Offsets For Astrodon and Auto Focus


I have read up what I could find about this subject but have a question I cannot seem to answer

It’s my understanding that when you go into your “Filter Setup” screen that the focal point box is actually the “Offset” box if you have the “Auto Adjust Focus Per Filter” option used.

It is also my understanding that auto focusing through the NB filter is not the best method and that we should use our LUM filter first and then apply the offset for the chosen filter in order to get it into focus.

So with that being said. How are you determining what your offset is?

I’m see that people are running an auto focus stream like L,L,HA,HA….L,L,OIII,OIII…ETC

Since the software has a hard time focusing through a narrowband filter would it be best to let it try to get it as close as it can and then manually dial it in until it is sharp and use the resulting number as the offset?

I’m guessing that you want to do this on the clearest night possible to get the best focus you can in order to compete with the changing seeing conditions outside.

Lastly can’t the changes in seeing conditions have an effect on the focus and thus messing up the offsets?


I have two different rigs… On one I use offsets and they work a treat… Using Astrodon 3nm filters. On the other rig I couldn’t get my offsets right and so I now focus each narrowband filter. While it takes longer, I don’t for one minute think that the focus is worse… So I would say that its OK to not use offsets… You won’t get better focus using either method.


I appreciate the feedback. Normally I do the same thing. The rub is when I go off to bed and need it to refocus late late at night. I’ve noticed that when the target gets way up in the sky that late that the focus needs to be adjusted. Its the best time to get data too.

Normally I sit out with it and do it myself but it would be nice to have the option to go off to bed and let it do it’s thing.

Thanks Again, Sean


It is an old thread but I would like to share my observations. I had purchased all Astrodon filters together, directly from Don. For a fast focus determination, I used FocusMax and ran the series like LLLRRRLLLGGGLLL… and so on, until LLLSIISIISIILLL at the same night. L remained fairly constant. The LRGB set itself is almost parfocal, with B requiring to focus inwards by 8 steps (=18.4 µm) for my FSQ-85 at native f/5.3. The Ha (5nm), OIII (3nm), SII (3nm) set is almost parfocal, too, but the gap between the LRGB and the narroband sets is quite large, namely 30 steps (69 µm). I guess that this will facilitate using offsets in SGP.