5238RE: [ccd-newastro] Re: Resolution
- Jan 1, 2002You don't need high resolution to get really nice images; the FSQ images
taken at 3.5 arcseconds per pixel illustrate that fact. Nothing has been
done to such images to enhance resolution; those are 3.5 arcseconds images.
Good optics and a steady mount play a big role in getting the most out of
whatever resolution you are imaging at.
The New CCD Astronomy
From: mpastro2001 [mailto:martin-pugh@...]
Sent: Tuesday, January 01, 2002 4:04 AM
Subject: [ccd-newastro] Re: Resolution
I take your point about the limitations pertaining to a LX200....and
indeed I intend to buy a new scope this year and I plan to spend a
lot of money on it including a top-of-the-line mount. I am not,
however, going to change out my camera, having only just bought it.
I see fabulous images posted by many owners of a FSQ106, and if I
have my maths correct, you cannot get any better than 3.5
arcsecs/pixel with an ST8E at F5.
In fact, up until now, I was considering a Takahashi purchase, but
neither the 106, 102, 128 or 152 are able to achieve those hi-res
sampling rates with either a ST8E or even ST10E. Yet, most of the
superb images you have taken with your FSQ106 have been with an
ST8E.....so what methods have you used to increase the resolution?
I move to Portugal in 6 months time for 3 years, where I hope the 9
months of sunshine a year and better seeing conditions than here in
Belgium are going to increase my imaging activity a hundred fold.
Would you advise that I wait to see what sort of seeing conditions
exist before I make a big purchase?
--- In ccd-newastro@y..., "Ron Wodaski" <ronw@n...> wrote:
> There is little or no agreement on this issue. <g> I prefer to
take a simple
> and practical approach to the discussion. The bottom line,
> the same: you can image at a _very_ wide range of image scales
> There are several issues to consider if high-resolution imaging is
> * How accurate is my mount for tracking?
> * How accurately does my mount make guide corrections?
> * How good is the seeing at my location on average and best nights?
> (Corollary: do I want to buy a camera so I will be able to image
> nights, or will I choose a high-res cameras and settle for imaging
> the best nights?)
> Answers to the above questions will tell you what resolution to
> With mounts, you mostly get what you pay for in terms of tracking
> guiding accuracy. The LX200 mount is about at the entry level for
> of astrophotography, so its ability to do high resolution is
> tracking and guiding accuracy vary from one sample to the next, so
> LX200 owners image using a focal reducer rather than at the full
> length of their scope.
> You can perform some simple tests which will tell you how capable
> is. Tracking can be measured in a large number of ways, including
> while offset from the pole and recording guide corrections to disk
> corrections are turned off in both axes. Both techniques are
> the book.
> For guiding, you can't really measure your guiding accuracy until
> optimize your mount in a number of areas. Recent messages have
> this in some detail, and again there is extensive material in the
> this. Guiding is not a simple point and shoot operation; the more
> spend optimizing your mount for guiding, the better the results
> get. The basic idea is to do whatever you can to optimize your
> tracking performance (superb polar alignment; optimal physical
> optimal backlash compensation; etc.) so that the least amount of
> needed. Every guide correction introduces noise as well as a
> the fewer and smaller the guide corrections that are necessary,
> your results will be.
> That's a lot of work; not everyone wants to go through all of
that. <g> Thus
> the general recommendation for someone new to imaging has been to
> camera that has about 2 arcseconds per pixel of image scale. That
> pixel "sees" a square of sky that is about 2 arcseconds on a side.
This is a
> reasonable compromise between ease of operation, typical seeing
> the limitations of mounts costing under $4000, and decent
> To work at a smaller image scale, you should be prepared to invest
> heavily in your mount; you should confirm that you have a
> of days with sufficiently good seeing to support your desired
> you should consider whether you would be better served by a larger
> and larger pixels on your camera; and you should make sure that
> willing and able to put in the additional work required to get
> at that desired high resolution. And so on; the higher your
> greater the number of factors you must consider.
> Given the system you are working with, it is reasonably likely
that you are
> already working at or near the resolution limits of your system if
> working at 2 arcseconds per pixel. You might be very fortunate and
> to go higher, but that would be an exception for a mount at the
> of the LX200. Every mount has a limit to its accuracy, and every
> has a limit to the seeing you will get. These are your primary
> factors. You can try to push as close as possible to these limits,
> gets harder and harder to do so as you approach the limits.
> Ron Wodaski
> The New CCD Astronomy
> -----Original Message-----
> From: mpastro2001 [mailto:martin-pugh@t...]
> Sent: Monday, December 31, 2001 10:54 AM
> To: ccd-newastro@y...
> Subject: [ccd-newastro] Resolution
> Hello all
> Once again I would ask for someone to clarify what has become a
> confused issue for me .and I hate to start the long debate about
> resolution/sampling etc.
> It concerns resolution and there was a recent couple of posts,
> together with earlier posts which I believe are contradictory.
> First of all though, I thought I did enough research before I spent
> thousands of pounds on an SBIG camera, thinking that it was matched
> to my 8" LX200. I only bought the camera in Aug, yet it now seems
> that aiming for an optimum resolution of around 2 arcsecs per
> the advice at the time, is outdated, and 0.5 arcsec per pixel is
> target for hi-res deep sky imaging. For me, that means imaging at
> 4000mm (F20) and the resultant miniscule FOV (and the other
> that will arise imaging at this FL).
> In summary, the recent posts have said
> "...that between 1 and 1.5 pixels FWHM represents severe
> I assume that to reach arcsecs/pixel you would need to divide by
> image scale of the set up you are using.
> "For an 8" SCT, 1 arcsec/pixel is optimum, with 0.5 arcsec/pixel
> required for hi-res deep sky imaging."
> I am also told that CCDOPS V5 does a good job (after Mira and MAXIM
> V3) of measuring FWHM accurately. So I took an f3.3 image in
> CCDSOFT, reduced it and saved it as an SBIG image and loaded it
> CCDOPS. The value of 'seeing' was given at 3.7 arcsecs which I
> divided by the image scale (2.8 arcsecs/pixel for a FL of 660mm)
> giving 1.3 pixels. Therefore, according to current thinking, this
> is severe undersampling. Yet 2.8 arcsecs per pixel, 6 months ago,
> was pretty much on the button.
> Another piece of advice was to sample so that the value of FWHM is
> 2.5 - 3.5 pixels. Well, if I multiply this figure by the image
> scale above, I get 8.4 arcsecs FWHM. Yet the average seeing for
> latitude sites is about 2.5 arcsecs .how useful is an FWHM of 8.4
> arcsecs for this?
> New Astronomy, Chapter 4 Section 4, if I am reading it correctly,
> clearly says that seeing which would support sub arcsecond imaging
> rarely occurs.
> So what am I to do? Is my 8" LX200/ST8E camera no longer a usable
> combination? Should I now aim to image at 1 arcsec/pixel, as has
> been suggested?
> Another post said that pixel size has minimal effect on the S/N of
> an object, yet I understand that binning 2x2 quadruples sensitivity
> with the resultant increase in S/N? Or do I have this wrong also.
> I would very much appreciate it if someone could summarise what
> steps I need to take in order to use the software I have (CCDSOFT
> V5, CCDOPS V5, MAXIM V2.14) to assess seeing (in FWHM pixels and
> arcseconds) and then adjust my set up to match the target sample
> rate (for which I have no idea now what the optimum should be)
> Thanks a lot
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