Well I always seem to learn something every time I service my C11. This time was the importance of the position of the secondary mirror. I recently installed the Starizona Fastar secondary holder on my C11. I was very careful about taking everything apart and noting the position of everything. Put everything back together and re-collimated under the stars. On axis images were fine visually. Onto the ST8XME for prime focus imaging. Something seemed to be off in the images. Stars were not uniform across the image plane. One edge of the image was always more distorted than the other side. Successfully used the collimation feature of CCD Inspector with the same result. Frustrating because in the past, that instrument always took good images. Mirror flop was my next thought so I redistributed the grease on the baffle tube (OTA is a carbon fiber from 2005 I believe and that had never been done). Same result (although less mirror flop!). Looking at the images, it seemed like the "sweet spot" was shifted to one side. Using CCD Inspector seemed to confirm that thought (see attachment).
On to the Hotech SCT Collimator. Not cheap for sure. After a learning curve, I finally got the hang of the Hotech collimator. Reading the instructions, it stressed the importance of secondary mirror centering. The collimator told me that the secondary mirror was slightly off center. I repositioned one cork shim and the collimator showed the secondary centered. Went through the remaining collimation process then took it out under the stars. Star test revealed collimation just needed a very minor tweak. Visually, stars snapped into focus. Out of focus stars distorted uniformly near the very edge of the field of view (unlike prior). Now time for the CCD camera. Finally got a chance last night briefly before fog rolled in. Same target as before. Results speak for themselves. Stars were uniform across the entire image. Using CCD Inspector confirmed that the C11 appears to be once again, well collimated (see second post).
What did I learn? Pay attention to secondary mirror centering. I prefer to do things myself and the only way I know to do that is to have the proper tools (or send it out). You can collimate under the stars BUT you can't center the secondary mirror that way. The Hotech wasn't cheap but it did work and worked well. Sure seems to have solved a frustrating issue with an otherwise fine instrument.
On to the Hotech SCT Collimator. Not cheap for sure. After a learning curve, I finally got the hang of the Hotech collimator. Reading the instructions, it stressed the importance of secondary mirror centering. The collimator told me that the secondary mirror was slightly off center. I repositioned one cork shim and the collimator showed the secondary centered. Went through the remaining collimation process then took it out under the stars. Star test revealed collimation just needed a very minor tweak. Visually, stars snapped into focus. Out of focus stars distorted uniformly near the very edge of the field of view (unlike prior). Now time for the CCD camera. Finally got a chance last night briefly before fog rolled in. Same target as before. Results speak for themselves. Stars were uniform across the entire image. Using CCD Inspector confirmed that the C11 appears to be once again, well collimated (see second post).
What did I learn? Pay attention to secondary mirror centering. I prefer to do things myself and the only way I know to do that is to have the proper tools (or send it out). You can collimate under the stars BUT you can't center the secondary mirror that way. The Hotech wasn't cheap but it did work and worked well. Sure seems to have solved a frustrating issue with an otherwise fine instrument.
Attached Image:
