Thank you Scott Mitchell for your reply. I did have a chance tonight so setup my Tak FS-78 and my ZWO-ASI178MM camera and took my LRGB filters and my artificial star and about 50 feet away focused with the L and all with the Bahtinov Mask and used the ASI studio software and zoomed in of the 3 spikes and got the center spike dead center and then used the R, G and those were dead center but when I went to the B filter is was not longer centered and of course I can fix that but this would tell me that using the same scope in the same temp and same set focus that the B filter was off, a tab bit high like in between the center and the top spike.
FYI just for fun I also tested the L-eNhance & L-eXtreme and all 5 filters were dead centered spike. I tested if I could use the same filter by inserting it in the 2" ext tube and I can repeat with the same center Bahtinov Mask spike, so I think this is a valid test right?
I think I understand Der_Pit that "other factors affects the focus position" but if I use the same OTA and temp and repeating the filter focus position and only the Blue filter is not centered with the Bahtinov Mask spike then in the real world I should have the same slightly out of focus imaging with my LRGB & Optolong filters?
In a perfect world I would use a filter wheel so no parts being removed but I think this is a valid test? Also I can just have a different focus mark of value for the Blue filter, but I am sure that high end LRGB filters would all be perfect Parfocal as that what you are paying for.
I also would expect that focused at infinity a real star with be the best test but this should prove my thinking right? Appreciate any feedback or thinking?
I did use ASi Studio ZWO camera software and was viewing the Bahtinov Mask spike at 300-400% magnified, to see the spikes better.
So my test eq is
1. Tak FS-78
3. 6 2" filters LRGB & Optolong L-eNhance & L-eXtreme
4. Inside with a Bahtinov Mask on a tripod and artificial star.
5. ZWO EAF focuser motor.
Edited by cengell, 24 January 2021 - 01:37 AM.