Celestron C8 Page

Celestron C8

In addition to being my personal favorite telescope, the Celestron C8 is probably one of the most important amateur telescopes of the last 40 years. This was the first mass produced Schmidt-Cassegrain telescope, and greatly influenced amateur astronomy in the 1970's through to the present. I have constructed this web page to consolidate the information I have gathered about the C8, hopefully this will be of interest to other C8 owners or anyone interested in early Celestron telescopes. This web page contains much of the information I have learned about the C8 and includes links to several good historical backgrounds on the C8, information about C8 construction, maintenance, where to find manuals and spare parts, homebuilt C8 accessories, and links to other C8 websites. 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.

Below are three photos of my 1979 C8 with a piggyback C90 on the original fork mount. The far right photo shows the C8 and C90 on an Advanced VX mount

C8 Historical Background

I had originally intended to write a historical background on Celestron and the C8, but there are already several very good sites for Celestron history. Below are links to several good webpages about Celestron and C8 history:

Celestron C8 Overview and History

Company Seven's Celestron Overview and History

C8 Optical Tube Assembly 

The C8 was launched in 1970 and was the first mass produced Schmidt-Cassegrain telescope. The C8 is an 8" aperture f/10 SCT, with a 1.25" optical back and a 6x-30 mm finderscope (also orange). The classic orange tube model was sold between 1970-1983, after which it was changed to a black tube. Early models had a smooth orange finish, which was switched to a pebbled orange peel finish in the mid-late 1970's (note that the orange paint is lead based!). The C8 is an all metal telescope, but still compact and light: 9" x 13" x 24" (swung down) and only 21 lbs. Celestron supplied the orange tube C8 with a 3-3/4 pound set of six counterweights that could be used separately or threaded together into a stack (below left photo). The counterweights screwed into the front accessory holes on the top and bottom of the C8 corrector plate ring (below right photo). There are screw holes for mounting accessories on both the front and rear cells. The rear cell accessory holes are located on the bottom (side by side, separated by 1-3/4") and are threaded 8-32. The front accessory holes (top and bottom) are located on center (12-1/4" from the rear accessory holes) and are threaded 10-24. The accessory screws extend 5.9 mm into the C8 front and rear cells.


he 1970's to early 1980's Celestron C8 came with a very stabile, non-collapsible tripod. What makes these tripods so stabile is that the legs are composed of triangular spring steel elements. Each leg is a triangle that is assembled by bending the spring steel rods and inserting them in top and bottom brackets that hold the triangular shape. Because the spring steel must be bent to fit into the brackets, the legs are under stress and resist vibrations. Several tripod photos can be found on the Repairing a Broken C8 Tripod webpage.

The C8
OTA is similar to contemporary SCT optical tubes (below diagram). The corrector plate (pale blue) sits on a small ledge, is centered inside the OTA by three cork pads, and secured by a rubber sided retaining ring (brown). Light enters through the corrector plate, reflects off of the primary (dark blue) and secondary (light blue) mirrors, and exits the optical tube through the center light baffle (dark red) to the diagonal (yellow) and eyepiece. The secondary mirror is glued to a metal mounting plate (tan) that pivots inside the plastic secondary holder (purple). The secondary metal mounting plate pivots on a plastic peg (blue) and is held in place by three collimation screws. The plastic secondary holder is glued to the outside of the corrector plate, protrudes through a circular opening in the corrector plate center, and is glued to the secondary light baffle. The telescope is focused by turning the black rear knob, which slides the primary mirror forward or backwards on the light baffle. A retaining ring prevents the primary mirror from falling off of the light baffle. The remainder of this section contains pictures illustrating the C8 OTA construction.

The below left photo shows the C8 with the corrector plate removed. When assembled, the corrector plate sits on the small ledge and is centered by the cork spacers. The below right photo shows the primary mirror and primary mirror light baffle. Additional photos of the opened OTA and corrector plate retaining ring can be found on the Removing the Secondary Holder, Freeing Locked Collimation Screws, Bob's Knobs Replacement, and Corrector Plate Realignment webpage.


The below photos show the corrector plate, secondary mirror holder, collimation screws (original factory screws), and secondary mirror light baffle.


The below photos show the secondary mirror mounting plate (left) and the plastic secondary holder (right). The secondary mounting plate has an factory
control number written on the secondary plate (I have obscured several numbers for privacy reasons); this same number is engraved on the edge of the corrector plate. The corroded collimation screws in the left photo were removed and replaced with Bob's knobs. The below right photo shows the plastic secondary holder with the Bob's knobs installed. The secondary plate pivots on the central peg and the collimation screws hold the assembly in the correct orientation. Additional photos of the corrector plate and secondary assemblies can be found on the Removing the Secondary Holder, Freeing Locked Collimation Screws, Bob's Knobs Replacement, and Corrector Plate Realignment webpage.


C8 Fork Mount and Drive Base 

The orange tube C8 came fork mounted on a circular drive base. The mid-late 1970's mount was monotone grey with solid forks, but early 1970's models had a two tone paint scheme and forks with circular cut outs. Both forks have declination (Dec) setting circles and tube saddles that attach to the C8 OTA. The left fork has a Dec clamp above the setting circle and a dec slow motion knob on the fork bottom-back side. The Dec slow motion knob turns a screw that moves a tangent arm connected to the left declination axis bearing. This allows very precise declination adjustments, but the tangent arm has a limited range of motion. The forks are detachable from the circular drive base, which contains a manual right ascension (RA) control knob, the RA clamp, the RA setting circle, and the clock drive. The clock drive is a dual synchronous motor system with spur gears that rotate the RA setting circle. Engaging the RA clamp locks the top of the drive base (forks and OTA) to the RA drive axis gear. The small plastic knobs on the RA setting circle allow it to be rotated for adjustment.


The drive base bottom (below left photo) contains the two 
Synchron® motors, the oval power cord receptacle, and three threaded holes for attaching the equatorial wedge (accepts 3/8-16 x 1" bolts). Removing the center 5/8" bolt separates the drive base into two sections (below right photo): a top plate with the right ascension axis setting circles and the main RA gear, and a bottom section containing two Synchron® motors. Each Synchron® motor turns a 12 tooth brass spur gear at 1 RPH. The spur gears extend into the drive base and engage the RA gear. I haven't counted the number of teeth on the RA gear, but it should contain 288 teeth (12 tooth spur gear x 1 RPH x 24 hrs. = 288 teeth).  The C8 Synchronous Motors Webpage contains additional C8 motor photos and information about: how synchronous motors function, finding replacement motors, and procedures for freeing stuck drive base motors.


Drive Correctors and Inverters

If you wanted to use a C8 for 35mm film astrophotography, then you required a drive corrector. The drive corrector compensated for imperfections in the gear train and errors in polar alignment, allowing the observer to vary the electric frequency to speed up or slow down the drive motors. Many drive correctors also included an inverter to produce AC current from a car battery, allowing operation in the field. The Celestron drive corrector was also orange (of course). My understanding of how these drive correctors operated was that they converted AC current to DC current and then used an oscillating switch to produce a square wave (mains electric is a sine wave). Adjusting the drive corrector speed setting changed how fast the switch oscillated, changing the electric frequency and speeding up or slowing down the drive motors. A description of how the synchronous motors function can be found on The C8 Synchronous Motors Page. A consequence of running with a drive corrector was that the square wave caused some motors to run noisier. Many drive correctors were dual axis systems, including a DC motor that turned the declination axis slow motion control knob. Below are two photos of my dual axis Accutrack drive corrector and inverter. This drive corrector probably dates to the very early 1970's when Orion Telescopes and Binoculars was known as Gieseler Electronics. It still powers up and functions.



The 1975 C5/C8 manual (33 pages) can be downloaded as a pdf file from Company Seven's Celestron library: 1975 C5/C8 Manual Download

Replacement Parts 

Celestron's Technical Support recommended the following website as a possible source of classic C8 information and replacement parts: Astro Parts Outlet.  Other sources of spare parts are: eBay, Cloudy Nights.com Classifides, and Astromart.com Classifides; where I have seen classic C90, C5, C8, and C14 parts.  

C8 Maintenance Websites

There are not too many websites (at least that I can find) giving classic orange tube C8 maintenance instructions. Following are links to the only two websites I have found: Unofficial C8 Homepage and Sideways Orion C8 Maintenance and Care. If anyone knows of other classic C8 websites, please send me a link.

C8 Maintenance 

Following are links to webpages describing the C8 maintenance I have performed:

Removing the C8 Secondary Holder, Freeing Locked Collimation Screws, Bob's Knobs Replacement, and Corrector Plate Realignment

C8 Synchronous Motor Information and Freeing Stuck Drive Base Motors

Repairing a Broken C8 Tripod

C8 Accessories

Following are links to webpages describing homemade C8 accessories:

Guidescope Rail with C8 Radius Blocks

Advanced VX Mount Pier Adaptor

Retrofitting an Autostar Autoguider to a C8

C8 Bahtinov Focusing Mask

Unofficial C8 Homepage-Accessories

Unofficial C8 Homepage-Tips and Tricks