I have always known that I had a bit of a back focus issue with my AT80ED. To be honest it never really concerned me that much, but it nagged away a bit although sheer laziness stopped me from tackling it. I always imaged with a filter (cut filter or l-enhance) in front of the field flattener, but recently acquired a filter drawer and an l-ultimate filter and an L3 to help tame blue bloat (it didn't provide a magic cure, although it did help a touch).
Last night I had the first clear night in ages (looks like the rains here are maybe getting to the end) so wanted to do some filter experimentation as well as guiding tuning and I had a few tiny thin spacers and wanted to play with back focus. I also had heard of aggressive emission filters causing problems with polar alignment and EAF functions, so I wanted a real play around night. Yup, the L-Ultimate threw some spanners in the works during polar alignment. So I removed the drawer and placed in an empty drawer (from my EF filter drawer) and I saw that my focus now had been thrown out considerably. I put the l-ultimate back in after PA and started my imaging and guiding experiments and boom! Best star shapes towards the corners that I have ever had.
I would be grateful if my thinking could be confirmed or not:
So my assumptions are - the Optolong IR/UV cut filter at 2mm thick and the l-enhance at 1.85mm thick being placed in front of the flattener never affected back focus as my understanding is that any filter in front of the FF does not affect back focus?
Introducing the l-Ultimate (again, 1.85mm thick) into my filter drawer which is now behind the FF has given me something like 0.61mm of additional "space" to my back focus? Whatever the case the difference (particularly towards the corners) in the star quality was pretty clear.
I guess the experiment tonight is to throw the 2mm thick Optolong IV/UR cut into the drawer and see if that tiny extra thickness is going to do the exact same.