I spent the weekend getting my RoboFocus installed and tested. Seems to work right out of the box (once I figured out that I needed to buy new bearings for the focuser!). I did have to move the primary mirror and install a new focuser on the tube, though. That took some time.
There is really very little documentation on how large the "steps" should be. The instructions spend a whole lot more time talking about installation and communicating with the computer, than about the settings. Actually, counting what I already know about focusing, and such, I did figure the following:
First--determine how large the "Zone of Critical Focus" is for a given F Ratio/Aperture. Using formulae available in several places, I figured that for my Newt (10 inch F5), it is about .0009 inches.
Second--Figure how big a "Microstep" is. I ran the procedure to determine that there are about 3300 microsteps from top to bottom of the focuser travel. My micrometer says this was .55 inches. This gave me something like .000167 inches per microstep.
Third--From what I have been told, you want the focuser to stop at least once, and maybe twice within that Critical Focus Zone. There are 5 "microsteps" through that zone, therefore, I want to set my motor to, say, four "Microsteps" per "Step." This makes sure that it stops every four steps (in my case .0007 inches), and does its sampling. Alternatively, I could set microsteps to two, and then run the autofocuser program through two steps at a time.
Well, does all that sound about right?
Then, when I went out to the telescope, I focused as well as I could with my camera viewscreen, parfocal eyepiece, and ronchi-focuser. The Robofocus reported it was at 506.
I ran the focuser in to 460.
Using the Autofocus program in DSLR Focus, I stepped through 10 sets (of three) exposures. They were spaced 8 steps apart. It reported the best focus was at about 480. (I was using 2 microsteps each)
I then told Robofocus to go to 470, and stepped back though again. This time it reported it at 486.
And I did it again and again. Each time, the "best" focus was different. It varied from about 480 to 496. The difference here is only .005 inches. While this does not sound like much--it is about five times the "critical focus" zone. (And probably a lot tighter than I ever did before.)
Does this sound about right? Is it still too sloppy? Is it just seeing? When you run your focusing routine do you always get exactly the same number?
I should say that the mount was tracking, but not guided. The stars were not obviously twinkling, but other than that, I did not notice seeing. And I was averaging three four second exposure for each trial.
At any rate--How does this sound? Any advice?
Alex
