Hi all,
Quick question: has anyone used ~7nm dual-band filters (Ha/OIII) on fast systems (f/3.8)?
I’m just checking if that could work with Askar 50P.
Does bandpass shift become a problem?
Is 7nm still usable, or should I look at high-speed filters?
Thanks! Clear Skies!
I’ve been using 6nm Astronomik MaxFR filters on my F/3.3 Epsilon 130 for years without any issues. They are rated down to F/2.2 with 90% transmission even at f/2.
I’m using 3.5nm Touptek (not fast scope optimised) on my F/2.8 Newtoninan and the results have been quite acceptable… I’m sure the transmission rate is like, 45%… Not optimal at all, but I’ve certainly not been ‘upset’ by the result… Problem is that I can’t really be arsed changing from my 3.5nm filters when using my F/4.6 scope, or my f/8 scope, then changing back to fast filters when I use my F/2.8 scope, so its a loss of transmission that I’m willing to accept…
Whilst I know that bandpass shift is absolutley, 100% a real, phyiscal occurrance, I am not so sure its actual effect is bad enough to warrant the amount of discussion it gets.
(note: if I only had the f/2.8 scope, I would definitely be buying fast focal ratio optimised filters)
I use Antlia Edge 4.5 nm on my f2.7 scope and they are in the 90% transmission range after the shift and no halos. Work great!
Thanks everyone for sharing your experiences — really helpful! I tried the bandpass shift calculator that Tony Gondola recommended, and it seems a 7nm filter should still perform well at f/3.8, with about 87–90% transmission (I checked SVBONY dual-band filters).
Thanks! Clear skies!
F/3.8 is moderately fast, but not in high-speed filter territory yet. You can use 4nm no problem and some 3nm filters. There may be slightly lower transmission but they give better contrast than wider bandpass filters.
