Autofocus asymmetry


AF has been working very well for me (well collimated 10" f3.9 reflector) but the AF curve is not symmetrical, though the parabolic shape is fine and consistent and the focus is accurate . The first HFR reading in an AF routine is typically above 5, while the last one tends to be much lower, like between 3 and 4. The number of steps is correct, meaning the first measurement is 100 steps above the focus value and the 9th measurement is 100 below it using 9 data points with a step size of 25. Just wondering what might cause this asymmetry, possibly optics issues?


Sorry…if it matters, I left off the rev…

Just uploaded but have not had a chance to try it.


I have the same kind of reflector, and the same behaviour. I don’t worry about that since the focus is always as accurate as possible !


Appreciate that information. Just curious what might cause it. Perhaps it’s the algorithm. Agree the focusing works well.


i see the same thing from time to time with my 6" RC. at first i thought my focuser must be slipping or that there was a ‘dead spot’ on the drawtube but subsequently i noticed that sometimes i get a very symmetrical focus curve… not sure what’s going on. as with both of you the focusing is working very well for me so… ?


I wouldn’t worry too much about it. Could be a handful of things but it’s almost certainly related to the light path somehow. I’m certainly no expert on optics but it seems there is a common thread here which is central obstruction. Our focus metric is calculated the same way for all scopes so if it was intrinsic to our algorithm you’d see it everywhere.



Jared makes sense regarding the likelihood that it’s optics-related rather than program. I do also occasionally get a perfect parabola from upper left to upper right on the graph. I though it could be drift in the focuser as well, though the Moonlite with the stepper is pretty tight other than a minimal amount of gear play. Not really a concern, more just curious. Doesn’t surprise me since I don’t get the exact out-of-focus image either side of focus. Probably some optical aberration.


I get a similar thing with my Schmidt Newt. The focus is very repeatable so I don’t think it is the moonlite focuser slipping.


I agree as long as the focus is good it’s not an issue…just a matter of curiosity.