Recently a coworker decided to gift me an old dobsonian telescope that they had hanging around in their basement. They bought it for a friend, but it was never really used and was now only taking up space for them.
As far as I can tell, the company that makes this telescope no longer exists. The tag on the dobsonian tube says Omnicon 620 but a quick google tells me that Omnicon doesn’t exist as it seems to have no internet presence.
Initial condition of the scope was not bad, but definitely could have been better. It was covered with dust, which was expected, but additionally there was rust on the steel elements. This includes on the focuser as well as all the screws holding both mirrors in place. There was also significant scratching on the finder scope body.
First thing to do was break the whole telescope down. Disassemble it into all it’s individual components. Luckily a dobsonian is a relatively simple telescope design and therefor was quite easy to take apart. It breaks down into the tube, the primary and secondary mirrors, the focuser, the guide scope and guide scope ring, as well as several threaded rods and nuts. From the figure below you can see just how few parts there actually were.
The screws and nuts needed to be completely replaced with new ones because they were so rusted over. Thankfully finding replacement parts at the nearby hardware store was quite easy. Additionally, I washed every part throughly to clean off the built up dust before I continued the restoration.
For the tube, there were a few things that needed to be done. On the inside of the tube there was a ring of what looked like corkboard glued to the tube with silicon. Perhaps this was part of the original shipping method when my coworker initially bought the telescope. In any case, it needed to be removed. Some silicon remover and a lot of elbow-grease and the inside of the tube was all cleaned up. For the outside of the tube, I sanded off the old paint and gave it a nice new coat of paint. I tried to match the colour of the new paint to the old paint. I also decided to put a little black ring around the top and bottom of the tube just for aesthetics.
For the finder scope, a similar repaint job was performed, but I also had to unscrew the scope and clean all the lense elements with a combination of isopropyl alcohol, Dawn dish soap, and room temperature water. Then the finder scope could be reassembled.
The steel elements on both the finder scope and the focuser did have a buildup of rust. These were cleaned with isopropyl alcohol and an old toothbrush to scrub with.
The most intensive part of this restoration were of course the mirrors. The primary mirror was so caked in dust that there were no longer any reflections from it’s surface. The 8” mirror plus the mirror cell was massive and rather than disassemble the mirror from it’s cell I decided to wash them both as a single unit. I filled a tupperware container with water, then added a generous amount of isopropyl alcohol and Dawn dish soap. The same mixture I used on the finder scope lenses. The mirror and its cell could then be submerged completely in the mixed solution. I left it to soak for about 10 minutes to loosen the dust. Then, using cotton balls, began to clean the surface of the mirror. I swabbed the mirror in horizontal streaks first, then vertical ones. Each cotton ball was only used for a single streak and was rotated as it was pulled along the surface of the mirror in order to prevent scratching of the mirror’s surface. Once complete, the mirror was rinsed with distilled water, and then dabbed (not rubbed) with kleenex gently in order to absorb any water that remained on the surface. This process was repeated for the secondary mirror as well.
I think the results speak for themselves. The figure below shows a comparison before and after the primary mirror was cleaned.
After that the telescope could be reassembled. Pretty easy job as everything just gets screwed back into the tube where they were initially removed from. The most important part was to make sure the primary and secondary mirrors were centred in the tube as best as possible.
The last step in this process was mirror alignment. Thanks to an inexpensive laser collimator this was also super easy. Just point the telescope at the wall, install the collimator into the eyepiece and see where the laser ends up. If it appears on the wall then just adjust the screws at the bottom of the primary mirror cell to tilt the primary mirror. Doing this will slowly move the laser dot on the wall towards the centre of where the telescope is pointing. At some point the laser will disappear from the wall since it is now colliding with the secondary mirror. Then look at the target on the laser collimator and keep adjusting the screws until the dot is centred in the collimator’s target.
ALL DONE!
The telescope is now cleaned, repaired, and realigned and ready to be used. The figure below shows the telescope after the refurbishment work has been completed.
