H-Alpha Auto Focus SGP

[04Stefan07]

First time using H-Alpha and I was having heaps of issues getting auto focus working with it. It works perfectly with my LRGB filters but with the H-alpha my graph was going nuts in SGP when auto focusing.

Has anyone had any issues like this before? Surely it can't be because of the full Moon, I thought NB is meant to block out most Moon light? I had the expsure set on 12 seconds compared to the 6 seconds of my LRGB for the Auto Focus Point I think its called.

[Adreneline]

I know there are plenty who will say otherwise but I gave up with NB autofocus and SGP - I wasted too many perfectly clear night.

I decided to devote one night to establishing the best focus positions for each filter with a BM, noted down the settings and fed them into SGP and/or offsets with respect to Lum. Sorted and time well spent. Temperature variations here are not that great that it's caused me any issues and I've not lost out on valuable imaging time.

That's probably not the answer you were hoping for, sorry  If you think Ha is a trial wait until you try to get OIII to work. Life is too short.

 from Melbourne, England

 

Edited October 1, 2020 by Adreneline

[Skipper Billy]

I haven't had any issues with narrowband compared to broadband in SGP autofocus but I do use significantly longer exposures. From memory I use 10 seconds for LRGB and 20 - 30 seconds for narrowband. Might be worth a try??

[blinky]

Yeah, increase the exposure and it should be fine, I use it for narrowband with no issues

[04Stefan07]

Thanks for the reply. I was at 12 seconds so I will try to increase it to 20, if no luck 25 then 30 if needed.

[symmetal]

If you're not doing so, it's worth binning 2x2 for auto-focus in SGP. With binning, I use 2s exposures for L, 5 seconds for RGB and 25 seconds for NB. Works every time. 😀 You also get faster downloads.

Alan

[ampleamp]

I had the same issues you describe. As has been stated, lengthening the exposure time, and binning 2x2, works well and I am up to 28 seconds now which usually works but I often don't get a nice V, more of a flattened U with literally anything going on at either end of the scale. (I get a perfect V with LRGB). 
I have noticed that low objects and/or sky darkness can still make focus a bit flaky but this improves with height and time. +1 for Adreneline's suggestion of working out focus offsets as this will save loads of time if you can focus with L (2-5 seconds versus 25-30) and let SGP adjust the focus point for each filter. So something I should do!

[kirkster501]

I also had this issue across three different rigs, 2x2 binning, twelve second exposures with object directly overhead and gave up and rely on the fact that my filters are claimed to be parfocal.  So I focus with lum and switch back to Ha/OIII.  Or I could work out the offset, focus with lum and apply the offset.  I CBA to work out the offset since it is precious imaging time faffing around, we get so little as it is!

28 seconds is a long time for an AF focus sub when temperatures are dropping.  Nine x 28s (actually, ten including the confirmation one at the end)  and we are looking at six minutes to do an AF run.  That's getting silly.

Don't know about anyone else, I can never "trust" AF and when it runs there's always some trepidation that I have to get involved.  It can work 99 times out of a 100. Then for almost no reason at all, it just starts playing havoc with gull wings and auto-abandoning the sequence "something terrible has happened"

As an aside, is there any way you can say that an AF score of 78% (say) is acceptable and to NOT run another AF session????  I do not see how to do that? Often I find the focus point to be perfectly acceptable and yet SGP wants to rerun.

 

[sloz1664]

50 minutes ago, kirkster501 said:

I also had this issue across three different rigs, 2x2 binning, twelve second exposures with object directly overhead and gave up and rely on the fact that my filters are claimed to be parfocal.  So I focus with lum and switch back to Ha/OIII.  Or I could work out the offset, focus with lum and apply the offset.  I CBA to work out the offset since it is precious imaging time faffing around, we get so little as it is!

28 seconds is a long time for an AF focus sub when temperatures are dropping.  Nine x 28s (actually, ten including the confirmation one at the end)  and we are looking at six minutes to do an AF run.  That's getting silly.

Don't know about anyone else, I can never "trust" AF and when it runs there's always some trepidation that I have to get involved.  It can work 99 times out of a 100. Then for almost no reason at all, it just starts playing havoc with gull wings and auto-abandoning the sequence "something terrible has happened"

As an aside, is there any way you can say that an AF score of 78% (say) is acceptable and to NOT run another AF session????  I do not see how to do that? Often I find the focus point to be perfectly acceptable and yet SGP wants to rerun.

 

Hi Steve

Can you bin at 4x4 as this will reduce the autofocusing operation by over 50% 

Steve

[04Stefan07]

OK so I did try the above suggestions but every now and then the focus will go out. I got HFR down to around 2.2 to 2.6 which seemed alright.

I did a refocus using my Red filter and bang worked perfectly. These H-Alphs are bloody hard to get right. And also at 25 seconds it takes so long to focus it.

Edited October 28, 2020 by 04Stefan07

[Adam1234]

I had a similar autofocus issue with Ha (using APT autofocus, I don't use SGP). Seemed to work more or less fine with the RGB filters and checking with Bahtinov mask, focus seemed ok and lovely small stars. 

When I tried with Ha the FWHM values went straight past the lowest value and settled on somewhere 5 - 6, checking with BM showed focus was way off and I had donut shaped stars. 

I don't know what settings are available in SGP but APT recommend setting the correct threshold, coarse and fine focus settings to get the best results, so going to have a play around with these on the next clear night 

[04Stefan07]

Gave it another crack and got it as best I can. Makes it hard since the exposures are a lot longer.