Autofocus with a central obstruction

Hi - I have read a few post recently via links from the digest which suggest that autofocus doesn’t work effectively as yet for scopes with a central obstruction. Is that a fair assessment of the situation? I have been using it with my Celestron C11, but i use it on the basis of I have to look at the stats it produces and decide best focus myself. It is still v useful, but not automated to the extent I can leave it to get on with things.

Thanks as always for great software from the SGP team.

best wishes

From what I have read, this is not an isolated issue for SGP, the other focusing algorithms suffer the same issues with donut stars. I am doing research at the moment for the next book and one of the topics is RCT collimation.

I invested in a GoldFocus collimation/focuser mask for determining collimation and at the same time, it can detect focus errors (in my case) of about 10 microns.
For this scope, I get SGP to prompt for its use. With a few minutes of calibration, I can take a stack of 5 5-second subs and it tells me how many focuser steps to move in or out. It’s getting me to within a few clicks (about 1/10th of the DOF) and I’m imaging again in less time than a 9-point AF routine. It does mean however that I have to stick around. In the fullness of time, some automation and a flip flat may solve this.

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hello ,
I have a fast scope with obstruction : focusing with lum filter is always very efficient once he steps
and parameters are properly set in sgp.
For the other filters , i find it not so good.
so today i’m using the calculated offset for the filters other than lum filter.
the focus is done each time with lum filter.

For info, I have found the GoldFocus mask (or a Bahtinov grabber) is a very efficient way of calculating these offsets.The diffraction spike mis-alignment pixel readout is linear with focus error, so with a few stacked readings and with some calibration, the precise offset is about 2-minutes work.

yep, in my experience with mesuring the offsets, oneneeds is to have a steady night with good seeing.
autofocus with rgbsho filters will be ok. veeeery slow, but ok, then the offets can be stored.

The focus masks are quite tolerant of seeing conditions. The GoldFocus software will automatically stack up to 15 subs and, as last night, even with less-than-perfect conditions, the repeatability was better than 10 microns (about 3 focuser steps) on my 10" f/8 RCT with a stack of 5, 4-second exposures. I normally use Polaris or a similarly bright star.

will try it, i think i will 3d print my bathinov mask

thanks for the info, I never thought of calculating offsets and that these would be constants. It is obvious when you think about it.

This will help the reliability of my capture runs considerably.