Astrophotography autofocus routines - Forums

Topic starter

Nov 23, 2021

I'm just about to set up in-sequence autofocussing for the first time (I'm using APT). I just wondered what strategies people use?

Autofocus on temperature change
Autofocus on filter change
Autofocus on a set time

or maybe a combination of all the above.

I have no experience in this space, so just looking for some initial guidance.

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Sean van Drogen

Nov 23, 2021

autofocus on temp change every 1 degree
also on HFR change 5% with 10 samples.
Don’t use autofocus on filter change as I use filter offsets, which I highly recommend as it saves a lot of time

Bogdan Borz

Nov 23, 2021

Well of course you need to focus on filter change (they are rarely perfectly parfocal, even high-end ones). Or use filter offsets if you can reliably obtain the step difference between filters. I usually use the temp and time criteria but the time criteria alone is sufficient once you get used to your system. My F4 newtonian has a low tolerance and 45min intervals are necessary for this fast system. You can start by using 1h for ex and see if the temp criterion brings any improvement. Temperature does not vary linearly during the night, the variation is pretty important in the early hours. Then it stabilises. Give it a try and get used to your system. It does not hurt to enable both time and degree criteria.

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AstroDivers

Nov 23, 2021

Have you tried already the autofocus feature without those options?
I say this because in NINA i can't make it work, in the sense that the position guessed as 'in focus'… in reality it's not.
I have tried all the videos online but no good results… for the moment I've given up on this

Seung-Jun Kim

Nov 23, 2021 · Edited: Nov 23, 2021

I just set autofocus on filter change.

Then, I make a sequence that changes filter every one hour or 30 min depends on my scope.

Alan Brunelle

Nov 24, 2021 · Edited: Nov 24, 2021

I have an f2.2 system and go with time, 1hr, and hfr delta of 10%, but can only manually change filters, but would strive to depend on offset, if I did. My location does not have big temp deltas.

I worry about choosing small hfr deltas. So work that out with your system. The danger is normal seeing issues could cause constant failures and redos, killing you session. Also, can 15% really be noticed? If you can see small differences when pixel peeping, ask yourself if it really matters at normal image viewing mags. Going for perfect can become very frustrating. Wise old adage: The enemy of good is not bad, it's perfect.

I should state that I use NINA and have had no problem. I also only do each focus step 3x. You could spend a lot of time if you go excessive. But you have to decide whether you want perfect focus for each sub, or whether your setup, plus desirable image scale requires such perfection. Again, you need to decide based on your system, needs, and wants.

Keep a careful eye on the routine's performance the first few times to catch failures/redundancies before trusting an untended night.

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Michael Feigenbaum

Nov 24, 2021 · Edited: Nov 24, 2021

Lots of great advice here… I use NINA and the autofocus module really works well. I do this routine and it seems to work well for me:

1) every 1°C temp change
2) with every filter change as I don't want to depend on the offsets and the routine only takes a couple of minutes to run

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Blue

Nov 24, 2021

I use Voyager and my focus routine is every hour, every 2 degrees temperature change (I have two scopes, neither are hugely impacted by temperature change) and on filter changes (For the mono cam, I have both OSC and mono) plus after a meridian flip.

I don't like HFR as a trigger as I find it to be too impacted by seeing. I let myself go down the rabbit hole recently as at the start of a sequence I saw a nice sub, then progressively increasing HFR to the point that the fourth sub (5 minute subs) looked obviously soft so I injected an early focus run assuming focus was shifting. That landed within two steps of the previous run and the next sub was still soft then over a couple more subs the HFR went back down to where I expected it to be.

I didn't really waste any time as the seeing had softened the subs to the point that I discarded them and I would have discarded them in the morning anyway so the extra focus run did not cost me any good data, but it did convince me that HFR variation is not a reliable trigger.

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CCDMike

Nov 24, 2021

I use both, APT and NINA and find it helpfull to check the HFR-change as well as the time. Like @Bogdan Borz already said, there can be steep temp drops in the evening (at least here in central europe) and the atofucus has much to do.
I highly recommend to chekc the routine first (step-size, settle-time etc.) thats will avoid any failures during the sessions.

CS
Mike

Christian Großmann

Nov 24, 2021

I use both, APT and NINA and find it helpfull to check the HFR-change as well as the time. Like @Bogdan Borz already said, there can be steep temp drops in the evening (at least here in central europe) and the atofucus has much to do.
I highly recommend to chekc the routine first (step-size, settle-time etc.) thats will avoid any failures during the sessions.

CS
Mike

I also use Nina and usually refocus after filter change, meridian flip and temperature change. Like Mike, I always use the HFR change but for safety reasons. I couldn't spend a huge amount of money on my scopes (yet)

. So the hardware is not super ridgid. I had times, when the focuser tilts a bit or the camera slightly shifts. I do not mean huge amounts. I just mean those small movements you can get, if your system is not perfect and there are some tolerances. In those cases HFR may increase and the autofocus routine prevents the setup from generating a lot of useless data. It is not a problem solver, but helps to get more useful subs.

CS

Christian

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