Re: [APML] IC1396 CCD with Schmidt-like FOV

[lkt <tan@cox.net>]

Rob,

I guess I should chime in here since the Schmidt was mentioned. I looked
at your CCD version and my Schmidt one at:

http://www.starryscapes.com/nebula/cepheus_ic1396.html

My thoughts on this are that the CCD is a better resolution detector
than Tech Pan. So for the same focal length scope, the CCD will be
better resolved, assuming equal optical quality. The CCD detector will
also have lower noise. The vdB142 section of your image is better
resolved than my Schmidt one by this argument since they both use a
300mm lens. My 8" f/1.5 Schmidt is also a 300mm lens. One other
difference here is, I believe the Schmidt is a much better optical
system than a Nikon ED lens. Since it's a mirrored system, the amount of
color is very minimal. You only get some color due to the corrector
plate. The Nikon ED lens has noticeable blue haloing on the bright blue
stars. Imagine how much nicer your image could have been w/o chromatic
abberation. The blue custom scientific filter allows light into the CCD
detector almost all the way down to 400nm. 

Of course, the big advantage of the Schmidt with TP is the field. The
Schmidt can illuminate a large image circle (relative to its aperture).
The image on my website has been cropped. Here is a smaller version
showing the whole field.

http://www.starryscapes.com/tmp/ic1396_small.jpg

You will need a Nikon lens with a rather short FL to match this field.
35mm film is 24mmx36mm and the KAF3200 is 10mmx15mm. Using the longer
dimension, 36/15=2.4. So to match the field of the Schmidt with TP, you
will need a 125mm lens. I would like to do some widefield work with a
CCD camera but I cannot find a very well corrected 125mm lens. I mean
one which is as well corrected as a mirrored system. Furthermore, I
believe (w/o proof) that the resolution of the ST10 with a 125mm lens
cannot rival TP with a 300mm Schmidt. But when you get to this FL, I
believe the CCD approach will lose in resolution to the Schmidt with TP.
If you can find a 125 colorless lens, you might do better. Of course,
one can do what you have done, that is to use a longer FL lens for
higher resolution at certain areas but this technique can apply with the
Schmidt as well. That is provided you can find a bigger Schmidt, one can
use the  bigger Schmidt to "fill in the details". 

Alternately as technology improves, you can get a much larger CCD array
than the KAF3200. I know these chips are available but not currently
affordable within reason. If you can get one the size of a 35mm film,
you can use the 300mm lens, provide the same field with better
resolution. 

Another issue with widefield images is the almost certainty that there
will be a very bright star in the field. Note, Antares in the Rho
Ophiuchus region. Here the linearity of the CCD is it's bane. Saturated
or bloomed stars are not very pretty and can be a severe limitation to
your exposure lengths thereby limiting the nebulosity you can attain by
longer exposures. You can do an h-alpha channel to help but I would
guess color accuracy might suffer a bit here. To me color accuracy is
very nebulous in our field of pretty pictures anyway. 

I am not claiming that either technique is better, just that there are
quite a bit of tradeoffs to be made. These are just my thoughts. Feel
free to disagree. 

Loke







Robert Gendler wrote:
> 
> Hi all,
> Using Nikon 180mm and 300mm lenses and the ST10
> I've created a widefield color image of the IC1396 complex.
> The inner detail of the nebula was imaged with the 300mm lens
> (using HA and red as luminance) and the outer portion was taken
> with the 180mm giving the generous FOV (straight RGB). I believe
> the FOV and detail compare favorably with schmidt camera and
> medium format film images of this object which demonstrates
> that large field color imaging can be done successfully with CCD
> and camera lenses. The widefield image is at
> http://www.robgendlerastropics.com/1396Large.html
> A cropped higher res version is at
> http://www.robgendlerastropics.com/1396Small.html
> The lenses were mounted on the ST10 using Steve Mandels adaptor
> and were secured on the mount using two losmandy adjustable tube rings.
> The 150mm ring fit perfectly around the ST10 while the 125mm ring
> supported the lenses. I have two pictures of the setup at
> http://www.robgendlerastropics.com/camera300.jpg
> http://www.robgendlerastropics.com/camera180.jpg
> Enjoy!
> Rob Gendler
> Email: robgendler@att.net
> Web site: http://www.robgendlerastropics.com/
> 
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