astronomyasylum.com    Homebuilt Astronomical Equipment and Observatory

I purchased a classic (orange tube) Celestron C90 for use as both a guide scope and a small "grab and go" telescope (below photo). My C90 arrived with a loose part inside the OTA requiring some repair.  It took me a little time and some postings on Astronomy Magazine's Reader Forums page to sort out the problem and a repair procedure. I contacted Celestron to ask a few questions about the threading on the optical back, and found out that the manufacturer could no longer answer these questions. Since repair information and step-by-step instructions seemed difficult to find on this 30 year old telescope, I decided to create this webpage to consolidate the information I found. Hopefully this will be of interest to other classic C90 owners. Included is basic information on where to find manuals and replacement parts. Details of the C90 Maksutov optical system can be found on my Telescopes and Optics webpage. Additional  C90 information can be found on the Telescope Bluebook's Celestron International Page.

 

Manuals

The  C90 manual (classic orange tube) is available as a pdf file from Celestron's web page:
Orange Tube C90 Manual Download.

Replacement Parts and Information

Celestron's Technical Support recommended the following website as a possible source of classic C90 information and replacement parts: Astro Parts Outlet Used Telescope.  Another source of spare parts is eBay, where I have seen classic C90, C5, C8, and C14 parts. One of the best sites I have found for information on all aspects of telescopes is Astronomy Magazine's Reader Forums pages. 

C90 Maintenance and Repair

The following section documents the maintenance and repair I have performed on my C90. There is always a risk of damage with any home repair and maintainance of optical equipment. It is always recommended to contact the manufacturer prior to performing any maintenance as this may void warranties and alter equipment performance. I assume no responsibility or liability for any damages resulting from what you may do to your own equipment. Anything you do is at your own risk, so be sure you know what you are doing and accept all risks prior to beginning.

Opening the C90 Optical Tube 

In order to open the C90 optical tube, the adapter foot (spotting scope model) must be removed. Turn the C90 over and remove the two screws on the adapter foot (below left photo). Next remove the two retaining screws underneath the adaper foot (below right photo); these screws limit the motion of the front optics and prevent the orange tube from being totally unscrewed and falling off. 


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Carefully unscrew the orange tube from the primary mirror cell. The telescope will separate into two sections as shown below.




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C90 Lubrication

If the large focus ring becomes difficult to turn, it may require cleaning and lubrication. Celestron's website gives instructions on opening the optical tube and lubricating the screw joint at the following link: C90 Lubrication Procedure.

Reinstalling the C90 Light Baffel

My C90 arrived with a loose part inside the optical tube. I  opened the optical tube and a small black plastic tube fell out. The below photo shows my C90 as received. The loose part was the small black plastic tube.



After placing a posting on Astronomy Magazine's Reader Forums, I learned that this tube is a light baffel that fits around the secondary mirror spot on the corrector plate. This baffel prevents the sharp edge of the primary mirror from giving an unwanted edge reflection onto the secondary mirror. I reglued the baffel around the secondary mirror spot. I avoided using super glue or silicon. The cyanoacrylic in super glue could fog the optics and silicon would out gas, possibly giving problems. I used an adhesive type glue and left the optical tube open for several days during drying. The secondary mirror spot with the reinstalled light baffel is shown below.


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