I new-to-me CPC 800 arrived here with an incredibly dirty (on the inside) corrector. It had a thick-ish film and several spots/rings formed around some kind of particulates. Shipping it to Celestron was considered, as was shipping it back to the seller. But in the end, I decided to clean the corrector myself. Shipping these scopes around the country is neither fun nor inexpensive.
Since I was going to be removing the corrector, and the secondary holder was already loose and spinning, I read everything I could find about alignment of correctors and secondaries.
Wow! There are a lot of different and conflicting reports about alignment marks and shimming.
So I thought I would document for posterity what I actually found on this 2015-era CPC 800:
- There was no etching or serial number anywhere to be found on the corrector plate. I looked carefully and closely more than one time. Nothing to indicate proper rotational alignment.
- There were no lines, marks, etc. on the secondary or the secondary holder to indicate proper rotational alignment. There is, however, a set screw at one location.
- There were fibrous (thick paper) shims (dark gray colored material) in 2 locations:
- Between the glass and the retainer ring. On my scope, this (single) shim went from 3 o'clock to 8 o'clock. It was not glued on -- just held in place by clamping action. The width of this shim was such that it was in contact with the aluminum front cell and also overlapped the edge of the glass. It's purpose is a mystery to me, since it does not alter the distance between the glass and the mating surface of the cell.
- Glued to the inside of the plastic retainer ring were 2 sections of shim. Together these shims covered almost the entire circumference of the retainer ring. Their purpose seems to be to provide a uniform cushion where the ring engages the glass.
- The shims did not seem to me to be providing any kind of optical alignment purpose.
- There are 4 nylon-tipped set screws around the circumference of the cell. The purpose of these set screws is to center the corrector and secondary with respect to the primary. If you look at a picture of a CPC front cell from the outside, there are 4 locations around the perimeter where it seems a screw is missing. These set screws are in those "empty" holes and are hex socket screws (2 mm hex key) with nylon tips.
- I was surprised how far off-center the corrector was positioned by these 4 set screws. The set screw at the 7 o'clock position was flush with the inside rim of the cell, so the edge of the glass is in contact with the aluminum for much of the lower quarter of the "clock". At the 1 o'clock position there was a gap > 1/8" as established by the set screw at that location.
I marked the corrector at the 3 o'clock position before disassembly, and noted and marked the location of the partial shim.
The secondary was loose and spinning, so the horse had already left the barn as far as marking the secondary goes.
After successfully cleaning the corrector, I reassembled. I put the corrector back where it was before disassembly. I rotated the secondary so that its set screw was at 3 o'clock, which is opposite the focuser on the CPC 800. Information on the Celestron web pages suggest that the 3 o'clock position (opposite the focuser) is the master reference for the optics on this scope. Hopefully positioning the secondary so that the set screw is aligned this way was the right thing to do!
I was able to collimate using my Hubble artificial star, and got nice looking diffraction patterns inside, outside, and at focus. So I am cautiously optimistic that the scope is going to perform well outdoors, despite the initial problems of a filmed-over corrector and spinning secondary.
(Photo shows the film on the inside of the corrector when I received it. The particulates that are circled are on the inside, too, and appeared to be small pieces of aluminum, probably left over from tapping the holes in the cell. Disappointed to see how much swarf there was inside the OTA.)