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73237Re: aperture or focal length most relevant to good/bad seeing ?

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  • echesak@flash.net
    Aug 22, 2013
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      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.
      > >
      > > 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]
      > >
      >
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