Hello,

I own and regularly use for more than ten years a SteelTrack Baader with Robofocus, attached either to a C9Edge (mine) or C14Edge (of a local club). Up to last year, I used a Sbig ST10 without Ekos. My focus routines were the one of Equinox Image and/or a private software developed by a friend, with them I could achieve sharp focus (fwhm below 1.5'' were not rare at 3950mm at Montagne de Lure, France) and, more important for the following, the procedures give repeatable results. The method looks very similar to Linear 1 pass, except that the V-curve is sampled by outwards moves, the solution being reached by an inward move. The Baader has an engraved scale for visual checking ranging from 0 to 30mm, I calibrated the Robofocus so that one 30mm = 3000 ticks. Hence the CFZ is 8 ticks.

Recently I bought a new camera (Zwo2600mm) and connected it to a Stellarmate Plus with Ekos 3.6.8 running on it. I am really impressed by the ergonomy of Ekos and the quality of the underlying algorithms. I made a test session few days ago with the C9, on targets such as M36 or M35 in order to have many (up to 100) stars to focus on.

My idea was to make repeated linear 1pass routines with the luminance filter (I mean with no time interval between routines) in order to check if I get consistant results. I made it either with backlash=30 or 15 (the actual backlash is certainly lower than that) and oversan=0, or backlash=0 and overscan = 40 or 15. It is noteworthy that the backlash is only "outwards", ie is applied only after inwards move. I found no way to enter an inwards backlash, anyway overscan mimics that.

In all cases, the results were puzzling. I could achieve repeatable focus (I mean, with a spread less than the CFZ with no trend) up to seven times, but otherwise I observe a frank decreasing trend. For instance, I got 1283, 1283, 1276, 1272, 1257, 1245, 1242, 1232, 1213, 1207, 1211, 1210, 1211, 1210, 1204, 1184, 1169, 1156. A visual check on the engraved scale yields a value about 1300 (more than 1 mm shift would have been noticed), which is about the right value to comply with the EdgeHd back focus. I never experienced variations that high over a whole night, at most it was a 30 ticks shift because of temperature change. So Ekos delivers variable results, but the Baader remains more or less at the same position.
The most intriguing for me is the erratic behavior: it can be stable for few focus routines, then a decreasing trend (I have not observed an increasing trend). The V-curve always seems to be correct (R2 above 0.98) and the hfr at solution is consistent from a run to another, up to the unavoidable seeing variations, at least points on the V-curve off to 15 ticks or more from the solution are clearly worse. I tried to change the polling (Default is 1000 ms, I changed to 10 sec and 100ms), with no change. I also tried to focus on only one star, I also stopped and relaunched the Indi servers... no change.

An idea ? I attach the log.

Again, thanks for that so well designed tool.
Philippe