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

[lkt <tan@cox.net>]

Sean, Rob,

 In general film does have good resolution for general photography. The
larger film formats produce more resolution due to less enlargement. I
feel this applies in general to terrestrial photography. But for
astrophotography where we try to glean the smallest amount of
information by processing, the best films are not as good a detector as
a CCD is. In my experience, under low light conditions, CCDs have higher
effective resolution and much less noise. Furthermore, since the CCD's
characteristics are repeatable for fix temperatures and readout times,
one can use flat fielding to effectively remove the noise in the pixels.
The pixels themselves do not disapear but their non-uniform response
gets normalized and this source of noise is effectively removed. So in
effect if we take an example 800x800 CCD detector, it truly has this
resolution. One can find an equivalent film size to match the resolution
of this CCD detector but in general it will be larger in physical
dimensions and would require a larger scope. I think this is only true
for resolution. I do not know of any film which has the dynamic range of
CCDs. 

 Nevertheless, I still see a place for the Schmidt camera. It's not that
the ST10 cannot achieve a smoother or higher resolution image with the
same focal length instrument. As stated before, my opinion is that the
ST10 beats Tech Pan in both these areas if we use the same focal length.
For me, the one main issue with doing widefield CCD imaging has to do
with bright stars in the field.  These bright stars limits one's
exposure length due to one's attempt to limit the amount of blooming.
Even if you use an anti blooming detector, there will still be digital
artifacts. As far as I know I have never seen a color CCD image of
Antares and its surrounding nebulosity. One can use a h-alpha filter but
the nebulosity is yellow-orange and not red. The problem is that the
dynamic range of Antares vs. its nebulosity is significantly more than
16bits. Film's logarithmic response which is it's disadvantage for faint
objects helps out here by effectively performing dynamic range
compression on the star directly during imaging. Most pretty picture CCD
images also do the same thing using DDP in a post processing step. 

The other issue with widefield CCD imaging is the need for short focal
length well corrected optics. I am sure they are out there so this is
just a matter for finding the right one. Alternately one can use a much
larger detector and this is just a matter of time as well.

Loke





Sean Walker wrote:
> 
> 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, I have to disagree with you here- Schmidt camera shots look
> quite impressive at web resolution, but when blown up, the grain of the
> film starts to become quite apparent. Rob's response sums it up for me
> also.
> 
> Sean Walker
> 
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