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Old Celestron C90 500mm f5.6 need visual back

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#1 jstrandberg

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Posted 28 June 2019 - 10:27 PM

I came across an old C90 500mm f5.6 mak today for a song and want to try it out for lunar photography. It accepts .965 eyepieces and is threaded for a visual back. A search of the internet provided a site that showed how the original visual back can be used to attach a T-ring to use the lens for photography. However, I cannot find an original visual back for the scope. I did find a Celestron T-adapter #93633-A, but from what I have seen on the net, this doesn't fully seat on the old c90 back. If anyone know where to find a visual back for this scope, or has an alternate suggestion, I would appreciate hearing from you.



#2 bella luna

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Posted 29 June 2019 - 11:22 PM

I have the Celestron T-adapter-Mak and a generic T-adapter that screw right into the back of my Celestron C-90,  F11.

both do not screw in ALL the way but the GAP is minimal.



#3 alvacouch

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Posted 03 July 2019 - 06:03 AM

I have one of these (the "telephoto lens c90") and have managed to put a visual back on it. The screw threads for mine are the same as for the Meade ETX visual back adapter. This might sound silly, but it works well with 2" diagonal and eyepieces!

 

I will have to check, but I think I ordered this one: 

https://optcorp.com/...-etx-telescopes

This adapter fully seats. This makes it compatible with 2" adapters for celestron and meade. 

 

Then, to put a camera on it, one has to adapt this for 42mm T-mount threads. Instead, I put my existing 2" diagonal (originally for a Celestron C11) on it. 

 

Looks silly. Works great. I used it for the most recent lunar eclipse in that configuration.


Edited by alvacouch, 03 July 2019 - 06:05 AM.


#4 jstrandberg

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Posted 04 July 2019 - 10:17 PM

I got the Meade Back Cell ETX to SCT adapter you showed. Screws right on the C90, but I think the outside threads are just a little different than the Celestron 93633-A T-adapter because although the adapter starts to thread on, it binds up after only about a half turn. Anyone have any ideas? I would really like to use this lens.



#5 alvacouch

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Posted 05 July 2019 - 07:06 AM

It took me a few years to finally figure out that the ETX back adapter was compatible. I have a thread pitch gauge. The way I figured this out before was via use of that gauge on the inner threads. Let me use my gauge on the outer threads and get back to you on this. 



#6 jstrandberg

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Posted 06 July 2019 - 03:40 PM

Don Rothman of ASTRO PARTS OUTLET USED TELESCOPES came through with an original.



#7 Geo.

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Posted 10 July 2019 - 11:45 AM

The OP really means he wants a a 1.25" visual back adapter. The threaded socket on the rear of the C-90 IS the visual back.

 

I think Baader has one that works. Telescope Adapters makes one. The Meade ETX large accessory ring is hit or miss. The C-90 is an imperial thread and the ETX is metric. If you get a Meade that fits it was probably made with worn tooling.

 

For my own needs I cut down an old Tele-extender and fit a T-to 1.25" adapter. The front thread of the Tele-extender is the same as C-90's VB thread and the threads on Meade and Celestron SCT VBs.

 

C90_VB_2.JPG

 

C90_VB.JPG



#8 alvacouch

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Posted 16 July 2019 - 06:55 AM

My solution to this would be the celestron c11 t-mount with the ring I suggested before. Something like this one: 

 

https://www.bhphotov...xxoCu5QQAvD_BwE

 

Since that ring fits the celestron 2" visual back, this one should fit it as well. 

 

I have had this particular c90 a very long time (~ 25 years) and have accumulated a lot of relatively unobtainable parts for it. from a variety of sources. I have two visual backs that fit the (500mm) c90: the 2" I mentioned, and a 1.25" visual back that screws in directly and accepts a 1.25" t-mount, as others have mentioned above.

 

My most bizarre c90 part is an extension tube that -- due to a mechanism I don't understand -- converts the lens to an EFL of 1000 mm without optics. I have no idea how or why this works. 




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