C8 Locked Collimation Screw and Secondary Holder Removal

Warning Prior to Any Maintenance

The following webpage documents the maintenance and repair I have performed on my C8. 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.

Background: C8 with Locked and Corroded Collimation Screws and Incorrectly Rotated Corrector Plate

I Purchased my C8 on-line, and it is still in very good condition for a 30+ year old telescope (estimated from the 1979 manufacturing date on the Synchron motor housings): the optics are clear, the motors function, and the telescope is in "like new" overall condition. However, I immediately noticed some poor maintenance from the previous owner. The corrector plate was rotated out of the original factory position with the top collimation screw pointing to the 8 o´clock rather than 12 o´clock position (below photo). The C8 was slightly out of collimation (this was expected after shipping), but I quickly discovered that the collimation screws were locked fast and could not be rotated-most likely from corrosion during storage. I believe that since the C8 collimation screws no longer turned, the previous owner attempted to collimate the C8 by rotating the corrector plate. I fortunately found the original factory markings to reset corrector plate orientation (see the last webpage section) and tried collimating the C8. Since my hex wrench set is metric, it fit a bit loosely into the US hex screws. I was unable to free the collimation screws and was concerned that too much force and using a slightly loose metric hex wrench would strip the hex screws. I ordered a SAE hex wrench set, but still couldn't free the collimation screws. I tried using a soldering iron to heat the screw heads and break the corrosion, but this also failed. I eventually decided to drill out the original collimation screws and replace with Bob's Knobs. The following webpage sections detail this repair. 


It is difficult to find detailed construction information on the orange tube C8 series telescopes, so I was very uncertain about how the secondary assembly and light baffel fitted onto the corrector plate and how to remove them....... so there was some trial and error during this process. I learned a lot about how the C series telescopes are constructed and, based on what I have learned, I would recommend several modifications to the procedure I used. This section sumarizes the procedure I used (which worked) and gives suggested modifications to my procedure based on what I have learned. Detailed instructions and photographs follow in subsequent webpage sections.

My Trial and Error Procedure (Which Did Work):
  1. Locate Factory Markings for Realignment of the Rotated Corrector Plate (Since my Corrector Plate was Rotated, it made no Sense to Mark its Incorrect Position)
  2. Remove the Corrector Plate
  3. Carefully Mark the Secondary Holder Position within the Corrector Plate
  4. Remove the Secondary Holder and Light Baffel from the Corrector Plate (This is a Delicate and Very Difficult Job Since its all Glued Together!)
  5. Drill Out the Locked Collimation Screws and Shear off the Screw Heads with a Metal Tap
  6. Remove the Secondary Mirror from the Secondary Mount
  7. Remove the Locked Collimation Screws from the Secondary Mounting Plate
  8. Clean and Retap (if necessary) the Secondary Mounting Plate Screw Holes
  9. Reinstall the Secondary Mirror (Highly Recommend Replacing the Collimation Screws with "Bob's Knobs" or Equivalent)
  10. Reglue the Secondary Holder and Light Baffel onto the Corrector Plate (Position Exactly According to the Alignment Marks from Step 3.)
  11. Reinstall the Corrector Plate onto the C8 
Recommendations Based on What I Learned During this Process:
  1. Mark the Exact Position of the Corrector Plate on the C8 (This Assumes that the Corrector Plate has Never Been Rotated to an Incorrect Position) 
  2. Remove the Corrector Plate
  3. Place the Corrector Plate on a Solid Surface (Collimation Screws Facing Up) and Place Something on Top of the Corrector Plate to Protect it From Damage During the Next Few Steps
  4. Get Someone to Help You Carefully Hold the Corrector Plate and Drill out the Collimation Screws without Removing the Secondary Holder from the Corrector Plate (This Will Save You a Lot of Work)
  5. Follow Steps (5)-(9) in the Above Procedure
  6. Reinstall the Corrector Plate onto the C8 
Note: removing the secondary holder is a delicate operation because it is glued onto the corrector plate front side and the light baffel is glued onto the back side and actually around the secondary holder. I chose to remove the secondary holder because I didn't feel comfortable trying to simultaneously hold the corrector plate and drill out the collimation screws; I was very concerned about losing control of the power drill and damaging the corrector plate. Had my drill press been slightly larger, I could have used it to easily drill out the collimation screws while still attached to the secondary. 

Corrector Plate Removal

A six screw metal retaining ring holds the corrector plate onto the OTA (below left photo). The corrector plate sits on a small metal ledge and is centered in the OTA by three cork strips (below right photo, corrector plate removed).


Before removing the corrector plate, be sure to elevate the OTA to point nearly to the zenith; this will prevent the corrector plate from falling out of the OTA. Next remove the 6 retaining ring screws, but be very carefull because you are using a screwdriver close to the corrector plate! Carefully lift the corrector plate out of the OTA. Immediately cover the open OTA to prevent getting dust onto the primary mirror. The corrector plate top with collimation screws and the corrector plate underside with the secondary mirror and light baffel are shown in the below left and right photos, respectively.


Removing the Secondary Holder and Light Baffel from the Corrector Plate

Below (left) is a diagram showing the secondary mirror (yellow), secondary holder (orange), the light baffel (dark blue) and the corrector plate (tan). The secondary holder is glued to the outside (front) of the corrector plate and extends through a central hole in the corrector plate. Inside the OTA, the light baffel fits around the outside of the secondary holder and forms a recess that contains the corrector plate. The entire assembly (secondary holder, light baffel, and corrector plate) is glued together. In order to remove the secondary holder, it is necessary to very carefully cut around the seam between the light baffel and secondary holder with a very sharp hobby knive or scalpel. The below middle and right photos show the seam between the light baffel and secondary holder; the seam is marked in yellow on the right photo, note that the corrector plate is removed, but the recessed space where it fits is clearly visible. Once the light baffel is removed, the secondary holder can be gently pried loose from the corrector plate (the plastic secondary holder to glass bond isn't too strong).


Before removing the secondary holder, precisely mark its position on the corrector plate so it can be correctly reinstalled (also mark the outside surface so the corrector plate isn't reinstalled backwards). Using a very fine pointed hobby knife or scalpel, carefully separate the glue seam between the secondary holder and the light baffel. This is delicate and time consuming work-so be patient. Once the light baffel is removed (below left photo), carefully separate the secondary from the corrector plate. The glue dosen't bond too well to the glass corrector plate, so you can carefully separate this seam using the hobby knife. Then remove the secondary holder from  corrector plate (below right photo).


Drilling Out the Corroded Collimation Screws

Before drilling out the collimation screwheads, I reassembled the light baffel and secondary holder with a large cable tie strip in the recess normally occupied by the corrector plate (below left photo). This will allow some pressure on the secondary and light baffel. Without the cable tie strip, pressure would cause the secondary holder to sink into light baffel. I used a drill press to drill out each collimation screwhead, screwed in a metal tap, and sheared of the screw heads. The below right photo shows the secondary holder and sheared off screwhead in the metal tap. As soon as the last colimation screwhead is removed, the secondary miror will fall out of the secondary holder. Be certain to hold the secondary mirror holder with the mirror facing upwards when you shear off the last screw and be prepared for the mirror to come free.


The secondary mirror is glued to a thick metal mounting plate with a central pivot hole and three collimation screw holes (below left photo). There is a 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 (see the last photo on this webpage). The below right photo shows the inside of the secondary holder. The secondary mirror mounting plate pivots on the central pin and the three collimation screws thread through the holes and into the secondary mounting plate.


Using a large pliers, I removed the factory collimation screws. The factory collimation screws were extremely difficult to remove and there was a tar like resin inside the screw holes; my best guess is that the previous owner applied some sort of grease that eventually dried to a tar like consistency!  After the collimation screws were removed, I thoroughly cleaned out the screw holes. The below left photo shows the old collimation screws and the replacement Bob's knobs. The below right photo shows the light baffel, secondary mirror, and secondary mirror holder prior to reassembly.


Secondary Reassembly

Reassembly is pretty straight forward. I installed the Bob's knobs, exactly repositioned the secondary holder in the corrector plate according to my markings prior to disassembly, and glued everything together. 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 corrector plate off of the OTA and on my workbench for several days during drying.

Corrector Plate Alignment

If your corrector plate is properly installed, just mark its exact position prior to removal and reinstall to the exact orientation. My corrector plate was incorrectly rotated due to some sort of previous owner maintenance. I opened the OTA and found what appear to be factory installation markings on the corrector plate. A number matching the secondary mirror mounting plate number "__66" is engraved on the edge of the corrector plate and there is also a blue alignment mark (below left photo). I discovered that if I aligned this mark (yellow arrow) with the screwhead on the outside of the C8 OTA (red arrow), then the secondary holder serial number plate was exactly horizontal (below right photo)-please excuse the dust on the corrector plate.....I took this photo prior to final lens cleaning. The last job was reinstalling the metal retaining ring over the corrector plate. Be sure to position this with the rubber side against the corrector plate and do not overtighten the screws-I only tighten to just past finger tightness.


C8 Status After Maintenance and Collimation

The below photo shows two images I took of Jupiter using the C8 and a SPC900NC webcam. The left image was taken before the maintenance described on this webpage and the right image was after installing the Bob's knobs and with only a rough collimation on an artificial star.

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