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Saturn Dione Tethys 28 August 2021

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Re: C-90 comparisons

Started by Orion61, 03/20/2014 05:49AM
Posted 03/20/2014 05:49AM Opening Post
Hi Billy, great to hear from you again.
The optics of the C90 have always been good.
The main advantages of the new C90 is the focuser, and
Modern coatings. The old system used a helical system that had the whole top part of the
C90 screw in and out for focus, Those can get very stiff in Cold weather.
Plus the grease in those are all drying out now.
If you want to replace the grease there is a hidden screw under the tripod mount that acts as a stop, and has to be removed. White lithium grease works well and stays fairly smooth. Doing this can also help with collimation from the tube sagging.
The old grease should be cleaned out with a petrol base cleaner, followed by an Orange oil
The old coatings are close to 30 years old and are getting tired. A great number of the early C90's do not have Multi Coatings on the Meniscus.
There are rumors the newer optics are a bit better and collimation is better.
I hope this helps.
Larry Duane

1977 Orange tube Celestron C8 JMI Computer
12" Meade LX200
7" Meade LX200 updated to Autostar
102mm Meade ED/APO on CG5 GEM
6" Celestron NexStar se
127mm Celestron NexStar SLT
6" DX6 Dynamax (dont laugh superb optics)
6" F8 Criterion RV6 Dynascope
4" B&L 4000 Portable Observatory
ETX 90 Astro Origonal 1990's
80mm Celestron C-80 refractor
Jason Comet Chaser