[APML] (OT) IC1396 CCD with Schmidt-like FOV

[Joe Pedit <pedit@email.unc.edu>]

I haven't been following this thread very closely, but some of the 
issues discussed in this thread were recently addressed in a "Luminous 
Landscape" article at:
<http://luminous-landscape.com/tutorials/dq.shtml>
If we could collectively come up with reasonable MTF and SNR estimates, 
then we have some basis for comparing CCD and film in terms of a 
"normalized image quality" index.

Joe
-- 
Joseph A. Pedit, Ph.D., Research Associate
E-mail: pedit@email.unc.edu
Research: http://www.unc.edu/~pedit/CASEhome.html


> Rob,
> 
> Thanks for your comments. Perhaps the criterion you have in mind the is
> ability of an image to sustain enlargement while still appearing reasonably
> smooth. If so, then I agree that a CCD image can definitely sustain a
> greater degree of enlargement. But smoothness is not the only thing that
> counts. If it was then we could take a well executed ST-7E image that is
> extremely smooth and enlarge it to a 30x40" print and, by sole virtue of
> the fact that it remains visibly smooth, claim that the enlargement is a
> complete success. I would say that it is not. Why not? Because those
> field stars, which with a correct degree of enlargement appear as miniscule
> pin-pricks, instead appear the size of peas. It doesn't matter how smooth
> the image is, it has simply been over-enlarged. The limit of enlargement
> is the point beyond which further detail ceases to become visible. Go much
> beyond that, and the image structure starts to break down and to overwhelm
> the structure of the actual object we're trying to portray. I have seen
> this happen with a few of my own 13x18" CCD prints. This is the fallacy of
> the extravagent enlargement claims we sometimes see for CCD images, where
> apparently the only thing that counts is whether the image remains smooth,
> regardless of whether the actual object structure and star sizes retain
> their original integrity.
> 
> Also I wonder whether all such comparisons are like for like... It is
> accepted by default that a CCD image will consist of a large number of
> sub-exposures to reduce the noise - so much so that the sub-exposures are
> routinely forgotten, and we frequently see references to "2 hour exposures"
> and suchlike. Of course these are not 2 hour exposures at all, and the
> results are nothing like what would truly result from 2 hour
> exposures. But that's beside the point - what sort of film images are you
> using for comparison? In my case I will use an accumulation of not less
> than 4 images for similar noise reduction purposes. The result, with the
> correct compositing technique, is an image that is very smooth. I have a
> set of medium format images produced in this way that appear grain-free
> when enlarged to 30x40". Four of these 30x40" prints which have been on
> public display at various Astrofests, seminars, etc. Even on very close
> inspection these prints appear grain-free and field stars appear as
> pin-pricks with a size small enough to be indeterminate without the aid of
> a magnifying lens. That would be impossible in the case of an ST-10 image
> enlarged to 30x40" purely because of the physics involved in such a huge
> degree of enlargement.
> 
> To take this further... I produce such images with a 6x7 camera attached to
> the back of an Astro Physics 155 EDF, which has a focal length of 1144mm
> (with field flattener). The resulting images are around 112Mb. To
> properly discern all of the detail in such images I need to zoom in to 100%
> or 200% in PhotoShop. There is detail there very close to the pixel level.
> 
> I can produce a very similar looking image (at web resolution) by attaching
> a 200mm lens to an ST-10E. The image will cover a very similar area of
> sky. The resulting image will be 9.6Mb. Now, I don't think anyone is
> claiming that the 9.6Mb image will contain the same level of detail as the
> 112Mb image produced with the 155 EDF, are they? At least I hope not!
> :-) Again, bear in mind that the 112Mb image has very little noise and has
> detail down to nearly the pixel level.
> 
> The CCD image will enlarge proportionately much more than the film image,
> but it will still not come anywhere near the quality of the film image when
> enlarged to 30x40". Some of the almost "infinite enlargement" claims that
> we hear are claims too far. There is the physics of detector size to
> consider - enlargement is much more than a matter of image smoothness.
> 
> --Philip



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