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Re: [APML] Re: (OT) IC1396 CCD with Schmidt-like FOV
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
At 01:01 05/08/02 -0400, you wrote:
>Hi Philip,
>Resolution in the context of image size is really a meaningless concept.
>The ability of an image to hold up to enlargement and in particular printing
>is much more than a function of actual pixel count. The overall smoothness
>and
>in particular the S/N is much more important. I know Tony will back me
>up on this since he has done alot of printing for me. I have given him files
>that were barely 6MB but were very well exposed with excellent S/N
>and he has made quality 40 x 20" prints. CCD images if well exposed are
>very smooth in quality and the actual detail in the image can be much
>greater
>than comparable film images scanned at much higher resolution. Loke even
>pointed out that VDB142 in the CCD image had more detail than the film
>image despite identical focal lengths and much smaller aperture for
>the CCD image (8" vs 3"). Ultimately its not the pixel count but the amount
>of real information recorded by the medium that counts.
>Rob Gendler
>Email: robgendler@att.net
>Web site: http://www.robgendlerastropics.com/
> > The detail and FOV may compare favourably at *web resolution* with a
>medium
> > format image. But as you know (from your own excellent work with medium
> > format) the medium format image has to be hugely downscaled for web
> > resolution. The raison d'etre for medium format (with you also I assume)
> > is to produce large exhibition-quality prints while preserving the full
> > integrity of the image (little or no upscaling). By dowscaling for web
> > resolution, the detail of a medium format image is compromised
> > severely. When comparing like for like, a full resolution medium format
> > image (when scanned on your UMAX or equivalent) will yield a resolution of
> > around 8,100 x 6,900 pixels. I am not sure that an ST-10 image at 2,184 x
> > 1,472 pixels "compares favourably". Much more detail will be apparent in
> > the full resolution medium format image. The ST-10 image has nearly 18
> > times less area. And that is only the resolution aspect - in terms of
> > field coverage the difference is even more stark.
> >
> > --Philip
> >
> > Philip Perkins - philip@astrocruise.com
> > Wiltshire UK & Luberon France
> > Astrocruise - http://www.astrocruise.com
> >
> >
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Philip Perkins - philip@astrocruise.com
Wiltshire UK & Luberon France
Astrocruise - http://www.astrocruise.com
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