[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
Re: [APML] IC1396 CCD with Schmidt-like FOV
Loke,
I appreciate all the points you made. I'm not saying the
CCD/lens system is better and I'm well aware of the
limitations of the method I used. I'm only saying that it
can be done by a motivated individual using a CCD
who lacks access to a dark sky which eliminates film as
an alternative. I live 15 miles out of Hartford and my skies are
mag 4.5 at best. I don't have the benefit of Mt. Laguna or any other
dark site at this point in my life so I must make a go with what I have.
Yes there are some blue halos. I'm not sure its from chromatic aberration
since I focused carefully for each filter. In your image there are some
prominent red halos around some bright stars but I doubt thats chromatic
aberration either. Its probably just star shine which in the CCD world is
accentuated with DDP.
Rob Gendler
Email: robgendler@att.net
Web site: http://www.robgendlerastropics.com/
----- Original Message -----
From: "lkt" <tan@cox.net>
To: <astro-photo@seds.org>
Sent: Saturday, August 03, 2002 6:58 PM
Subject: Re: [APML] IC1396 CCD with Schmidt-like FOV
> 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/
> >
> > -- APML Archives at <http://astro.umsystem.edu/apml/> ---
> > Unsubscribe at <majordomo@seds.org>
>
> -- APML Archives at <http://astro.umsystem.edu/apml/> ---
> Unsubscribe at <majordomo@seds.org>
>
-- APML Archives at <http://astro.umsystem.edu/apml/> ---
Unsubscribe at <majordomo@seds.org>