Celestron C90 Page
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
a loose part inside the OTA
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.
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
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
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.
manual (classic orange tube) is available as a pdf file from
Celestron's web page: Orange
Tube C90 Manual Download. The Celestron
Memories website also contains downloadable C series telescope
and the 1978 Celestron catalog.
Celestron's Technical Support recommended the following
website as a
possible source of classic C90 information and replacement
Parts Outlet. Other sources
spare parts are: eBay,
Nights.com Classifides, and Astromart.com
Classifiedes; where I have seen classic C90, C5, C8, and C14
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.
order to open the C90
optical tube, the adapter foot
scope model) must be removed. Turn the C90
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.
Carefully unscrew the orange tube from the primary mirror cell.
telescope will separate into two sections as shown below.
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
C90 Light Baffel
arrived with a loose part inside the optical tube. I
opened the optical tube and a small black plastic tube fell
The below photo shows my C90
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
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
with the reinstalled light baffel is shown below.
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not copy or reproduce in any form without prior written consent.