Celestron C-90 for 4x5?

[drew bedo]

<p>I have acquired a Celestron C-90 spotting scope. It has threads for a T=mount adaptor. When used with a 35mm SLR it becomes an f-11/1,200mm lens. Can i use thhis on a 4x5 camera with na eyepiece to project an image on the GG? Anybody done something similar?</p>

[dsms photos]

<p>I don' think so. that was designed to fill a 35mm camera frame, not a 4x5" format plus the back focal distance would be way too short.</p>

[dan_fromm2]

<p>Second the motion, with vigor. Run away!</p>

<p>I bought one in 1978. It was the worst lens I've ever owned. Severe astigmatism (it couldn't focus vertical and horizontal wires in a screen at the same time), fuzzy (and not only due to motion blur), had a severe asymmetrical central hot spot, and had mechanical problems too. Don't even think of getting one.</p>

<p>I shot with it, never got a picture I was willing to show in public. Re coverage, nil image quality in the corners.</p>

<p>I eventually sent it back to Celestron. They replaced it under warranty. Good warranty, 25 years. But the replacement was no better than the first one.</p>

<p>Oh, yes. Shutterless. How would you get timed exposures with it?</p>

<p>Have I been clear?</p>

<p>Ain't no good and inexpensive long lenses for LF.</p>

[profhlynnjones]

<p>The 1930's design was from the Russian, Maksutov. He wanted a cassegranian telescope using a spherical mirror. A spherical mirror cant bring light to a sharp point! Schmidt and Maksutove both by different means wanted to deliberately defocus the incoming light rays so the the spherical mirror would bring light to a sharp point at the secondary mirror. Schmidt tried the same thing at about the same time, it was more successful if you could figure out how to make an aspheric corrector plate, Schmidt made som semi-successful correctors but never told anybody how he did it (Celestronwas the first company to do that in 1963).. </p>

<p>Maksutovs traditionally work reasonably well visually because the field curvature is pretty well compensated by the eye. In Cameras that doesn't happen, these instruments have field curvature, loss of light by 1 1/2 to 2 stops, and chromatic aberration because the thick meniscus corrector plate causes light dispersion (prism effect, separates the white light to rainbows).</p>

<p>Dr. Bouwers during WWII places a simple lens system into the baffle tube of a Maksutov correcting the color as well as the field flatness. This is the reason that photo mirror lenses actually work quite well. Dr. Back (American immigrant scientist escaping from the Germans) published the same work before Dr. Bouwers but Bouwers hid the discoveries so that the Nazis couldn't use this work for their war effort, Bouwers gets the credit. Dr. Back, however, was the most successful designer of Zoom lenses after WWII and made great mirror telephotos during WWII.</p>

<p>Lynn </p>

[pmarcus]

<p>Yea, but my Meade ETX (a rather good Maksuov scope) makes a great compact telescope! As I recall, my friend's Celestron never came to sharp focus because it was just plain bad. But that was thirty years ago, YMMV.</p>