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Is There Any Hope for this old C90 500mm f5.6

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#26 Jon Isaacs

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Posted 25 August 2019 - 07:20 AM

I put such design through raytrace, and, unless it has a subaperture corrector of some kind, it has to be heavily compromised optically.

 

 

I have never seen one but it is my understanding that there is a sub aperture corrector.

 

It's basically a standard C-90 with a focal reducer.

 

Jon


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#27 Vla

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Posted 25 August 2019 - 02:42 PM

I don't see how that would work. That corrector would have to be quite large to cover 2-inch field needed to provide 1-inch field for the faster ratio. It would have to be a reducer-flattener, and those are not simple correctors. Then, how would they go about focus accessibility? It's 2x reduction factor.



#28 Vla

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Posted 25 August 2019 - 02:46 PM

Thanks for taking the time to produce such a detailed report. Although I understand very little of the technical analysis, I think you were saying that this is a flawed design that cannot be adequately corrected and that I should toss this lens in the trash.

I wouldn't just jump on anything. You still don't know for sure what is the problem. If you could take a measurement of at least front radius, and primary's f.l. that could give some hint as to how close is this raytrace model to it. And if you could inspect the rear baffle end to find out if there are signs of a (missing) corrector lens - or that there is one, that could be helpful.



#29 jstrandberg

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Posted 27 August 2019 - 08:50 AM

Taking the lens apart, the entire optical train consists of the front lens, primary mirror and secondary mirror. All are in pristine condition. There is a baffle tube around the secondary mirror about 1" long, and a baffle tube about 2" long extending up from the hole in the primary mirror. There is no lens in either of the baffle tubes. I would think that if there was a reducer/flattener mounted in the primary baffle tube that had come loose at one time, it would have damaged the primary mirror or lens rattling around inside the lens and, like I said, the mirrors and primary lens are in beautiful condition.



#30 Sarkikos

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Posted 27 August 2019 - 11:10 AM

I have never seen one but it is my understanding that there is a sub aperture corrector.

 

It's basically a standard C-90 with a focal reducer.

 

Jon

Maybe better to get a C90 and install a Celestron 0.63x RC.

 

Mike



#31 Vla

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Posted 27 August 2019 - 02:02 PM

Taking the lens apart, the entire optical train consists of the front lens, primary mirror and secondary mirror. All are in pristine condition. There is a baffle tube around the secondary mirror about 1" long, and a baffle tube about 2" long extending up from the hole in the primary mirror. There is no lens in either of the baffle tubes. I would think that if there was a reducer/flattener mounted in the primary baffle tube that had come loose at one time, it would have damaged the primary mirror or lens rattling around inside the lens and, like I said, the mirrors and primary lens are in beautiful condition.

Can you measure depth of the front radius, meniscus edge thickness, primary r.o.c. (approximately), and the corrector to mirror separation? That would allow to reconstruct something close to the actual unit.



#32 sneeds

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Posted 03 September 2019 - 05:05 PM

Wow - I've seen versions of Schmidts that were 500 mm f5.6, but never a Mak.  We're sure this is a Mak?



#33 davidmcgo

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Posted 04 September 2019 - 08:37 PM

The original 500 f5.6 had a different curve on the meniscus than the standard C90, a larger secondary, and no reducing optics in the baffle tube.

 

Dave



#34 luxo II

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Posted 08 September 2019 - 02:04 AM

needs, I'm old enough to have seen these when new, and yes this is a mak - and there is a reason why they are rare - they weren't great, optically.

 

OP these were originally manufactured as a camera lens and subsequently re-packaged and badged by Celestron with minor mechanical mods to function visually. As such not particularly well corrected - they were well known for being soft. The bad news is there's no way you'll get a great image for visual astronomy let alone resolve a diffraction pattern.

 

It might be adequate as a finderscope but not much more.

 

Historically Questar made a 700mm mirror lens that (like their scopes) was in a class of its own and it inspired many cheap imitations such as this one, all inferior.


Edited by luxo II, 08 September 2019 - 06:28 AM.



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