
>
> A good reason for measuring purely atmospheric seeing is that it is the uncontrollable part of seeing. As I will be discussing at length at both AAIC and AIC, other aspects of seeing are controllable - the contribution of your location, your observatory, and your telescope can all be mitigated by taking the right steps, both passive (height above ground; open truss, etc.) and active (fans or air blades near your optical surfaces).
>
>
>
>
> On Aug 19, 2013, at 8:56 AM, CurtisC <calypte@...> wrote:
>
> > Dumb question for either you, Ron, or Stan, or maybe both: as a practical matter, how do you measure seeing so as to arrive at the 1 arc sec value or whatever?
> >
> > --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@> wrote:
> > >
> > > Having worked with 16", 20", and 40" telescope at this point, I would describe seeing not as a problem or a difficulty, but as appropriate or not.
> > >
> > > For example, if I want fine detail, then I'm going to wait until I have really good seeing. If I want to record an extremely dim object, I also need best possible seeing.
> > >
> > > But even with modest seeing, I can take satisfying images of many targets.
> > >
> > > You have to take the seeing you get. Everyone can decide for themselves when the seeing is "too bad for imaging."
> > >
> > > For example, I get really excited when the seeing drops below 1", because the one-meter telescope can take advantage of that.
> > >
> > > So "bad seeing" on that scope is anything over about an arcsecond. But if we didn't use the scope when the seeing was 1.2" or 1.5" or even 2", we wouldn't use it very much. :-)
> > >
> > > So I get excited when the seeing approaches the capability of a given telescope, but that's not the only time I use it!
> > >
> > > As I will be describing in a talk at the Australian AIC next week, there are also things you can do to control the net seeing effects at the focal plane. (Will also come out in book form later this year.)
> > >
> > > So that's the way I see it: just part of the job. You do what you can in terms of locating your observatory or observing location; you build your observatory to minimize seeing effects, you design (or modify) the telescope to minimize its contribution to seeing, you deal with temperature differences in the observatory, telescope, and/or optics, and so on.
> > >
> > > In my experience, you can gain from 0.5-1.5" of improved seeing by knowing what to do with the above variables. (More, if you can get a site at 10,000 feet, of course!) But at a given location, with the right skills, you can definitely improve your best FWHM.
> > >
> > > nature's part in the seeing can't be modified, but there are seeing forecasts to help you plan. :-)
> > >
> > > So a bigger aperture may not give you a smaller FWHM if your site can't deliver one. But it is a bigger aperture, so even if you can't get much additional detail, you are getting more light, and you can image more efficiently at the image scale that your site and equipment allow.
> > >
> > >
> > >
> > >
> > > On Aug 16, 2013, at 9:56 AM, lmbuck2000 <lmbuck2000@> wrote:
> > >
> > > >
> > > >
> > > > thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have under "typical" seeing conditions.
> > > >
> > > > Lee
> > > >
> > > > --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@> wrote:
> > > > >
> > > >
> > > >
> > >
> > >
> > >
> > > [Non-text portions of this message have been removed]
> > >
> >
> >
>
>
>
> [Non-text portions of this message have been removed]
>
• BTW: it loos like they are using some cutting-edge hardware and software for measuring seeing at Palomar, something called a Multi- Aperture Scintillation
Message 9 of 21 , Aug 19, 2013
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BTW: it loos like they are using some cutting-edge hardware and software for measuring seeing at Palomar, something called a Multi- Aperture Scintillation Sensor (MASS) combined with DIMM (the two-aperture technique I mentioned in my other response). In simplest terms, MASS measures the amount of light falling on a series of circular bands around the nominal star position, and then slices and dices that data to determine the atmospheric turbulence at various elevations about the observatory. These are weighted for distance to determine the actual seeing at the telescope pupil.

On Aug 19, 2013, at 8:56 AM, CurtisC <calypte@...> wrote:

> Dumb question for either you, Ron, or Stan, or maybe both: as a practical matter, how do you measure seeing so as to arrive at the 1 arc sec value or whatever?
>
> --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
> >
> > Having worked with 16", 20", and 40" telescope at this point, I would describe seeing not as a problem or a difficulty, but as appropriate or not.
> >
> > For example, if I want fine detail, then I'm going to wait until I have really good seeing. If I want to record an extremely dim object, I also need best possible seeing.
> >
> > But even with modest seeing, I can take satisfying images of many targets.
> >
> > You have to take the seeing you get. Everyone can decide for themselves when the seeing is "too bad for imaging."
> >
> > For example, I get really excited when the seeing drops below 1", because the one-meter telescope can take advantage of that.
> >
> > So "bad seeing" on that scope is anything over about an arcsecond. But if we didn't use the scope when the seeing was 1.2" or 1.5" or even 2", we wouldn't use it very much. :-)
> >
> > So I get excited when the seeing approaches the capability of a given telescope, but that's not the only time I use it!
> >
> > As I will be describing in a talk at the Australian AIC next week, there are also things you can do to control the net seeing effects at the focal plane. (Will also come out in book form later this year.)
> >
> > So that's the way I see it: just part of the job. You do what you can in terms of locating your observatory or observing location; you build your observatory to minimize seeing effects, you design (or modify) the telescope to minimize its contribution to seeing, you deal with temperature differences in the observatory, telescope, and/or optics, and so on.
> >
> > In my experience, you can gain from 0.5-1.5" of improved seeing by knowing what to do with the above variables. (More, if you can get a site at 10,000 feet, of course!) But at a given location, with the right skills, you can definitely improve your best FWHM.
> >
> > nature's part in the seeing can't be modified, but there are seeing forecasts to help you plan. :-)
> >
> > So a bigger aperture may not give you a smaller FWHM if your site can't deliver one. But it is a bigger aperture, so even if you can't get much additional detail, you are getting more light, and you can image more efficiently at the image scale that your site and equipment allow.
> >
> >
> >
> >
> > On Aug 16, 2013, at 9:56 AM, lmbuck2000 <lmbuck2000@...> wrote:
> >
> > >
> > >
> > > thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have under "typical" seeing conditions.
> > >
> > > Lee
> > >
> > > --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@> wrote:
> > > >
> > >
> > >
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
>

[Non-text portions of this message have been removed]
• I have found the clear sky chart to a be a fair qualitative reference for what is going on where I live. They use the following scale: Color Meaning Excellent
Message 10 of 21 , Aug 19, 2013
• 0 Attachment
I have found the clear sky chart to a be a fair qualitative reference for what is going on where I live. They use the following scale:

Color Meaning
Excellent 5/5
Good 4/5
Average 3/5
Poor 2/5
Too cloudy to forecast

I am not sure how they create this. I am sure they do not use the quantitative approach Ron references, but probably factor in the weather, jet stream etc. This clear sky clock obviously would not address the local items that Ron speaks of: telescope, observatory, and other items that can contribute to seeing (e.g. rocks , pavement, houses). I look to learn more on this topic from Ron's future sessions. Having said that, I found the CSC to be somewhat representative to what I can expect.

Jim Miller

----- Original Message -----

To: ccd-newastro@yahoogroups.com
Sent: Monday, August 19, 2013 10:53:07 AM
Subject: Re: [ccd-newastro] aperture or focal length most relevant to good/bad seeing ?

The most accurate method is to measure the high-speed scintillations. In simplest terms, one samples a star about 100x per second, and you determine the amount its centroid moves. The actual calculation is somewhat complex because the star is not going to move the same amount every time. :-) You need to develop statistics about the star's wanderings. The most common way this is expressed as the FWHM of the star's centroids.

This is not the same as taking an image of a star for one second and measuring the FWHM, since a seeing monitor attempts to measure purely atmospheric seeing. Your telescope has to deal with additional problems, such as dome effects, tube currents, etc.

Also, the height above ground where the seeing measurement is taken is critical, since most seeing effects are the result of mixing and temperature differences based on the ground and stuff attached to it, such as trees and buildings.

A seeing monitor typically uses either a double aperture (the light from one star goes through two apertures on the same telescope, and the differences between the air masses they pass thru result in a seeing calculation) or a single aperture (using TDI, Time Delay Integration, which is reading out the chip as light continues to hit it, thus spreading the star's light out into a series of lines of varying width) to measure seeing metrics.

Both of those methods develop much more accurate seeing information than an integrated exposure because of the series of short exposures, each of which contains information about a specific moment in the process.

These individual measurements are typically combined, use different statistical methods, over a period of time (say, a minute or two) in order to characterize the seeing in a useful way for 'long' exposures (which for professional telescope are often simply a few minutes duration at most).

A good reason for measuring purely atmospheric seeing is that it is the uncontrollable part of seeing. As I will be discussing at length at both AAIC and AIC, other aspects of seeing are controllable - the contribution of your location, your observatory, and your telescope can all be mitigated by taking the right steps, both passive (height above ground; open truss, etc.) and active (fans or air blades near your optical surfaces).

On Aug 19, 2013, at 8:56 AM, CurtisC <calypte@...> wrote:

> Dumb question for either you, Ron, or Stan, or maybe both: as a practical matter, how do you measure seeing so as to arrive at the 1 arc sec value or whatever?
>
> --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
> >
> > Having worked with 16", 20", and 40" telescope at this point, I would describe seeing not as a problem or a difficulty, but as appropriate or not.
> >
> > For example, if I want fine detail, then I'm going to wait until I have really good seeing. If I want to record an extremely dim object, I also need best possible seeing.
> >
> > But even with modest seeing, I can take satisfying images of many targets.
> >
> > You have to take the seeing you get. Everyone can decide for themselves when the seeing is "too bad for imaging."
> >
> > For example, I get really excited when the seeing drops below 1", because the one-meter telescope can take advantage of that.
> >
> > So "bad seeing" on that scope is anything over about an arcsecond. But if we didn't use the scope when the seeing was 1.2" or 1.5" or even 2", we wouldn't use it very much. :-)
> >
> > So I get excited when the seeing approaches the capability of a given telescope, but that's not the only time I use it!
> >
> > As I will be describing in a talk at the Australian AIC next week, there are also things you can do to control the net seeing effects at the focal plane. (Will also come out in book form later this year.)
> >
> > So that's the way I see it: just part of the job. You do what you can in terms of locating your observatory or observing location; you build your observatory to minimize seeing effects, you design (or modify) the telescope to minimize its contribution to seeing, you deal with temperature differences in the observatory, telescope, and/or optics, and so on.
> >
> > In my experience, you can gain from 0.5-1.5" of improved seeing by knowing what to do with the above variables. (More, if you can get a site at 10,000 feet, of course!) But at a given location, with the right skills, you can definitely improve your best FWHM.
> >
> > nature's part in the seeing can't be modified, but there are seeing forecasts to help you plan. :-)
> >
> > So a bigger aperture may not give you a smaller FWHM if your site can't deliver one. But it is a bigger aperture, so even if you can't get much additional detail, you are getting more light, and you can image more efficiently at the image scale that your site and equipment allow.
> >
> >
> >
> >
> > On Aug 16, 2013, at 9:56 AM, lmbuck2000 <lmbuck2000@...> wrote:
> >
> > >
> > >
> > > thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have under "typical" seeing conditions.
> > >
> > > Lee
> > >
> > > --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@> wrote:
> > > >
> > >
> > >
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
>

[Non-text portions of this message have been removed]

------------------------------------

[Non-text portions of this message have been removed]
• Well, it IS Caltech! :)
Message 11 of 21 , Aug 19, 2013
• 0 Attachment
Well, it IS Caltech! :)

--- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
>
> BTW: it loos like they are using some cutting-edge hardware and software for measuring seeing at Palomar,
• The Clear Sky Clock is a useful tool, as you ve discovered. It s a prediction, and it s accuracy will vary somewhat in any given area of coverage. It s based
Message 12 of 21 , Aug 19, 2013
• 0 Attachment
The Clear Sky Clock is a useful tool, as you've discovered. It's a prediction, and it's accuracy will vary somewhat in any given area of coverage. It's based on expected weather patterns, and it's general for an area. It's not very reliable at our Cloudcroft, NM observatory, but we have fairly unique conditions there that would change dramatically if you moved even 1000 yards from the spot. :-)

meteoblue.com is also a good resource.

On Aug 19, 2013, at 2:47 PM, jfmiller7@... wrote:

>
>
> I have found the clear sky chart to a be a fair qualitative reference for what is going on where I live. They use the following scale:
>
> Color Meaning
> Excellent 5/5
> Good 4/5
> Average 3/5
> Poor 2/5
> Too cloudy to forecast
>
> I am not sure how they create this. I am sure they do not use the quantitative approach Ron references, but probably factor in the weather, jet stream etc. This clear sky clock obviously would not address the local items that Ron speaks of: telescope, observatory, and other items that can contribute to seeing (e.g. rocks , pavement, houses). I look to learn more on this topic from Ron's future sessions. Having said that, I found the CSC to be somewhat representative to what I can expect.
>
>
> Jim Miller
>
> ----- Original Message -----
>
> To: ccd-newastro@yahoogroups.com
> Sent: Monday, August 19, 2013 10:53:07 AM
> Subject: Re: [ccd-newastro] aperture or focal length most relevant to good/bad seeing ?
>
> The most accurate method is to measure the high-speed scintillations. In simplest terms, one samples a star about 100x per second, and you determine the amount its centroid moves. The actual calculation is somewhat complex because the star is not going to move the same amount every time. :-) You need to develop statistics about the star's wanderings. The most common way this is expressed as the FWHM of the star's centroids.
>
> This is not the same as taking an image of a star for one second and measuring the FWHM, since a seeing monitor attempts to measure purely atmospheric seeing. Your telescope has to deal with additional problems, such as dome effects, tube currents, etc.
>
> Also, the height above ground where the seeing measurement is taken is critical, since most seeing effects are the result of mixing and temperature differences based on the ground and stuff attached to it, such as trees and buildings.
>
> A seeing monitor typically uses either a double aperture (the light from one star goes through two apertures on the same telescope, and the differences between the air masses they pass thru result in a seeing calculation) or a single aperture (using TDI, Time Delay Integration, which is reading out the chip as light continues to hit it, thus spreading the star's light out into a series of lines of varying width) to measure seeing metrics.
>
> Both of those methods develop much more accurate seeing information than an integrated exposure because of the series of short exposures, each of which contains information about a specific moment in the process.
>
> These individual measurements are typically combined, use different statistical methods, over a period of time (say, a minute or two) in order to characterize the seeing in a useful way for 'long' exposures (which for professional telescope are often simply a few minutes duration at most).
>
> A good reason for measuring purely atmospheric seeing is that it is the uncontrollable part of seeing. As I will be discussing at length at both AAIC and AIC, other aspects of seeing are controllable - the contribution of your location, your observatory, and your telescope can all be mitigated by taking the right steps, both passive (height above ground; open truss, etc.) and active (fans or air blades near your optical surfaces).
>
>
> On Aug 19, 2013, at 8:56 AM, CurtisC <calypte@...> wrote:
>
> > Dumb question for either you, Ron, or Stan, or maybe both: as a practical matter, how do you measure seeing so as to arrive at the 1 arc sec value or whatever?
> >
> > --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
> > >
> > > Having worked with 16", 20", and 40" telescope at this point, I would describe seeing not as a problem or a difficulty, but as appropriate or not.
> > >
> > > For example, if I want fine detail, then I'm going to wait until I have really good seeing. If I want to record an extremely dim object, I also need best possible seeing.
> > >
> > > But even with modest seeing, I can take satisfying images of many targets.
> > >
> > > You have to take the seeing you get. Everyone can decide for themselves when the seeing is "too bad for imaging."
> > >
> > > For example, I get really excited when the seeing drops below 1", because the one-meter telescope can take advantage of that.
> > >
> > > So "bad seeing" on that scope is anything over about an arcsecond. But if we didn't use the scope when the seeing was 1.2" or 1.5" or even 2", we wouldn't use it very much. :-)
> > >
> > > So I get excited when the seeing approaches the capability of a given telescope, but that's not the only time I use it!
> > >
> > > As I will be describing in a talk at the Australian AIC next week, there are also things you can do to control the net seeing effects at the focal plane. (Will also come out in book form later this year.)
> > >
> > > So that's the way I see it: just part of the job. You do what you can in terms of locating your observatory or observing location; you build your observatory to minimize seeing effects, you design (or modify) the telescope to minimize its contribution to seeing, you deal with temperature differences in the observatory, telescope, and/or optics, and so on.
> > >
> > > In my experience, you can gain from 0.5-1.5" of improved seeing by knowing what to do with the above variables. (More, if you can get a site at 10,000 feet, of course!) But at a given location, with the right skills, you can definitely improve your best FWHM.
> > >
> > > nature's part in the seeing can't be modified, but there are seeing forecasts to help you plan. :-)
> > >
> > > So a bigger aperture may not give you a smaller FWHM if your site can't deliver one. But it is a bigger aperture, so even if you can't get much additional detail, you are getting more light, and you can image more efficiently at the image scale that your site and equipment allow.
> > >
> > >
> > >
> > >
> > > On Aug 16, 2013, at 9:56 AM, lmbuck2000 <lmbuck2000@...> wrote:
> > >
> > > >
> > > >
> > > > thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have under "typical" seeing conditions.
> > > >
> > > > Lee
> > > >
> > > > --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@> wrote:
> > > > >
> > > >
> > > >
> > >
> > >
> > >
> > > [Non-text portions of this message have been removed]
> > >
> >
> >
>
> [Non-text portions of this message have been removed]
>
> ------------------------------------
>
> [Non-text portions of this message have been removed]
>
>

[Non-text portions of this message have been removed]
• The distinction here is that we re talking about measuring seeing as it actually occurs, not as it s forecast.
Message 13 of 21 , Aug 19, 2013
• 0 Attachment
The distinction here is that we're talking about measuring seeing as it actually occurs, not as it's forecast.

--- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
>
> The Clear Sky Clock is a useful tool, as you've discovered. It's a prediction, and it's accuracy will vary somewhat in any given area of coverage. It's based on expected weather patterns, and it's general for an area. It's not very reliable at our Cloudcroft, NM observatory, but we have fairly unique conditions there that would change dramatically if you moved even 1000 yards from the spot. :-)
>
> meteoblue.com is also a good resource.
>
>
>
>
> On Aug 19, 2013, at 2:47 PM, jfmiller7@... wrote:
>
> >
> >
> > I have found the clear sky chart to a be a fair qualitative reference for what is going on where I live. They use the following scale:
> >
> > Color Meaning
> > Excellent 5/5
> > Good 4/5
> > Average 3/5
> > Poor 2/5
> > Too cloudy to forecast
> >
> > I am not sure how they create this. I am sure they do not use the quantitative approach Ron references, but probably factor in the weather, jet stream etc. This clear sky clock obviously would not address the local items that Ron speaks of: telescope, observatory, and other items that can contribute to seeing (e.g. rocks , pavement, houses). I look to learn more on this topic from Ron's future sessions. Having said that, I found the CSC to be somewhat representative to what I can expect.
> >
> >
> > Jim Miller
> >
> > ----- Original Message -----
> >
> > From: "Ron Wodaski" <yahoo@...>
> > To: ccd-newastro@yahoogroups.com
> > Sent: Monday, August 19, 2013 10:53:07 AM
> > Subject: Re: [ccd-newastro] aperture or focal length most relevant to good/bad seeing ?
> >
> > The most accurate method is to measure the high-speed scintillations. In simplest terms, one samples a star about 100x per second, and you determine the amount its centroid moves. The actual calculation is somewhat complex because the star is not going to move the same amount every time. :-) You need to develop statistics about the star's wanderings. The most common way this is expressed as the FWHM of the star's centroids.
> >
> > This is not the same as taking an image of a star for one second and measuring the FWHM, since a seeing monitor attempts to measure purely atmospheric seeing. Your telescope has to deal with additional problems, such as dome effects, tube currents, etc.
> >
> > Also, the height above ground where the seeing measurement is taken is critical, since most seeing effects are the result of mixing and temperature differences based on the ground and stuff attached to it, such as trees and buildings.
> >
> > A seeing monitor typically uses either a double aperture (the light from one star goes through two apertures on the same telescope, and the differences between the air masses they pass thru result in a seeing calculation) or a single aperture (using TDI, Time Delay Integration, which is reading out the chip as light continues to hit it, thus spreading the star's light out into a series of lines of varying width) to measure seeing metrics.
> >
> > Both of those methods develop much more accurate seeing information than an integrated exposure because of the series of short exposures, each of which contains information about a specific moment in the process.
> >
> > These individual measurements are typically combined, use different statistical methods, over a period of time (say, a minute or two) in order to characterize the seeing in a useful way for 'long' exposures (which for professional telescope are often simply a few minutes duration at most).
> >
> > A good reason for measuring purely atmospheric seeing is that it is the uncontrollable part of seeing. As I will be discussing at length at both AAIC and AIC, other aspects of seeing are controllable - the contribution of your location, your observatory, and your telescope can all be mitigated by taking the right steps, both passive (height above ground; open truss, etc.) and active (fans or air blades near your optical surfaces).
> >
> >
> > On Aug 19, 2013, at 8:56 AM, CurtisC <calypte@...> wrote:
> >
> > > Dumb question for either you, Ron, or Stan, or maybe both: as a practical matter, how do you measure seeing so as to arrive at the 1 arc sec value or whatever?
> > >
> > > --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@> wrote:
> > > >
> > > > Having worked with 16", 20", and 40" telescope at this point, I would describe seeing not as a problem or a difficulty, but as appropriate or not.
> > > >
> > > > For example, if I want fine detail, then I'm going to wait until I have really good seeing. If I want to record an extremely dim object, I also need best possible seeing.
> > > >
> > > > But even with modest seeing, I can take satisfying images of many targets.
> > > >
> > > > You have to take the seeing you get. Everyone can decide for themselves when the seeing is "too bad for imaging."
> > > >
> > > > For example, I get really excited when the seeing drops below 1", because the one-meter telescope can take advantage of that.
> > > >
> > > > So "bad seeing" on that scope is anything over about an arcsecond. But if we didn't use the scope when the seeing was 1.2" or 1.5" or even 2", we wouldn't use it very much. :-)
> > > >
> > > > So I get excited when the seeing approaches the capability of a given telescope, but that's not the only time I use it!
> > > >
> > > > As I will be describing in a talk at the Australian AIC next week, there are also things you can do to control the net seeing effects at the focal plane. (Will also come out in book form later this year.)
> > > >
> > > > So that's the way I see it: just part of the job. You do what you can in terms of locating your observatory or observing location; you build your observatory to minimize seeing effects, you design (or modify) the telescope to minimize its contribution to seeing, you deal with temperature differences in the observatory, telescope, and/or optics, and so on.
> > > >
> > > > In my experience, you can gain from 0.5-1.5" of improved seeing by knowing what to do with the above variables. (More, if you can get a site at 10,000 feet, of course!) But at a given location, with the right skills, you can definitely improve your best FWHM.
> > > >
> > > > nature's part in the seeing can't be modified, but there are seeing forecasts to help you plan. :-)
> > > >
> > > > So a bigger aperture may not give you a smaller FWHM if your site can't deliver one. But it is a bigger aperture, so even if you can't get much additional detail, you are getting more light, and you can image more efficiently at the image scale that your site and equipment allow.
> > > >
> > > > Ron Wodaski
> > > >
> > > >
> > > >
> > > > On Aug 16, 2013, at 9:56 AM, lmbuck2000 <lmbuck2000@> wrote:
> > > >
> > > > >
> > > > >
> > > > > thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have under "typical" seeing conditions.
> > > > >
> > > > > Lee
> > > > >
> > > > > --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@> wrote:
> > > > > >
> > > > >
> > > > >
> > > >
> > > >
> > > >
> > > > [Non-text portions of this message have been removed]
> > > >
> > >
> > >
> >
> > [Non-text portions of this message have been removed]
> >
> > ------------------------------------
> >
> > [Non-text portions of this message have been removed]
> >
> >
>
>
>
> [Non-text portions of this message have been removed]
>
• ... Setting aside serious measuring machines (scintillation monitors, DIMM and such) a practical method is to really know your instrument and site thru
Message 14 of 21 , Aug 20, 2013
• 0 Attachment
--- "CurtisC" <calypte@...> wrote:
> as a practical matter, how do you measure seeing...

Setting aside serious measuring machines (scintillation monitors, DIMM and such) a "practical" method is to really know your instrument and site thru experience and measurements.

A key measurement is the FWHM for the better short exps on the best night. This value can be adjusted and used as the instrument's resolution limit. The instruments resolution limit can then be used to calculate the seeing on any other night by quadratically subtracting it from the FWHM of short exps.

short exp FWHM = sqrt(instrument^2 + seeing^2)
thus
instrument = sqrt(exp^2 - seeing^2)

In the absence of impendent measurements, it is necessary to make some reasonable assumptions. Seeing (measured in any way) is very rarely below 0.5" and most non-professional sites rarely experience less than 1". So it is not unreasonable to assume that a very good night has base seeing <=1"; be warmed that there are some chronically bad sites that never approach this. So use 1" as a first approximation:

Instrument limit = sqrt(exp^2 - 1)
Examples:
if best night FWHM = 1.5" then the instrument limit approx = 1.1"
if best night FWHM = 2.0" then the instrument limit approx = 1.7"
if best night FWHM <= 1" then the seeing was less than 1" and the instrument is very good.

Once you have some experience and confidence with the instrument then lesser seeing can be easily estimated via short exps:

seeing = sqrt(fwhm^2  instrument^2)

examples:

if instrument = 1" and short exp FWHM = 2.0 then seeing = 1.7"
if instrument = 1" and short exp FWHM = 2.5 then seeing = 2.3"
if instrument = 1" and short exp FWHM = 3.0 then seeing = 2.8"

if instrument = 2" and short exp FWHM = 2.5 then seeing = 1.5"
if instrument = 2" and short exp FWHM = 3.0 then seeing = 2.3"
if instrument = 2" and short exp FWHM = 3.5 then seeing = 2.9"

Estimation error increases as (exp - instrument) decreases.

Stan
• I often see people in astro forums confidently stating that their seeing is a certain value. But, having observed what they go through at Palomar to derive
Message 15 of 21 , Aug 20, 2013
• 0 Attachment
I often see people in astro forums confidently stating that their seeing is a certain value. But, having observed what they go through at Palomar to derive seeing, I often wonder how the average amateur astronomer can be so positive of what they're getting. I often check FWHM values on my downloaded images, mainly to verify good focus, but I never see FWHM values anywhere close to 1 arc sec. As to what the Night Assistant reports at the 200-inch Hale Telescope, the 1.2 average figure I cited for the night of Aug 18-19 is pretty typical for this time of year. Sometimes it's a bit better (down to, say, 0.9), sometimes quite a bit worse, especially in spring. My site here in Anza, CA, is similar enough to Palomar that my experience of "good" vs "poor" seeing usually parallels what they get, but I can only see it after the fact ("Oh, they had a lousy night too!")

--- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@...> wrote:
>
> --- "CurtisC" <calypte@> wrote:
> > as a practical matter, how do you measure seeing...
>
> Setting aside serious measuring machines (scintillation monitors, DIMM and such) a "practical" method is to really know your instrument and site thru experience and measurements.
>
> A key measurement is the FWHM for the better short exps on the best night. This value can be adjusted and used as the instrument's resolution limit. The instruments resolution limit can then be used to calculate the seeing on any other night by quadratically subtracting it from the FWHM of short exps.
>
> short exp FWHM = sqrt(instrument^2 + seeing^2)
> thus
> instrument = sqrt(exp^2 - seeing^2)
>
> In the absence of impendent measurements, it is necessary to make some reasonable assumptions. Seeing (measured in any way) is very rarely below 0.5" and most non-professional sites rarely experience less than 1". So it is not unreasonable to assume that a very good night has base seeing <=1"; be warmed that there are some chronically bad sites that never approach this. So use 1" as a first approximation:
>
> Instrument limit = sqrt(exp^2 - 1)
> Examples:
> if best night FWHM = 1.5" then the instrument limit approx = 1.1"
> if best night FWHM = 2.0" then the instrument limit approx = 1.7"
> if best night FWHM <= 1" then the seeing was less than 1" and the instrument is very good.
>
> Once you have some experience and confidence with the instrument then lesser seeing can be easily estimated via short exps:
>
> seeing = sqrt(fwhm^2  instrument^2)
>
> examples:
>
> if instrument = 1" and short exp FWHM = 2.0 then seeing = 1.7"
> if instrument = 1" and short exp FWHM = 2.5 then seeing = 2.3"
> if instrument = 1" and short exp FWHM = 3.0 then seeing = 2.8"
>
> if instrument = 2" and short exp FWHM = 2.5 then seeing = 1.5"
> if instrument = 2" and short exp FWHM = 3.0 then seeing = 2.3"
> if instrument = 2" and short exp FWHM = 3.5 then seeing = 2.9"
>
> Estimation error increases as (exp - instrument) decreases.
>
> Stan
>
• ... Very few amateurs do! Amateur DS FWHM = 1.0 was a holy grail for many years. I know of only a mere handful of credible achievements, most of them
Message 16 of 21 , Aug 21, 2013
• 0 Attachment
--- "CurtisC" <calypte@...> wrote:
> I often see people in astro forums confidently stating that
> their seeing is a certain value...
> but I never see FWHM values anywhere close to 1 arc sec.

Very few amateurs do! <g>

Amateur DS FWHM = 1.0" was a holy grail for many years. I know of only a mere handful of credible achievements, most of them stretching the definition of "amateur" or "deep space" (e.g. short exps from professional grade meter class RC on mountain top observatory).

Benoit Schillings and I combined our knowledge, skills and equipment to struggle for this goal over a very long time and finally broke thru that barrier with room to spare. Using Benoit's home-built 20" Newtonian (operating at f/20) and my home-assembled photon intensified camera at a good seeing camp site, we achieved FWHM near 0.5" for the core of M13:

http://www.stanmooreastro.com/IntensifiedAstronomicalImaging.htm

Since then I have been able to get near 1.0" on a few more occasions using C-14 with DS lucky imaging. (I need to update my web site with newer pics). So I have a pretty good idea of what it takes. It is not easy...

Stan
• I d like to try this out, Stan. But I have two questions: 1) Your second equation seems to mix units of seconds (time) with arc-seconds. The usual sidereal
Message 17 of 21 , Aug 21, 2013
• 0 Attachment
I'd like to try this out, Stan. But I have two questions:

1) Your second equation seems to mix units of seconds (time) with arc-seconds. The usual "sidereal" conversion factor doesn't seem to make sense here - what's the deal?
2) How long is a "short" exposure - something like 1 second?
3) Would it make sense to take, say, twenty 1 second exposures and use an average?

Thanks.

Bruce

--- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@...> wrote:
>
> --- "CurtisC" <calypte@> wrote:
> > as a practical matter, how do you measure seeing...
>
> Setting aside serious measuring machines (scintillation monitors, DIMM and such) a "practical" method is to really know your instrument and site thru experience and measurements.
>
> A key measurement is the FWHM for the better short exps on the best night. This value can be adjusted and used as the instrument's resolution limit. The instruments resolution limit can then be used to calculate the seeing on any other night by quadratically subtracting it from the FWHM of short exps.
>
> short exp FWHM = sqrt(instrument^2 + seeing^2)
> thus
> instrument = sqrt(exp^2 - seeing^2)
>
> In the absence of impendent measurements, it is necessary to make some reasonable assumptions. Seeing (measured in any way) is very rarely below 0.5" and most non-professional sites rarely experience less than 1". So it is not unreasonable to assume that a very good night has base seeing <=1"; be warmed that there are some chronically bad sites that never approach this. So use 1" as a first approximation:
>
> Instrument limit = sqrt(exp^2 - 1)
> Examples:
> if best night FWHM = 1.5" then the instrument limit approx = 1.1"
> if best night FWHM = 2.0" then the instrument limit approx = 1.7"
> if best night FWHM <= 1" then the seeing was less than 1" and the instrument is very good.
>
> Once you have some experience and confidence with the instrument then lesser seeing can be easily estimated via short exps:
>
> seeing = sqrt(fwhm^2  instrument^2)
>
> examples:
>
> if instrument = 1" and short exp FWHM = 2.0 then seeing = 1.7"
> if instrument = 1" and short exp FWHM = 2.5 then seeing = 2.3"
> if instrument = 1" and short exp FWHM = 3.0 then seeing = 2.8"
>
> if instrument = 2" and short exp FWHM = 2.5 then seeing = 1.5"
> if instrument = 2" and short exp FWHM = 3.0 then seeing = 2.3"
> if instrument = 2" and short exp FWHM = 3.5 then seeing = 2.9"
>
> Estimation error increases as (exp - instrument) decreases.
>
> Stan
>
• I ll throw-in my 2-cents worth. I m not an expert in seeing, and my method does not give a definite seeing value. It s also affected by mount alignment, worm
Message 18 of 21 , Aug 22, 2013
• 0 Attachment
I'll throw-in my 2-cents worth. I'm not an expert in seeing, and my method does not give a definite seeing value. It's also affected by mount alignment, worm accuracy/PE. But since I align my mount to the same accuracy for each use (<1 arc-min), and my Mach1 is a fairly accurate mount, I figure that it's my night-to-night comparisons should be fairly accurate.

When I guide, using Maxim, I see the macro effects of seeing, by looking at the guider corrections on the scope. I can't really say with certainty that my seeing is X, but I can see definite differences between good nights and poor nights. I approximate the seeing by the variation in my guiding graph. What I concern myself with is that my "guider seeing" is better than what I need for my scope/camera combo.

Since I shoot with an FSQ-106ED and an FLI11K CCD, my image scale is 3.5 arc-sec per pixel. So I figure that as long as my guiding chart indicates less than +/- 2 px in guiding, I should be near 1 pixel in image scale resolution. I also save each night's guiding, so if I see problems in post-processing, I can review the graph to see if there was any anomalies.

With this technique, my "guiding seeing" is usually around 1-2 arc-sec. On the rare evening, the atmosphere calms to where my guiding graph is less than +/- 1/2 Arc-sec.

Maybe a rough way to look at it, but at least I have fairly repeatable way to see the night-to-night variations in atmospherics.

Again, a rough technique, but one that seems to fit my wide-field imaging requirements.

Eric

--- In ccd-newastro@yahoogroups.com, "CurtisC" <calypte@...> wrote:
>
> Dumb question for either you, Ron, or Stan, or maybe both: as a practical matter, how do you measure seeing so as to arrive at the 1 arc sec value or whatever?
>
> --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@> wrote:
> >
> > Having worked with 16", 20", and 40" telescope at this point, I would describe seeing not as a problem or a difficulty, but as appropriate or not.
> >
> > For example, if I want fine detail, then I'm going to wait until I have really good seeing. If I want to record an extremely dim object, I also need best possible seeing.
> >
> > But even with modest seeing, I can take satisfying images of many targets.
> >
> > You have to take the seeing you get. Everyone can decide for themselves when the seeing is "too bad for imaging."
> >
> > For example, I get really excited when the seeing drops below 1", because the one-meter telescope can take advantage of that.
> >
> > So "bad seeing" on that scope is anything over about an arcsecond. But if we didn't use the scope when the seeing was 1.2" or 1.5" or even 2", we wouldn't use it very much. :-)
> >
> > So I get excited when the seeing approaches the capability of a given telescope, but that's not the only time I use it!
> >
> > As I will be describing in a talk at the Australian AIC next week, there are also things you can do to control the net seeing effects at the focal plane. (Will also come out in book form later this year.)
> >
> > So that's the way I see it: just part of the job. You do what you can in terms of locating your observatory or observing location; you build your observatory to minimize seeing effects, you design (or modify) the telescope to minimize its contribution to seeing, you deal with temperature differences in the observatory, telescope, and/or optics, and so on.
> >
> > In my experience, you can gain from 0.5-1.5" of improved seeing by knowing what to do with the above variables. (More, if you can get a site at 10,000 feet, of course!) But at a given location, with the right skills, you can definitely improve your best FWHM.
> >
> > nature's part in the seeing can't be modified, but there are seeing forecasts to help you plan. :-)
> >
> > So a bigger aperture may not give you a smaller FWHM if your site can't deliver one. But it is a bigger aperture, so even if you can't get much additional detail, you are getting more light, and you can image more efficiently at the image scale that your site and equipment allow.
> >
> >
> >
> >
> > On Aug 16, 2013, at 9:56 AM, lmbuck2000 <lmbuck2000@> wrote:
> >
> > >
> > >
> > > thanks, stan. appreciate the explanation. would be interested to hear from some users of 16-20inch aperture scopes (@f/7-f/9) how much difficulty they have under "typical" seeing conditions.
> > >
> > > Lee
> > >
> > > --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@> wrote:
> > > >
> > >
> > >
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
• ... in the eq, exp is meant to refer to the FWHM of the exp ... The full answer would be a long discourse because seeing is a time based dynamic. Seeing
Message 19 of 21 , Aug 22, 2013
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> 1) Your second equation seems to mix units of seconds (time)

in the eq, "exp" is meant to refer to the FWHM of the exp

> 2) How long is a "short" exposure - something like 1 second?

The full answer would be a long discourse because seeing is a time based dynamic. "Seeing wander" is often a significant component of image FWHM but it is not measured by instruments like DIMM because such devices normalize wander and only measure relative wavefront distortions. Also, wander is a function of aperture (larger apertures produce less wander) so including it in the measurement produces an aperture dependent seeing value.

To derive the seeing independently of wander via a normal scope/exp requires very short exps, such as 10-20ms. But such short exps produce varying, irregular non-Gaussian PSF that yield unreliable FWHM. A stream of a few hundred 20ms exps can be aligned and stacked to produce a virtually wanderless FWHM and I do that frequently with intensified video. But that's not practical for slow CCD and not even possible for charge transfer CCDs (shortest exp time is >100ms for many such cameras).

So 1-5 seconds is usually enough time to get a near Gaussian PSF and short enough to escape most mount issues (except vibrations), especially if you take a dozen or so exps. But it does include wander and as such may over-estimate the core seeing. However, it should be fairly consistent for that scope and can be a useful measurement.

> 3) Would it make sense to take, say, twenty 1 second
> exposures and use an average?

Yes.

Stan
• ... That is an easy and useful method of estimating conditions relative to particular equipment and practices. However, as you note it does not produce an
Message 20 of 21 , Aug 23, 2013
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--- "echesak@..." <echesak@...> wrote:
> I see the macro effects of seeing, by looking at the guider
> corrections on the scope...

That is an easy and useful method of estimating conditions relative to particular equipment and practices. However, as you note it does not produce an accurate seeing FWHM that can be reliably compared with other systems and measurements.

Guide exps are usually several seconds long in order to avoid having the mount chase the seeing. Longish guide exps blur most of the seeing effects to distill pointing errors. So bad seeing usually does not produce large guide errors since this method deliberately minimizes seeing effects. Guide errors can have a seeing component but are more often due to factors such as mount, vibrations, OTA response to wind, etc.

AO guiding is another matter. AO guide exps are usually near 100ms. So most of the report error RMS is due to seeing wander. AO can remove a substantial amount of coherent seeing wander.

Stan
• ... (continued) Image wander is produced by refractions from atmospheric seeing cells and these cells have a large range of sizes and speeds. The power
Message 21 of 21 , Aug 23, 2013
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--- "echesak@" <echesak@> wrote:
> I see the macro effects of seeing, by looking at the guider
> corrections on the scope...

(continued)

Image wander is produced by refractions from atmospheric seeing "cells" and these cells have a large range of sizes and speeds. The power spectrum of seeing effects occur over a broad range of time scales. Most of the disturbances are fast and will not be detected by guide errors (they simply blur into each other in a single exp) but the large cell effects have low frequencies (>=1hz) and this low end of the power spectrum can show up as a guiding error. But this is undesirable because it causes the mount to chase an imaginary deviation so if you experience guide error attributable to seeing then you should increase the guide exp time and/or lower the guider aggression.

This slow seeing wander can be attenuated with AO.

Stan
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