I have found that my AstroDon E G2 Blue filter seems to require a -1 step focus offset compared to the other filters in my LRGB set. I use an Atik EFW2 filter wheel, and the driver supports a focus offset feature. This allows me to configure a focus adjustment per filter in focuser steps. I set it to -1, thinking that would do the trick, however every time I get to my blue filter, the focus is slightly off.
This wouldn't be a huge issue except that I have some very difficult to resolve tilt issues which result in the slightly defocused blue stars to elongate diagonally in a couple of the corners. Looks terrible. I am using a Canon EF 600mm f/4 L II telephoto camera lens, big aperture, very fast, refractor...and all of the EF adapters on the market that fit within the 44mm of backfocus never have enough tension to handle the weight of a filter wheel and CMOS camera. I've tried to shim up the tilt, however it needs to be fairly precise, and as I continue to track throughout the night, the orientation of the FW to the pull of gravity changes as does the tilt.
The only solution has been to maintain dead-on focus, which is actually not all that hard with SGP's AF feature. However, it seems to ignore the ASCOM focus offset I have configured in my EFW2 ASCOM driver settings. Is this just not a supported ASCOM feature? If it is, is this a known bug (I couldn't find anything with a quick search of the forums.)
If this is not a supported ASCOM feature...is there any chance support could be added? I have found it is basically essential for my blue filter to work. I cycle through events, rather than complete each event one at a time, as this evenly distributes atmospheric conditions throughout all filters, both seeing and LP gradients. This helps a lot when combining the data as it avoids any real issues with atmospheric bloat across the channels (and my lens is very well corrected across the spectrum).