It has already been two years since the Astrotech 12'' RC became the workhorse of Asteria observatory. It has already logged tens of hours of imaging during that time, specifically close to 150 hours. They say you're not supposed to clean the optics at all if possible, due to the thin aluminum coating that may be affected. However there came a point where I could see a layer of gunk caked on the mirror's surface, which would eventually start to affect performance by lowering the mirror's reflectivity. I think being located in the woods with all the pollen around expedited the need for a cleaning.
I finally got my nerve up to tackle this project after watching Gary Honis's excellent video on modifying the AT12, which includes the cleaning steps.
The video is very detailed and covers everything very well. It set the expectation, and by the time I was done watching, I gained the confidence needed to tackle the project. Therefore a big thanks goes out to Gary for his very informative video. Here is a photo timeline of the project:
August 11, 2018
Telescope disassembly & mirror cleaning
Before listing the steps, my first impression was what a high quality build this instrument has to the point where taking everything apart was really straight forward and predictable. Also my goal was to put everything back in the same exact position. I therefore used masking tape to mark the orientation of the various components.
1. Remove the focuser assembly and focuser adjustment ring
2. Remove the screws around the mirror cell and remove the tube assembly along with the secondary mirror and spider
3. Remove the baffle tube
4. Remove the primary collimation screws that hold the mirror cell to the back of the scope
5. Remove the safety clips around the mirror
6. Removed the rubber O-ring from the center of the mirror cell.
7. Used fishing line to break the RTV glue holding the mirror to the cell on the sides
8. Carefully pried along the back to detach the mirror from the cell, held down by glued cork pads.
9. Cleaned the mirror as follows:
a. Laid the mirror on a flat water proof surface
b. Poured distilled water to cover the surface and let it sit a few minutes
c. Used the weight of cotton balls to gently clean the surface
d. Rinsed with more distilled water
e. Removed water droplets using a blower until completely dry
10. Stored the mirror ensuring no dust will get on the surface by covering it.
August 23, 2018
It turns out I did not have all the materials needed to finish the job on the 11th, so the 23rd was the next best opportunity. I bought the velcro pads listed in the video to act as mirror retainers. They are oval pieces of velcro spread out evenly on the mirror cell. Continuing from above:
11. Used a scraper to completely remove the remaining glued pieces of cork from the back of the mirror and mirror cell.
12. Used 91% Isopropyl alcohol to completely remove the glue from the mirror and mirror cell.
13. Placed the velcro pads evenly along the cell.
14. Dropped the mirror back into the cell, paying special attention to the correct orientation.
15. Re-attached the rubber o-ring to the middle of the cell, and only applied snug pressure.
16. Re-attached the 1mm set screws to the o-ring and tightened.
17. Re-attached the safety clips
18. Re-attached the mirror assembly to the back as follows:
a. Attached the collimation screws and springs and used tape to hold them together in the upright position.
b. Lined up the mirror assembly to match the masked tape on the telescope back, which aligned it correctly with the collimation screws.
c. Loosely tightened each one of the collimation screws.
d. Fully tightened collimation screws so the mirror assembly and telescope back looked parallel to each other.
This was the trickiest bit getting the mirror assembly and scope back to line up correctly.
19. Re-attached the baffle tube
20. Re-attached the telescope tube to the back using the phillips screws, in the correct orientation.
21. Re-attached the focus ring, plate, and focuser.
Clean mirror
Collimation (optical re-alignment)
Focuser and secondary mirror collimation
Since the telescope was disassembled, an optical re-alignment was necessary. The secondary mirror was never moved, so theoretically ensuring that the laser hit the center of the secondary mirror also ensured good alignment for the focuser. Sure enough, the center of the secondary matched the return beam at the center of the laser.
Primary mirror collimation
The primary mirror as expected was way off. In an RC scope it is very easy to use your eye during the day to get a rough alignment. I think I got it very close. My method consists of doing an optical rough alignment by visual, and final alignment using the CCD camera and CCD Inspector. CCDI showed the visual alignment was only around 4.5'' out, which is very good (and lucky). I improved this by making minor adjustments to the collimation screws in the direction required.
Cameras and focus motor re-attached and cable management done.
Under the stars during the final collimation session.
Final CCDI collimation image
First light, M103 open cluster in Cassiopeia
The question in the back of my mind remains how well collimation hold after the cell modification. I will be testing this by slewing the scope to different areas in the sky and checking the alignment in CCDI. Stay tuned!
