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
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
maintenance and repair I have
performed on my C90.
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
optical system (details
the C90 Maksutov-Cassegrain
system can be found on my Telescopes
and Optics Tutorial webpage).
available in three versions: the spotting scope (pictured
below), the single fork
mounted astro version, and as a black
C90 optical system was identical in all three versions.
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
between the primary
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).
manual (Classic Orange Tube C90 Instruction Manual)
is available as a pdf file from Oceanside
Photo and Telescope's web page.
Celestron's Technical Support
recommended the following
website as a
possible source of classic C90 information and replacement
Parts Outlet. Other
spare parts are: eBay,
Nights.com Classifides, and Astromart.com
Classifiedes; where I have seen
classic C90, C5, C8, and C14
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.
order to open the C90
optical tube, remove the adapter foot
scope model). Turn the C90
and remove the two
screws on the adapter foot (below left photo). Underneath the adapter
foot are two retaining screws (below right photo).
retaining screws limit the motion of the front optics and prevent the
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
longer than the other screw and can be left in place.
Carefully unscrew the orange tube from the primary
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
previously gave instructions for lubricating the screw joint
Mobil 1 grease (unfortunately this document is no longer available from
Celestron's archives). I purchased a synthetic grease
Lube®, which contains
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
the OTA completely together, and then separated
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
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.
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
loose part was the small
black plastic tube, which is
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
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.