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5235Re: Resolution

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  • mpastro2001
    Jan 1, 2002
      Thanks Ron

      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?

      Martin







      --- 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,
      however, remains
      > the same: you can image at a _very_ wide range of image scales
      successfully.
      >
      > There are several issues to consider if high-resolution imaging is
      your
      > goal:
      >
      > * 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
      on most
      > nights, or will I choose a high-res cameras and settle for imaging
      only on
      > the best nights?)
      >
      > Answers to the above questions will tell you what resolution to
      aim at.
      >
      > With mounts, you mostly get what you pay for in terms of tracking
      and
      > guiding accuracy. The LX200 mount is about at the entry level for
      all forms
      > of astrophotography, so its ability to do high resolution is
      limited. Actual
      > tracking and guiding accuracy vary from one sample to the next, so
      many
      > LX200 owners image using a focal reducer rather than at the full
      focal
      > length of their scope.
      >
      > You can perform some simple tests which will tell you how capable
      your mount
      > is. Tracking can be measured in a large number of ways, including
      imaging
      > while offset from the pole and recording guide corrections to disk
      while
      > corrections are turned off in both axes. Both techniques are
      described in
      > the book.
      >
      > For guiding, you can't really measure your guiding accuracy until
      you
      > optimize your mount in a number of areas. Recent messages have
      gone over
      > this in some detail, and again there is extensive material in the
      book on
      > this. Guiding is not a simple point and shoot operation; the more
      time you
      > spend optimizing your mount for guiding, the better the results
      you will
      > get. The basic idea is to do whatever you can to optimize your
      mount's
      > tracking performance (superb polar alignment; optimal physical
      backlash;
      > optimal backlash compensation; etc.) so that the least amount of
      guiding is
      > needed. Every guide correction introduces noise as well as a
      correction, so
      > the fewer and smaller the guide corrections that are necessary,
      the better
      > 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
      purchase a
      > camera that has about 2 arcseconds per pixel of image scale. That
      is, each
      > 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
      conditions,
      > the limitations of mounts costing under $4000, and decent
      resolution.
      >
      > To work at a smaller image scale, you should be prepared to invest
      more
      > heavily in your mount; you should confirm that you have a
      sufficient number
      > of days with sufficiently good seeing to support your desired
      resolution;
      > you should consider whether you would be better served by a larger
      aperture
      > and larger pixels on your camera; and you should make sure that
      you are
      > willing and able to put in the additional work required to get
      good results
      > at that desired high resolution. And so on; the higher your
      resolution, the
      > 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
      you are
      > working at 2 arcseconds per pixel. You might be very fortunate and
      be able
      > to go higher, but that would be an exception for a mount at the
      price point
      > of the LX200. Every mount has a limit to its accuracy, and every
      location
      > has a limit to the seeing you will get. These are your primary
      limiting
      > factors. You can try to push as close as possible to these limits,
      but it
      > gets harder and harder to do so as you approach the limits.
      >
      > Ron Wodaski
      > The New CCD Astronomy
      > http://www.newastro.com
      >
      > -----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
      pixel,
      > the advice at the time, is outdated, and 0.5 arcsec per pixel is
      the
      > target for hi-res deep sky imaging. For me, that means imaging at
      > 4000mm (F20) and the resultant miniscule FOV (and the other
      problems
      > that will arise imaging at this FL).
      >
      > In summary, the recent posts have said
      > "...that between 1 and 1.5 pixels FWHM represents severe
      > undersampling."
      >
      > I assume that to reach arcsecs/pixel you would need to divide by
      the
      > 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
      into
      > 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
      mid-
      > 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
      >
      > Martin
      >
      >
      >
      >
      >
      >
      >
      >
      >
      >
      >
      >
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