This posting discusses how I used the AF routine to tweak my focuser’s temperature coefficient.
As I watched my auto focus routine run, I would notice that often times the final V curve would not be symmetrical. I did not give it much thought however, since my subs are always well focused. But it started bugging me – why are they not symmetrical? Then it became obvious – the V curve will only be symmetrical when the camera is already in perfect focus when the AF routine starts.
My focuser has very good built-in temperature compensation but its coefficient must be carefully determined. These asymmetrical V curves showed my focuser was not keeping up with the temperature shift between AF runs – close but no cigar.
Here is a typically AF run I was seeing:
So I decided to change my TC coefficient by 25% and see the effect:
These were NB subs, so there was 30 minutes between AF runs. Clearly I had adjusted it too much and in the wrong direction as the asymmetry was worse and in the same direction. So I moved it back to its original value and then moved it 10% in the other direction:
A tad too much but pretty close!
A final small tweak and good enough!
By ensuring good TC on your focuser, you can have confidence that 30 minute narrow band subs will hold their focus throughout.
Charlie Miller
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Hi Charlie,
thank you for sharing your observations.
My focuser has very good built-in temperature compensation but its coefficient must be carefully determined.
You are using a focuser with built-in TC and let it run during exposures. This requires that neither backlash nor lateral shift will occur during focuser movements. These are high requirements that are not fulfilled by the most. Apparently your focuser does.
May I ask, which brand and model of focuser are you using?
How is backlash compensation accomplished with your focuser? Obviously it must be different from the backlash compensation implemented in SGP, or do you rely on steadily falling temperatures during each exposure?
Bernd
@bulrichl
Yes, my focuser’s TC runs continuously making small focusing steps during the exposure. I do not use TC or backlash inside of SGP.
I am using the Optec Gemini Focusing-Rotator, which has 0.11 micron focusing steps. It only has 12mm of focus travel, which results in excellent stiffness and repeatability. It also has built-in backlash adjustment.
The TC and backlash is configured in the Gemini ASCOM driver and it works independently of SGP. When it is time for SGP to start an AF run, SGP disables TC in the focuser. After the AF run completes, SGP re-enables TC in the focuser.
My goal was to be able to do AF only on filter change or after an hour of exposures. I am able to do that regularly.
Prior to purchasing the Gemini, I used Optec’s TCF-Si focuser, which works the same way and is very robust in its own right.
Charlie
Thank you again for these informations. Wow, that really seems to be a fine focusing solution, congratulations! I can imagine that this is the right equipment for long exposed narrow band subs.
Certainly downside is price and weight. So (sadly) I will have to stick to my rack & pinion focuser and step motor, hoping that the revised TC inside SGP (v3.0.1.0) will work correctly now. Unfortunately, until now I had no chance to test this new version due to fog, rain and storm that do not seem to come to an end here…
Bernd
