Does auto focus with an Ha filter require different focusing steps and/or a much different offset than without the filter? I tried out my L-Extreme Pro Ha/OIII filter for the first time last night, and wasn’t able get things focused with the auto focus routine. Instead of the usual V-curve, the curve started out high, dropped like it was going to be a V-curve, but then stayed flat as additional points were added. Works great with no filter, so it’s almost certainly my fault in the auto filter setup.
I didn’t have much time to play around, but my guess is that although the frame and focus exposure times seemed fine, the Ha/OIII filter needed a different focus starting point or different step sizes than the no-filter case. Anyone have any guidance for doing this? I’ll do some experimenting tonight (hopefully), but it would be good to have an initial starting point in order to save some time.
Yes, filters cause refraction which causes the focus point to move - you should first manually focus your L-Extreme and then run SGPs auto focus to see how it performs.
Once you have determined the difference in focus points between filter vs no filter (or between your filters, if they are not parfocal), you can store those offsets in SGP and have it automatically adjust as you move between filters / no filter.
Excellent - thanks very much for the info! Assuming the clouds clear, I’ll run some tests and get things set up tonight. Thanks again!
To have an corect V curve, you need to increase focus frame exposure, because OIII/SII filter are dark.
You need to have enough star to have an ok V curve.
Also you need to have 50 to 80 steps, or around.
Test and let as to know.
For NB filters I have to use about 30 second exposures to get autofocus to work at all. Even 2x2 binned subs require quite lengthy exposures to produce enough stars for the autofocus routines to work. It’s not a deficiency in SGP, it’s just that the filters are doing what they’re supposed to do and rejecting a lot of light. I have also found that I need to slew to a rich starfield and focus there first if I’ve changed my imaging train. Otherwise I might not have enough stars in the FOV to get close enough to focus that the autofocus routine will work in other parts of the sky. Of course none of those challenges exist with wideband filters.
Aren’t LED streetlights fun?
Thanks for the tips. I use 1 second exposures and 4x4 binning for non-filter autofocus, and that works well. I think my problem with the L-Extreme was not only underexposed frames, but also the focusing shift due to the filter being so far off that it couldn’t do much. I’ll try manually focusing using 30 second exposures and 2x2 binning to get “close”, then figure out the correct step size, and go from there. Of course it’s been cloudy for the past few days and is forecast to be that way for the next several…
I have a mono camera with NB filters 6nm and I need around 1 minute exposure to perform focus with these filters For LRGB filters, 8 sec are sufficient…
The skies finally cleared last night and I was able to do some experimenting with the L-Extreme filter. As suggested above, both a longer exposure and offsetting the focus starting point were needed, but after doing so I was able to get good focus.
With a Pegasus focuser on a W-O ZS-61 scope, the settings required were an exposure length of 10 seconds with 4x4 binning (no filter is 1 second with 4x4 binning). The best focus was achieved by moving the focuser -330 steps, which moved the draw tube inwards by about 0.6 mm. Autofocus with the L-Extreme filter works great now, so I’m good to go.
Thanks again for all the suggestions.
You could also try binning the NB exposures. Contrary to popular belief, it is not necessary to focus with 1x1 binning, especially with modern small-pixel CMOS sensors. Doing that will bring the exposure time down. On an f/7 scope, I have 15 second NB exposures for focusing.