Johann and I had another session with the 6" objective yesterday, and made some significant progress in objective alignment and DPAC.
Last weekend, we had several unresolved problems at the end of the session:
- Signficantly skewed Ronchi lines on the DPAC
- Newton rings that were skewed away from the center of the objective
- An objective that fit so tightly into the cell that there was no slight "rattle" heard after objective reassembly
David G. gave us some good advice on troubleshooting, which we followed yesterday:
- Removing the cell from the objective, and checking the shims and air gap between the crown and flint
- Shims were in good shape, and air gap was even around the objective perimeter
- Using a template, found all three to be symmetrical (120 degrees apart, plus or minus a degree)
- Checking the three support points at the bottom of the cell for proper placement
- Using a template, found all three to be symmetrical (got similar spacing results as for the shims)
We also cleaned the inside of the cell:
- Removed a lot of dirty film from the inside cell walls
When we re-assembled the objective, we ensured that the objective shims and the lower support points were aligned. The objective went in on the first try with no difficulty. We also ensured that the objective retaining ring was installed "hand-tight, then backed off slightly"...and we did get the "slight rattle" that had been missing previously.
Newton rings were symmetric and centered...and we had three straight lines on the DPAC testing! (...and there was great rejoicing! ). It's possible that one element may have very slightly cocked in the cell during last week's activities, due to the dirty residue on the cell walls...that, and/or the misalignment of the objective shims to the cell support points, may have been the cause of last week's problems.
As a final (unrelated) step, we cleaned and tested the 3" f/6 objective for the finder on the 6" Spacek. I had not been able to disassemble the finder previously, but with Johann's help we got it apart, and found that one of the objective shims was out of the air gap entirely (was on the edge of the objective...we reinstalled it after cleaning both elements). During reassembly, Johann demonstrated for me what DPAC results for a reversed objective and flipped element looked like...as Dave G. says, DPAC will immediately show you those conditions if they exist! Finder is reassembled, shows fine images, and is reinstalled on the 6" OTA.
All in all, a great day! Many thanks to Johann, for his support and his patience teaching a refractor "newbie" about optical maintenance and testing!
Edited by John Higbee, 25 February 2019 - 04:53 AM.