Dither when focusing every frame?

Is there a way to dither if you use robofocus every frame?
I need to refocus every 30 minutes and I am taking 30 minute subs. I am using PhD guider and have set Voyager and it to do dithering.
At the end of the 30 minute exposure, I do a robofocus and then take the next sub. No dither takes place. My mount is very accurate such that in most cases the pointing is good after this to 1 arc second. So effectively no dithering occurs.
Any suggestions?

Probably you talk about RoboFire and focus on a single star using RoboStar.

Pointing a star for focus means to come back to target with a precise pointing. So what you ask its not possible for how you want the focus.

Solution can be to not focus every 30 minutes but on temperature change and with a more larger interval for example 60minutes. Or use LocalField autofocus.

All the best

Thank you for that information. So the dither will work if I do a local field focus? Is this just as accurate?
Thank you.

I checked Voyager code, if a focus pending dithering will be not done also in LocalField mode because guide will be stopped and reacquired after focus, dithering will not have sense.

All the best

Oh. Then I will use 15 or 20 minute subs and refocus after two of these.
Dithering makes a big difference to quality with CMOS cameras like the asi6200 when the signals are weak.
Thank you.

Use the high gain of CMOS to avoid this long exposure or change the focus in 60minutes and each 1.5°C of change in temperature.

Just curious, what mount do you have and what imaging scale are you working with? It’s VERY impressive that your mount bring the target dead on within 1 pixel with every GoTo :+1:


I use an fsq106 with a asi290 guide camera binned at 1.
The mount is an astro-physics AP1100 without encoders but with a pointing model .
So each precise pointing gets usually down to 1 arc second error. Worse case is 3 arc seconds. But usually 1.

I have little experience in how often the focus should be done. I am sure you have much more experience in this. I did a few calculations: I am using the Moonlite Nitecrawler with an FSQ 106. Over 30 minutes, the focuser position changed -50 microns last night (according to specs). Does this seem a reasonable value? Any idea what would be the range that is acceptable?
Many thanks

The FSQ is F5?

If so with the ASI6200’s 3.76um pixels and assuming the previous focus run nailed the dead center of the critical focus zone (Which would be about 60um wide at F5) it would be just getting outside the critical focus zone. You would probably be lucky to see a difference before and after focus from 20um or so outside the CFZ. Sky conditions would probably make more impact.

I let myself get sucked into injecting a new focus run to a sequence recently as a sub looked visibly out of focus. Firstly, the next sub (After which the injected focus run occurred) was much sharper and second, the focus run landed within a couple of focuser steps of the previous one anyway. So sky conditions can make a bigger difference than a small focus error.

Yes. FSQ105 f/5.
So it looks good to do a refocus every 1.2 degrees or so. Using 20 minute subs will most likely allow some dithering too. Good info!
Thanks everyone.

I find the FSQ106 pretty sensitive to temperature changes so have Voyager set to refocus at 1c delta. Strangely I also run an FSQ85 and that is much less sensitive.

I am writing here because I have read in another forum about this thread things that as regards the philosophy of Voyager have no application. No guide is activated during the autofocus with both OAG and guide scope, Voyager stops the guide just for not having it. And that’s something that will never be in Voyager because it is a source of problems. If you autofocus every sub, the dithering doesn’t make much sense, the selection of the guide star and the pointing always matter a random shift Opposite one wonders if having the focus every sub is a way of shooting that is not effective.

For the user who asked, I usually refrain from giving astrophotography advice but in this case I make an exception to the rule. I have been using the FSQ in astrophotography for 15 years, I have had 3 of which one still today. I have never done 30 minutes poses and I have had all the possible CCD and CMOS cameras directly and indirectly. I never went beyond 15 minutes. I invite you to take into consideration the fact of going down on the times of the single pose. As for the temperature, the material the FSQ is made of foresees an autofocus at least every 1.5 ° in my experience. Going down, also seen the reading fluctuations takes time especially if the poses are long. Then everyone is free to do as he thinks of course.

Everything that is done in Voyager is extremely thoughtful even if it doesn’t necessarily have to be shared by everyone (I don’t expect everyone to be in tune). So some things aren’t there by choice.

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As a follow up, the advice here has been very good. Last night I had 4 hours of 20 minute shots where the sequence used focusing every 1 degree C change in temperature. The temperature did not change very rapidly last night so many subs had the dither which I wanted.
Thanks for making things clear for me.

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