Unsuccesful autofocusing


Hi, autofocusing does not work probably due to the high sky glow in my urban area. Simply does not work, It does a random v-curve detecting false stars with low HFD.
The configuration I use is:
Sbig STF-8300M (Luminance filter used in autofocus)
Sharpsky (ASCOM) focuser
I’ve tried shutter times from one to 45 seconds.
I’ve also tried PinPoint FWHM, nothing different.
False positives are detected on sky frames with any number of stars.
The critical focus step is 17 (188 microns) and I’ve tried focus steps from 10, to 100 with similar results.
Now I’m using the MaximDL autofocus that works flawlessy, but each time I have to switch the camera between the two applications is bored and sometimes the switch causes system hangs and other minor problems. I would like to non load MaximDL togheter with MainSequence.
Any help?
Thanks, best regards, Vincenzo, Rome


Your best bet is to increase the nebula rejection. That would likely help SGP to stop picking up the sky glow.



Thanks Jared, I’ll try on next clear night, I’d like very much to use just one software for the whole session.
I’ll let you know the progress.
Regards, Vincenzo


Seems like you have experimented with most of the settings so I don’t have
much in terms of ideas that might help you. You have effectively
eliminated (in your situation), star detection algorithms from SGPro and
DC-3 dreams (Pinpoint’s star finding algorithm) as effective for you. When
you are using the HFR metric, does the image have boxes over real stars or
just junk like hot pixels?


I know some folks over on SGL (may even be mentioned on Cloudy Nights) have used the Hutech IDAS LP filters in front of LRGB, set-up dependent of course, with success to reduce LP. It is an expensive filter to experiment with though. I’ve not tried this myself so I cannot comment from first-hand experience.

Just another idea . . .



@Ken, yes I’ve tested a lot of configurations hoping in a solution but without significant results. Seems that some parts of sky are detected as tiny stars and the HFR is corrupted in some ways. I’ve also used dark frames, hoping in hot pixels but the result does not change. This night obviously is cloudy… so next time I’ll try raising the nebula detection and I’ll let you know the results. If I remember, the software permits to save the complete focusing session including the frames, right? In that case I’ll use that feature and I’ll send you a kindly gift :smile: , may be you can found the issue with your experience.
@Barry, I prefer to non add another piece of glass in my optical train, also because I’m short in backfocus (expecially with the focal reducer) and I cannot add further thickness.


I assume you are prefocused manually to be close to begin with.

I have to ask, if you are imaging from a light polluted area, are you using narrowband filters for the actual final images?

If so, have you tried focus through those with long exposures?

If not, why bother imaging at all since getting decent shots with Luminance or RGB in such an environment ranges between difficult and hopeless.

If you do insist on doing LRGB anyway, how about getting a wide bandpass Ha filter and focusing with that and using an offset to the LRGB. A bit backwards from the normal way and slow, but might work.


OK Vincenzo. I know it is all set-up dependent and will or may not be possible with your reducer or back focus. For example, I use a WO Flattener 4 and this has 2" push fit which allows 2" filters to be fitted and so wouldn’t add any mechanical difference to the optical train, apart from the additional one third filter thickness to compensate.



yes I prefocus with FocusMax or directly with MaximDl without any problem than I switch to SGP and the focus fails creating a random vcurve.
Apart this problem I use with profit LRGB without the need of other narrowband filter. My wheel accepts just 4 filters (2") and for now I do not have room for narrowband.
I like very much SGP workflow and I’m confident of the solution with nebula detect tuning.
Thanks for all answers.


Another bit, the plate solving works like a charm and also the solve and centering. Just the focusing (and the famous random black frames…) bother me.