Re: Back Focus Troubles #Absolute_Encoders
> have an ATIK 16200 with an EFW3 FW. I ordered the TV NP101is Field Flattener (LCL-1069), which per TV has a recommended Back Focus of 2.75" (69.85mm) for the ATIK 16200 34.6mm diagonal sensor.
>Although, this may be a less of a sin for a wide field scope. I may have to go OAG route, but I would like to see if I can make what I have work first.
Andrew, one last chime in and I’ll leave you to it…
I have that scope, and I think you will love it. Their backfocus distance recommendation is a bit unusual, as they later say for “larger [than 35mm] formats, use 2.67in instead of 2.75in”. I talked to TV and when describing my setup with a ASI6200 (which is 35mm format) he kept saying 68mm (2.67” = 67.8mm). I made the mistake of ordering from precise parts without ability to adjust, but minimum length (with OAG below) and am at 68.7mm, and I think ended up slightly too long based on corner star shape. So I think you are wise to preserve some additional adjustment room. In my discussions with them (David Nagler if I recall, not sure) they emphasized a certain amount of variability unit to unit.
I added a Celestron OAG – which is definitely NOT made for that OTA’s backfocus as it is really thick at 29mm – as I use it with a C11 as well. I added it after using a 60mm guide scope on top of the rings. On the TV double ring set the guide scope was pretty secure there, but I thought I would get better guiding both from a flex standpoint and magnification with an OAG, and I did, improving by maybe 20-30% of RMS (with the usual caveat that there’s so much randomness night to night it is hard to trust such measurements). Plus I can disconnect from the TV101is and move the entire setup to the C11 without losing focus.
My real reason for writing though is the mounting of all this. The TV Ringset is very short, and does not give any room above the dovetail, indeed negative room to the focus knobs. And especially with a filter wheel and OAG this is all very back heavy, and I find the dovetail forward enough that the focuser would hit the saddle. I ended up putting in a pedestal to raise the rings about a half inch or so to allow the focus knobs room if I slid the dovetail forward in the saddle.
For perspective here it is mounted on my MyT (still waiting for the AP1100 but the issue will be similar as they don’t even extend above the dovetail much less saddle without some kind of pedestal).