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Early orange C90 trouble

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#1 DMala

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Posted 06 October 2016 - 10:08 PM

A couple days ago I decided to do some work on the early (I guess, s/n 9136) orange C90 I got with the C8 trunkful package I picked up about a month ago. I collimated the finder and did a star test, that showed rather rather dim images (same with daylight: maybe the mirror shows its age?), and moderately offset Airy disks. Since the focuser mechanism feels totally dry and is quite loose, to the point that it wiggles a bit, I thought that greasing the threads may help to line things up a bit better.

Based on these instructions ( http://www.astronomy...um.com/c90.html ) , I expected to find two focuser retaining screws under the adapter foot, but there is actually none. Evidently there was a different design earlier on.

 

I noticed a small set screw where indicated by the screwdriver tip, that was fully seated and felt very loose. I tried to unscrew it but it does not seem to retain the focuser. I decided to screw it back and....it fell inside.   :bangbang:

 

So, the question is, how do I get to it now? Thanks

Attached Thumbnails

  • Celestron C90_Orange_1.jpg
  • Celestron C90_Orange_2.jpg

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#2 DMala

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Posted 07 October 2016 - 08:44 AM

Problem solved, I found a small hex screw under the golden band with the serial number, that's the focuser retainer. Will post pictures later. It seems that later in production they switched to larger phillips screws.


Edited by DMala, 07 October 2016 - 08:45 AM.


#3 terraclarke

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Posted 07 October 2016 - 09:27 AM

Wow! That is an early one! I have not seen another that early, next to mine (#9429). Mine however produces surprisingly bright images considering its Maksutov design and 1000mm focal length.


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#4 Geo.

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Posted 07 October 2016 - 10:22 AM

Yes, those holes are for the mounting points. The retaining screw is usually hidden by the camera mounting block. 

 

Cleaning the helical thread on these is a PITA due to the proximity of the primary. Recently I simply added a few dabs of silicone brake grease to the front section and reinstalled it. This resulted in much better action. Usually I'd recommend through cleaning but not here. 

 

I managed to part with one at auction for an OK price by putting it on a mount.

 

P1015417.jpg

 

So, I'm down to six, I think. But I keep finding them.


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#5 DMala

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Posted 07 October 2016 - 10:37 AM

Thanks, maybe it seems dim to me because I never looked through a Mak f11 before...  I would like to soon compare it to the black made in USA C90 I found at a thrift store last week. I also got on Goodwill a nice tripod that I hope may be of appropriate age to match. More fun details to be posted after work.


Edited by DMala, 07 October 2016 - 10:38 AM.


#6 terraclarke

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Posted 07 October 2016 - 02:02 PM

The image is actually brighter than a Lomo 95mm Mak I used to have. That was F15 tho.


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#7 ftwskies

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Posted 07 October 2016 - 02:07 PM

My C90 is the later design with the screws under the mount block.  It was purchased in 1979, I still have the paperwork.  SN 900881.  It's well used, but it's only been opened up maybe 3-4 times in its life (mostly in the last ten years).  It gives nice, bright views to this 10" dob pusher.  :)   How do the coatings look on your primary?  Also, are both baffles intact?  The secondary baffle especially has a bad habit of falling off.  Don't ask.  ;)


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#8 DMala

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Posted 07 October 2016 - 04:13 PM

OK here we go with the details:

 

This morning I noticed no tips in response to my post (not surprising at it seems that such early orange C90s are not that common), therefore I found some time to carefully look for a solution. First I tried to take off the finder thinking that maybe the screws of the base would double as focuser retainers. No way (yes it was a far shot). Then I took out the two slotted set screws below and to the right of the serial number (see first picture posted), no result. Finally I carefully raised the edge of the serial number gold band, and there it was, a small hex screw! After that I disassembled the front tube, retrieved the dropped screw, and got depressed seeing some marks on the otherwise nice looking mirror, that I initially thought were scratches caused by the screw I dropped.  :mad:  :mad:  Only after a while I realized they were actually grease residues, pfieuuuuu. I carefully tested one edge of the mirror and found that they came off easily with some Zeiss cleaner first (which I find a bit heavy in detergent, but it dissolves the dirt very well), followed by a few very small drops of glass plate cleaner to remove any residue of grease and detergent and a very, very light and brief wipe with a microfiber cloth. The baffles seem stable and not loose but frankly I did not shake them to find out.... I then dusted off everything with a very light stream of air from my compressor (which has a humidity condenser prior to the nozzle), used purple Loctite to re-position the screw I had dropped (which is the end stop of the focuser), applied fishing reel drag white grease to the threads, and reassembled everything. Now the focuser has no wobble and it moves with the right amount of friction for my taste.  :)  :)

Attached Thumbnails

  • Celestron C90_Orange_5.jpg
  • Celestron C90_Orange_4.jpg
  • Celestron C90_Orange_3.jpg

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#9 DMala

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Posted 07 October 2016 - 04:35 PM

The tripod story:

 

The black, US-made C90 I found last weekend was on a Habitat for Humanity thrift shop shelf next a an old tripod made in Japan, medium-light duty, which seemed to match the right period. I bet they came together. Unfortunately the tripod was not in good shape, with missing parts, and I did not pick it up. I have a Velbon medium-duty tripod that is OK for still photos, but the plastic head pan motion is as smooth as the wheels of my car when I brake and the ABS kicks in.... you know what I mean? 

So I searched Goodwill and found a tripod that seemed decently robust, and from the solid metal head, metal crank handle and cork pad of the shoe base I guessed it to be more or less of the right age period (for the black tube C-90) . It is a Quickset model S-95565-3, Made in USA.  I got it and I was very pleasantly surprised because it is much heavier duty than I expected. The altitude control is damped by a spring mechanism, that makes it hard to point very high. The simple solution was to disengage it. I may actually remove the spring completely. So my two C90s now have a properly functional and solid base, for a very modest cost. Nice. :elephant:

Attached Thumbnails

  • Celestron C90_Orange_6.jpg
  • Quickset_1.jpg
  • Quickset_2.jpg

Edited by DMala, 07 October 2016 - 04:38 PM.

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#10 twhite

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Posted 07 October 2016 - 08:05 PM

Wow! That is an early one! I have not seen another that early, next to mine (#9429). Mine however produces surprisingly bright images considering its Maksutov design and 1000mm focal length.

Mine is in that same serial range - 94xx (61, I think -- I looked at it earlier after reading this thread but that was several hours ago and I'm just now getting back to it, but I'm pretty sure that's it).  Mine is an astro model, too.


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#11 ftwskies

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Posted 07 October 2016 - 11:06 PM

One other change that I've noticed is the finders. I look at a lot of pictures of orange C90s and some have a black finder eyepiece while others (like mine) have chrome.  I used to think maybe it distinguished the spotter version from the astro version, but maybe it was just a change made over time.  Any body know for sure?  I wonder if the screw design changes were broken in at the same time...


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#12 DMala

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Posted 08 October 2016 - 10:39 AM

The C-90 shown in the link below seems to have the later focuser retaining screw design, but a finder with black eyepiece. 

 

http://www.astronomy...um.com/c90.html



#13 terraclarke

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Posted 08 October 2016 - 03:50 PM

Mine came with no finder, nor was it ever tapped for one.



#14 ftwskies

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Posted 08 October 2016 - 08:00 PM

So you have the telephoto, then?



#15 terraclarke

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Posted 09 October 2016 - 10:56 AM

Yes. It is marked C90 F11 Maksutov, 1000mm, and mirror lens on the gold band and has extremely sharp optics.



#16 terraclarke

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Posted 07 November 2016 - 06:30 PM

I took mine apart and cleaned and regreased the focuser with white lithium grease. I used it last night and noticed a little sticking and slop. It was time. Thanks Dionigi for the excellent instructions.


Edited by terraclarke, 07 November 2016 - 06:33 PM.

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#17 NMBob

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Posted 04 November 2018 - 03:58 PM

These computer's remember everything! Now I know what that DMala clown did to this poor scope. He kept grub screws in it!

 

I bought #9136 a couple weeks ago from the ads and it will be living comfortably in New Mexico. I got it as a pet for my Edge 8. It's still in perfect shape, has 4 .965 eyepieces and Barlow and diagonal in a grey cardboard box, and has what looks to be one of the original carrying cases. At least the foam in it is the same as in my '79 C8 case. I bought a .965 to 1-1/4" adapter and will use it for fun from time to time. Cool!

 

Bob :)


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