Secondary size & CCD imaging

Considering all this, could one not
replace the 4" secondary currently installed with something much
smaller, such as a 2.6" or 3"?

Hi:

You could. HOWEVER.... I doubt you'll notice any improvement in your images,
and if you're not using the scope visually, anyway...

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
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Considering all this, could one not
replace the 4" secondary currently installed with something much
smaller, such as a 2.6" or 3"?

I think this is an excellent idea. Depending upon how far outside the tube
you need to bring the focal plane for the CCD, you could easily get by with a
2-2/3" minor axis diagonal. It would completely illuminate a 1/2" field with the
CCD plane about 2" outside the light path and reduce your obstruction from 25%
down to only 17%. I think that is significant for giving improved contrast to your
CCD imagery.
Clif

On 27 Jul 2003 07:59:07 -0700, (Brian) wrote:

I am hoping someone out there can help me think through a question
that has come up in my mind for some time now; My 16" Newton is used
primarily for asteroid astrometry and photometry of variable stars and
is seldom "looked" through anymore. Having a full computerized goto
drive system, the need to remove the CCD to center an area of question
has all but vanished. At prime focus (f/4.5), the CCD provides a FOV
of about 9x7 minutes of arc. Considering all this, could one not
replace the 4" secondary currently installed with something much
smaller, such as a 2.6" or 3"? Using this scope with the narrow FOV
of the CCD is only a small portion of the central light cone. The
benefit of more contrast, and even slightly more light gathering by
reduction of the secondary size seems more than worthwhile to me. I
realize that visually, at low powers, a secondary of say 2.6" would
not show a fully illuminated field, but with the tiny FOV of the CCD,
I'm not sure this would even be an issue. Can someone enlighten me on
where I may be going wrong on this thought? Or, better yet, let me
know if I'm right in my thinking!

You won't see any difference in your images. The effects of an obstruction on
contrast are subtle and confined to visual observing. The best imaging
telescopes in the world have very large central obstructions, because image
processing completely eliminates the tiny loss of contrast. Yes, you'll collect
a few more photons with the increased aperture area, but I doubt you'll notice
the difference without making a special effort to measure it.

In fact, since CO is so unimportant for imaging, you might want to consider
removing the secondary mirror altogether and putting your camera at the prime
focus. This will give you a wonderfully balanced tube assembly.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Chris L Peterson" wrote in message
...

In fact, since CO is so unimportant for imaging, you might want to
consider
removing the secondary mirror altogether and putting your camera at the
prime
focus. This will give you a wonderfully balanced tube assembly.

If you did this, woud you still need to have a focus mechanism of some kind?

On Mon, 28 Jul 2003 08:30:08 -0400, "Stephen Paul" wrote:

If you did this, woud you still need to have a focus mechanism of some kind?

Unless your tube/struts have a zero coefficient of thermal expansion. I suppose
that would be the trickiest thing about implementing a prime focus imager-
coming up with a clean way to support a camera and focuser. That's quite a bit
of weight for an ordinary spider.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Chris L Peterson" wrote in message
...
On Mon, 28 Jul 2003 08:30:08 -0400, "Stephen Paul"
wrote:

If you did this, woud you still need to have a focus mechanism of some
kind?

Unless your tube/struts have a zero coefficient of thermal expansion. I
suppose
that would be the trickiest thing about implementing a prime focus imager-
coming up with a clean way to support a camera and focuser. That's quite a
bit
of weight for an ordinary spider.

I figure you have to remove the spider and bolt the camera closer to the
open end of the tube in order to bring the camera far enough away from the
primary to be at prime focus. But, I can't really see a reason to, as long
as the photons will hit the detector I think I'll just keep it simple and
stick with the 2" Focuser and the Starlight XPress MX series cameras.

The MX bodies fit right down into the 2" focuser. I haven't tried it yet,
but I'm guessing it will go in far enough to reach focus. Then, all I need
is a dual axis EQ Platform with autoguider input, and to piggy back the ST80
as a finder scope.

Does weighing the options never cease?

-Stephen

Thanks to all for the replies. Good points were made in each and
every one of them, and I do appreciate the ideas and thoughts...

I may experiment with both my 4" sec and a 2.6" (or 3.1")to see if
there is any detectable difference in the images. I did forget to
mention that I also do planetary imaging from time to time, and I
suspect if there is any difference, this is where I will see it most.

Thanks Again,

Brian Sherrod