(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?)
Yes, I think your experiment is valid. That only the blue filter deviates is a consequence of the 'other factors' I mentioned. If you look at a typical focal curve of a triplet lens, e.g., here, then you notice that it runs off really fast at the blue end close to 400nm. If your blue filter has a lower cutoff limit at (or even below) 400nm such a shift is normal. Even for excellent APOs like your TAK. Yes, you can directly convert that to 'real world' and conclude that you should slightly refocus when taking blue images. Another option would be to get a blue filter with a higher cutoff, like Astronomik, Antlia, CHROMA, and a matching L.
Thanks that make perfect sense, what's funny NOT, is I asked the company if their LRGB filters were Par Focal and they said yes. I know these are the lowest cost LEGB at $250 for the set but I can't see paying $500 or $1000 for a set of LRGB but maybe what you are saying is the reason those cost 2x to 4x more?
I attached is both spectral curve and it shows that the B starts at 375 nm but at 90% starts at 400 to 500, which I think is very close to those "Astronomik, Antlia, CHROMA" filters? I attached the Antlia LRGB that cast $350 and the curve is more straight up and down, so that's what you are saying and that's why the focus is not the same point as the LRG filters?
(I'm quite sure that the filter is fine, and even highest quality filters would show that slight offset (unless they cut off at 420nm+). Most acquisition software can take care of adjusting the focus automatically on filter change (if you have a motorized focus, that is wink.gif)
Which software are you talking about that would take care of the adjusting the focus on a filter change, you mean Auto Focus like NINA or SGP?
If you set up such a filter offset, yes, you'll have to do the measurement on real stars, focused at infinity. Also make sure to look at backlash in the focus drive.
Yes I understand that and that makes perfect sense as real world conditions, now the backlash is normally not an issues with softwares like NINA when you input an offset? Well that's from Cuiv with NINA as he wrote that code.
Thank you for your help and I hope this also helps others!