Auto focus settings for narrow band


Just starting to move to narrow band imaging and initial testing shows that focusing with the NB filters is probably going to be slow. So, I am thinking that I want to always focus using the luminance filter binned and then have SGP make a relative adjustment for each NB filter. I have an accurate focus position for all four of the filters defined in the filter setup dialog box. These were all done at 24.1 C. These are the options chosen in the Auto Focus Options:

I have also selected “Auto adjust focus per filter”

It kind of seems like some of these options might be mutually exclusive but I would like to verify that I have chosen correctly. When I tested this configuration Friday night, I did not get a good focus with the NB filters but there could be other causes for that.



As long as your focus positions are accurate you should be good to go! Those will be used by the “Auto adjust focus per filter” option.

Have you used temperature compensation before? I don’t find it particularly useful (or accurate as the temp sensor isn’t in a great place on most setups).



A few questions/observations:
Do you really need a step size of 2500 steps? That seems outlandishly huge
to me (not saying it’s not legit, I’ve just never seen one that high).
Why do you have smart focus disabled?
Without knowing your equipment, I might suggest reducing the minimum star
size to 5 or 4.
Do you have a focuser that constantly changes focus during temp changes?
If not I would suggest using "auto adjust focus every XXX degree change"
rather than “temperature compensation”.

Other than that it looks good to me. Your current settings should work
well, but these are some other things to try.


I use an Optec Gemini Focusing-Rotator that has 0.11 micron focus steps. Using 2500 steps causes the V curve to span about HFR 5.2 to HFR 1.5. I have smart focus disabled since I use a 12" Meade f/8 LX850 OTA and smart focus has never seemed to provide any benefit. The focuser appears to have an accurate temperature readout. Sometimes I disable SGP’s temperatrue compensation and just let the Optec do TC continuously in the background. The Meade OTA has a significant TC requirement.

I will repeat the auto focus routine for each filter and compare the results to the previous ones just to verify I’m in the ball park. I will also monitor the auto focus runs more closely to verify that the move to the filter position is performed after the auto focus using the Lum filter. I assume that will show up in the SGP logs.

Thanks for the info.



This is interesting. I just did an auto focus with the Lum and then immediately did one with the Ha filter and got almost identical focus positions. The first screen shot is Lum and second is Ha. This is what I expected before since the Astrodon filters are described as parfocal. So, some additional tests are in order. I’m only using Ha tonight, so will start with these values.



Just to follow-up – did an imaging run from 10PM until 4AM on the Elephant Trunk Nebula (IC1396) in Ha. 20 subs, 2x2 binning, 15 min exposures. Imaging at 2500mm produces a serious close up of a target more commonly imaged at short focal lengths. This 6 hour run was fully unattended and included an automatic meridian flip with camera rotation as well as auto focusing on every sub. This was my first attempt at Ha imaging and processing in PixInsight. The results seems decent:

Elephant Trunk Nebula at Astrobin



Very nice Charlie. Given the original topic of this thread, are you focusing using LUM and offsets? If so I’d say it is working well for you, at least with the Ha filter.