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07-25-2008, 03:50 AM   #46
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QuoteOriginally posted by christinelandon Quote
The corrector element in the center of a Schmidt-Cassegrain design reduces spherical aberrations; as noted elsewhere in this thread by procyon,
It doesn't. That is exactly the job of the Schmidt corrector plate. The Schmidt plate, which is highly aspherical, was invented to reduce the spherical aberration of the spherical main mirror.

There is a heavy curvature of field (I used the wrong expression "spherical aberration" in another post in this thread) in a pure Schmidt system and the Schmidt-Cassegrain manufacturers counter this up to a certain degree with the help of a slightly modified Schmidt corrector and generally by producing slow systems (like the f10). A real Schmidt telescope can be anything between f1.5 and f5, but shows the heavyly curved image plain. Therefor you use film cassettes which are accordingly curved and are fixed inside the Schmidt telescope.

As I don't not know this particular photographic system, I can only assume, that the central lens group serves at least the purpose of focusing (usually Schmidt-Cassegrains are focused by moving the main mirror or the corrector/secondary mirror assembly front- and backwards). The central group might also be used as a distant field flattener, though I would guess, that there is an additional (or the only) field flattener at the rear, just at the camera mount. This has the added advantage of closing the tube against dust.

Ben


Last edited by Ben_Edict; 07-25-2008 at 06:02 AM.
07-25-2008, 05:23 AM   #47
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I missed this one - wow double bigma - a megama ! Now the wildlife and landscapes of the Maritimes don't stand a chance - congrats and have fun.
07-25-2008, 05:33 AM   #48
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QuoteOriginally posted by saladin Quote
can i ask what may be a stupid question - are these mirror lenses fixed aperture? ie- is it always f/10 or can it be stopped down to f/32 etc? just curious, as i have never handled, or even seen, one.

i reckon it would be a great lens for moon shots.
Not such a stupid question.

We are discussing here in reality a telescope with a T mount and pentax K adaptor. They are fixed apature. There are mirror lenses made specifically for cameras, and some of these have a set of ND filters that can drop in in front of the mount to reduce lighting, but not control DOF.
07-25-2008, 06:04 AM   #49
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This is fantastic information everyone. Ben you certain have a good understanding of the subject as do many other members. Good info on the math. The 2x shot was very useable but I had to take it portrait style and twist the tripod ball head to get it in the frame (see below). At this magnification, even my rock steady tripod is showing vibration.
It's a whole new area for me and it would seem I got luckier than I realized with this lens.
I hope I'm not driving people crazy with this stuff. It's been pretty exciting learning to use this. I keep thinking about the missed shots from the past and the future possibilities.

As for the T mount adapter. This is the one on mine and what you need Frogroast:
NEW T Mount Adapter for PENTAX K Metal MX K1000 - eBay (item 280249108684 end time Jul-29-08 20:45:28 PDT)
I'm not sure what the thread pitch is but it's a standard created by Tamron (hence the "T"). it's the same diameter as an M42 but a different pitch to the screw threads. So don't try to mount an M42 lens to it. You'll strip the lens threads and or the adapters as well. Get out and use that thing. I'd love to see some results from it. Post them here when you do. I hope others would like to know what our experiences have been.

As for the lens flare thing, It simply doesn't. No CA or PF either. I took several shots last night with the sun just off the frame, in the frame and other angles. Nothing, not one small reflection, no colour shifts or banding. In the other thread I took a sunset shot that would just be impossible with any other lens I've tried. (probably would be blind too with a faster optic).


https://www.pentaxforums.com/forums/post-your-photos/32992-sometimes-little-luck-also-helps.html

That 2x 1000mm moon shot is not cropped. You can see that the orientation (this was 30 minutes after moon-rise EST) is totally off and the moon should be leaning the same way as the shot earlier. But not too bad in terms of sharpness with a Star D 2x TC.


Last edited by Peter Zack; 07-25-2008 at 07:02 AM.
07-25-2008, 06:32 AM   #50
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Wildman, This thread https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/30288-folks-ov...n-request.html and the images you posted on that POTW thread a few weeks ago was what really got me interested in this area. Those bird shots you posted were amazing. Do you have a web site with more shots? You should share them with everyone. I'd watermark them in some way since they are so good.

This thread also was interesting to see some other telescopes. https://www.pentaxforums.com/forums/general-pentax-photography/30336-technique-moonshots-2.html

Last edited by Peter Zack; 07-25-2008 at 06:50 AM.
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