nightster

Olly, you are such an excellent teacher of these most difficult skills and theories that I, as well as others, get excited to see a reply from you. I appreciate the time you give a mere beginner such as myself. I hope you recover quickly and fully. Thank you,

Well that didn't work, and since I cannot edit my posts yet that garble will have to stay, not quite sure how to post a table to the thread.

I spent some time working with Vincent at Highpoint Scientific (my favorite stateside astro dealer.) He helped me with the backfocus calculation for my scope and the new CCD today. We found the error in the Astronomik thickness, so I was correct on that line. He also contacted the maker of my OTA, Flattener and comfirmed the setback distance and the allowable margin of error. The dealer stressed the importance of a tight tolerance to the prescribed 55mm distance. I thought I'd share the worksheet he sent me as I thought this a great way both understand and calculate this space for any novice such as myself. Item Part # Backfocus remaining Stellarvue flattener SFF6-25 55 55 Male T-threads (M48 spacers can be used) Stellarvue Spacers M48 Spacers 10 45 10mm thick M48 to M48 spacer Stellarvue Spacers M48 Spacers 3 42 3mm thick M48 to M48 spacer 0 42 Starlight Xpress SXUFW-1T 29 13 Telescope side (female T-threads), Camera side (male T-threads) Astronomik NB+LRGB -0.33 13.33 Assuming 1mm thick filters (1/3rd filter thickness) Atik 460EX 13.5 -0.17 Ends in female T-threads

And I was just trying to save the 19 dollars for an additional 1mm spacer when I order the mono ccd tomorrow.

And if I knew how to edit my post on SGL I could rescind the previous post as Olly says it subtracts from the backfocus. My dealer told me it adds also. The QSI report says "Back focus reduced by (glass thickness / 3)." At this point this topic seems clear as mud.

Ok, so it Adds to the Backfocus. Since the Astronomiks are thick it add 1/3mm which is negligible. But the Astrodons @3mm would add a full 1mm for a total of 56mm behind my specific flattener. To further my understanding, and I understand every flattener different, there has to be a small amount of leeway on the 55mm backfocus, right?

I talked with my dealer to prepare the equipment list for the move to mono, the 460ex. As we were determining the backfocus for the spacer requirements, he told me that you need to add for the filters. He said 1/3 of the filter thickness. The Astronomiks list their glass at 1 mm, which is inconsequential. But my dealer said they are 3mm hence an additional 1mm backfocus. This is something I haven't read about before. 1. Has anyone heard of filters adding to backfocus? 2. Is the 3mm for Astronomiks correct?