Exposure length for DSLR focusing with NB filters

Just after some pointers from anyone using a similar set up.
I use a modded (and cooled) dslr and want to use NB filters with autofocus and wondered what a good starting point would be for exposure length. I have initially set 25seconds at max ISO (6400 in my case) but wondered if this would be sufficient given the reduced sensitivity of the CFA.
Trial and error would no doubt sort this out but wondered if anyone had tried this and could confirm a suitable base point.

The other option I suppose is to focus unfiltered and then use a Baht mask to focus with the NB filter to work out the offsets and use the Auto Adjust Focus per Filter?? but im not sure how that works…need to read the manual again I think


I can’t really say that we support auto focus with a narrowband filter when used with a Bayer matrix. I honestly think you will just be frustrated with this experience. Restricting a star to only 25% of its normal data will introduce unwanted side effects into the focus routines I think.


I think you’re on the right path by using focus offsets once you get it set… I wouldn’t use auto-focus at all. There was a conversation before the ‘crash’ that covered this in detail… it’s just too noisy.

Ok so if I have this correct, the best approach is to do an initial autofocus run unfiltered (or do it manually with a Baht mask & Baht Grabber) to get my unfiltered focus point.
Then do the same (manual focus) for the 2 NB filters so as to get the offsets, preferably on a night with a steady temperature.
Then type these into the filter profiles of each filter and save with a suitable profile as a record of the steady state offsets.
I then need to tick the “Auto adjust focus per filter” box.

Then when I go imaging just do an unfiltered autofocus run to get that nights FP and then under the focus point settings select the “set focus for no filter” option and SGP will then calculate relative offsets based on the saved profile offsets?
and SGP will the simply move the focuser to each filters focus point eliminating the need to autofocus on a filter change yes??

so lets say on a night of steady temp I have my unfiltered FP as 5000, Ha at 5020 and Oii at 5050 giving offsets of 20 and 50.
Then on another night my AF run gives me an unfiltered FP of 4950 SGP will automatically move the FP to 4970 when I choose the Ha filter and 5000 for the Oiii…
Just want to make sure I have undersood this correctly so apologies for the numpty questions.

I did some Ha imaging with my DSLR and SGP. Autofocus just won’t work. I set my exposuer to 100 sec (max allowable) and iso6400 and it still wouldn’t detect stars, just hot pixels. What I did was figure out what my temperature compensation is while color imaging. I then do the initial focus for the Ha filter using a mask, 20-30 sec exposure will usually suffice and run temp compensation from there.


This sounds fairly accurate… just validating your findings here since this seems to be an increasingly popular question and might come in handy for folks searching for info on the topic.

Hmm, My DSLR is cooled so not that noisy but sounds like its more trouble than its worth. In that case ill just AF unfiltered and leave it at that.
If im a tad out of focus on the NB filters, I can live with it.
May try the offset route though just to see if it works.
Thanks for the heads up as this will save me a lot of grief and frustration :smile:

Forgive me, the Auto Focus discussed here is not DSLR autofocus with (IE) an OEM lens?

Milli, no it’s not. He’s running auto-focus via a telescope focuser of some type.

That’d be neat… There are some packages working towards that.

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We can focus a Canon Lens with SGP through the camera (this is an unreleased feature), but we haven’t taken the time to figure out how to run auto focus with it. I’m sure it’s technically possible at narrower fields. I’m not sure how things would work at very wide fields. It’s on the (ever growing) todo list.



At this point, even being able to tweak the focus manually through the SGP interface every couple frames would be nice. And so if this message could be counted as “anybody interested” I’m waving both hands :wink:

Your best bet for focusing a DSLR through narrowband filters is likely going to be manual focus.

Unfortunately DSLRs (and OSCs) are at a pretty big disadvantage where narrowband is concerned. Most Bayer patterns are only 1/4 red in the first place…so there goes 75% of your sensitivity. Add to that the IR cut filter in front of your sensor and you lose even more…even if you have a modified camera it still rejects some of the hydrogen Alpha band.

All that to say it makes getting useful focus data in a short period of time fairly difficult and nearly impossible. That doesn’t mean you can’t pickup hydrogen alpha…it just means it will take you longer to do it.

Really using a bahtinov mask and a (very) bright star is the best way to go here. Unfortunately it’s not automated but hopefully this isn’t something you’re doing for the majority of your imaging time. You can set focus targets in SGP so that getting to that bright star and getting back on target is faster. You can find these options in the telescope area of the CP.

To be completely honest I don’t focus through my narrowband filters either. I setup focus offsets and focus with my lum filter. However most DSLRs and OSCs don’t have filter wheels so this isn’t really an option for most.


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Thanks Jared
I’ve had (and seen) surprisingly good results from DSLR’s using NB filters albeit with longer exposures
I think i’ll use a bright star and mask method to work out my initial offsets and just use the offset method as I have a drop in filter box which makes changing filters pretty simple.