The focussing tool is brilliant - thanks to John and the team for continuing to improve and add functions and options. However, - I have only had consistent auto focussing success with my Celestron 9.25HD SCT and Celestron focuser if I select sub-frame and manually select the star - which means that I can’t set up regular autofocus updates during the night while I’m asleep.

I find that if I select Full Field I get very inconsistent HFR, and large error bars, with flat or no hyperbolic response on the V-plot.

If I choose sub field and auto select, the single star selected is usually very faint - resulting in the same large error bars and flat or weak V-curve

If I choose sub field and manually select a reasonably bright star then everything works perfectly - it is critical to choose a reasonably bright star - not too bright because it may be clipped, and not too weak because hfr will vary randomly between focus frames

I may have missed a tick box or parameter, but the auto-focuser’s choice of the single SubFrame star seems to always be faint, with low signal/noise. Could the auto focuser be coded to select a brighter star from the field? This would result in successful auto focus outcomes every time.

Thanks for your thoughts.

Hi John,

I'm not very familiar with subframe focusing as I don't use it. But I did just take a look at the code. The Autoselect star functionality basically works on HFR (rather than brightness - although usually the brighter the star the larger the HFR).

If star detection method = SEP...
1. Look at all stars except those within 10% of the border of the sensor.
2. Order the stars by HFR.
2. Go 5% down the list (from largest HFR).

If star detection method is not equal to SEP...
1. Picks the star with the largest HFR.

So you could try a star detection method other than SEP and see if that helps.

Another thing you could try is selecting a star before running Autofocus as there is code that looks like it tries to maintain the same star detection. (I've not tried this so I'm not sure it will work).

So these might help a bit.

If you're interested we could see if I can help optimise your setup for using full field. With your SCT at 2000mm FL the issue I imaging you're having is detecting enough stars for the statistical averaging to work effectively. I did a video on optimising star detection that you might find helpful:

Alternatively if you send me the focus frames and log from a full field autofocus run I can take a look and see if I can improve parameter settings.

HTH

Thanks John - I’ll try your suggestions on the next clear night. In the meantime - building on your suggestion, I checked the SEP parameters in the Focus Options Profile Editor (next to the 1-Focus Default option). I see that the SEP star selection is set be default to cut the brightest 10%, cut the dimmest 20% and cut any stars above 90% saturation. I’m going to try cutting, say, the dimmest 50%, keeping the brightest, and retaining the saturation cutoff. Just a thought - if the code then cuts an additional %, there may be a degree of overkill on whittling down the set of available stars.

I see that the guide module and the align modules retain pretty much all stars - the ‘cutoffs’ are zeroed.

Thanks again for your suggestions and help

The defaults are quite conservative wrt processing power so stars numbers are limited in the default profiles.

Its a good idea to remove the brightest restriction as they are the ones you need! And I agree keep the 90% saturation.