Bullet-proof auto focus with platesolving

Would love to see SGP autofocus use platesolving to select known stars. This is how the Planewave folks do it. It works all the time for any target. No need for initial calibration.

Not sure what you mean by initial calibration in this post. SGP auto focus, at least the version I am using (I have not gone to the newer 3.1 versions because of QSI driver issues) does not require any calibration.

SGP Auto Focus requires you to select a minimum star size to help select “stars”. But you can still pick up very small objects like globular clusters or galaxies. This skews the calculated HFR and makes for a less than perfect focus. Platesolving resolves this my guaranteeing you always use stars. When I compare SGP focus results to those done by Planewave, the Planewave results are far more precise and repeatable.

There is a focus target feature in SGP but it is often overlooked

Even if a small galaxy or globular is included, the HFR average value over the whole FOV from one autofocus sample to the next should be “relatively” valid for that FOV. Thus the focus setting for the minimum HFR point found should still be a valid “best focus”. It there is bad seeing or scope motion from one sample to the next, then the results might be skewed.

I’m interested in K_G’s finding that the Planewave focus results are “far more precise and repeatable”. Do you mean better accuracy such that your imaging results are superior? Is the SGP result jumping around so much that you have low confidence that the imaging results will be good? I have this problem from time to time but it is typically due to bad seeing or wind blowing the scope during autofocus.


Hi Kent, with the Planewave platesolving focus technology I see focus vary from ideal +/- 50 micron. This is +/- 200 micron with SGP. I’m guessing this is due to using non-star objects, but I’m not 100% sure.

Hi K_G,

With SGP's new least-squares parabolic fit, the vertex should be

fairly accurately determined depending on step size and number of
steps. With my setup, when the seeing is good and no wind, the
best-focus determination seems to be within +/- 3 steps or so from
one run to the next immediate run. If I’ve done my arithmetic right,
a rough calculation based upon the Robofocus stepping motor rotation
(3600 steps / rev) and diameter of my Crayford-style focuser roller
(3.18mm) says that the precision (not necessarily the accuracy) is
on the order of +/- 3 microns run-to-run. BTW, I had to change my
step size when the new algorithm came out as the results were
inconsistent. I have an F/4.2 Newtonian which requires more accurate
focusing than, say, an F/8.

You should still get good focus even with a field of fuzzy objects

as you would just get higher HFRs but the minimum should still
represent good focus. Unless, of course, the curve turns out flat
due to the fuzzies. Good focus also presumes stable conditions
during the autofocus run.

I'm always looking to improve focus/tracking on my system, although

it seems to work quite well, thus my interest in your experiences.


Whiskey Creek Obs Logo 7.jpg

I’m similar - I monitor the autofocus and I’m within +/- 5 steps of optimum, each one being 4 microns. Tracking and seeing conditions have more effect on actual exposures.