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Re: Re: [APML] IC1396 CCD with Schmidt-like FOV
Rob,
I heartily agree <g>. I am not saying that either technique is better, just that there are trade offs with both ways. I would like to make a difference between the various different types of halos. Tech Pan has halation effects which cause a halo like effect on all bright stars. This is not due to chromatic abberation. This effect is akin to the blooming streak in CCDs. It is also analogous to the vertical and horizontal haloing in CCDs. I am not sure if you know what I mean by this but it is noticeable on bright stars as well. Typical example is the bright star near the FLame nebula in Orion.
Loke
Loke
Loke
>
> From: "Robert Gendler" <robgendler@worldnet.att.net>
> Date: 2002/08/03 Sat PM 09:02:24 EDT
> To: <astro-photo@seds.org>
> Subject: 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/
> > >
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