Celestron C90 Page

Celestron C90  

I purchased a classic (orange tube) Celestron C90 for use as both a guide scope and a small "grab and go" telescope (below photos). My C90 arrived with a loose part inside the OTA requiring some repair. 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 maintenance 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 the C90, where to find manuals and replacement parts, and details about the maintenance and repair I have performed on my C90. Due to recent instances of my equipment photos being posted without my permission on eBay (used in adds for C8's, C90's, and SCT accessories), I have had to watermark my equipment photos.

The C90 was launched in 1978 and is a 90 mm aperture f/11 Maksutov optical system (details of the C90 Maksutov-Cassegrain optical system can be found on my Telescopes and Optics Tutorial webpage). The C90 was available in three versions: the spotting scope (pictured below), the single fork arm mounted astro version, and as a black telephoto lens. The C90 optical system was identical in all three versions. The C90 was sold with a 5x24 finderscope, a 0.96" optical back, and came with 18mm and 30mm Kellner eyepieces. The C90 optical tube is very compact (5"diameter x 8" long) and weights about 3 lbs. The C90 is focused by turning the black focusing ring attached to the orange tube; this screws the orange tube forward or backward, changing the distance between the primary mirror and corrector plate


The C90 visual back accepts 0.965 inch. eyepieces. Options for upgrading to standard 1.25 inch. eyepieces and accessories are a hybrid 0.965 to 1.25 inch. diagonal or adaptor, or a 2 inch. NX4 (C90)/ETX expanding ring. Addition of the expanding ring adaptor converts the C90 visual back thread into a 2 inch. SCT male thread. Below are several photos of the C90 with a Baader 2 inch. NX4 (C90)/ETX Expanding Ring + Baader Universal Photo-T Adapter C90 for NX4/ETX, a standard T-ring, and a Canon DSLR camera. With the Canon DSLR camera and both Baader adapters installed, The C90 comes to focus (both with and without a f/6.3 reducer corrector in the optical path).  



The  C90 manual (Classic Orange Tube C90 Instruction Manual) is available as a pdf file from Oceanside Photo and Telescope's web page.

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 OutletOther sources of spare parts are: eBay, Cloudy Nights.com Classifides, and Astromart.com Classifiedes; where I have seen classic C90, C5, C8, and C14 parts. 

C90 Maintenance and Repair

The following sections document 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, remove the adapter foot (spotting scope model). Turn the C90 over and remove the two screws on the adapter foot (below left photo). Underneath the adapter foot are two retaining screws (below right photo)These retaining screws limit the motion of the front optics and prevent the orange tube from being totally unscrewed and falling off, or advanced too far into the primary mirror. Next remove the retaining screw nearest to the front optics.  The retaining screw nearest to the primary mirror is slightly longer than the other screw and can be left in place.


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


C90 Lubrication

If the large focus ring becomes difficult to turn, it may require cleaning and lubrication. Celestron's website previously gave instructions for lubricating the screw joint with Mobil 1 grease (unfortunately this document is no longer available from Celestron's archives).  I purchased a synthetic grease called Super Lube®, which contains Teflon, and used this instead of Mobile 1 grease. I opened the C90 as described in the previous section, but left the rear retaining screw in place. I removed the old grease from the screw threads,  added a light coating of Super Lube®, screwed the OTA completely together, and then separated the parts. I removed any grease clumps, added more grease as necessary, and repeated the process until all screw surfaces were evenly coated with grease. I avoided adding too much grease, as this could clump and fall inside the OTA. It's better to add small amounts of grease until you get the proper coverage instead of over coating and having clumps of grease form inside the OTA. The below left photo shows the threading on the mirror cell after grease application-note that the entire threaded surface is thinly coated with grease (without clumping). The below left photo also shows the retaining screw that prevents the orange OTA from being advanced into the primary mirror (leave this in place). The below right photo shows the lubricated OTA sections, ready for reassembly.


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, which is a light baffle 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.