Focus Woes

2004.12.24

I sent the STI Series IV Stiletto focuser back to STI for service since we couldn't get it to work. He said that it looked like the Ronchi screen was a little out of whack and sent me a new one (and a lollipop - thanks Richard). It arrived a couple days ago and tonight the clouds cleared up so I went over to Bob's place to test it out.

We set the telescope up Alt-Az in his backyard and trained it in on Sirius. First I checked the back-focus with the new short-length T adapter and it works great. We are now able to use the microfocuser and the focal reducer and still bring the camera to focus in prime.

Next we threw the Series IV on there and gave it a shot. Clearly something was wrong with the old one because it looked totally different this time. The diffraction lines were very dark and the background much brighter. This was an encouraging sign. As I used the coarse focusing knob on the scope I could see the lines get less numerous, etc. However, once the pattern got down to about 4 visible lines it was overtaken by wiggly motion and the lines were increasingly harder to make out. The entire focal region inside of 4 lines on either side of focus was basically unusable because I couldn't tell how many lines were actually there and whether or not I was focusing in the right direction to get to exact focus.

We guessed that maybe this was a result of bad seeing, so we made a cardboard mask for the objective that reduced our aperture down from 14" to 3" and tried again. It was better, but the central focus region was still not useful and we had to guess. We tried counting the time between where the bars were visible on one side and the other, then focusing back 1/2 of this time... no luck.

We then tried making a Hartmann mask out of the cardboard and focusing that way, but again there was a certain length where the two stars seemed to be on top of each other when viewed through the camera viewfinder.

Right now my best guess is to use the Hartmann mask and do a timed series exposure like I did with the 50mm lens. I could then view the image in the camera, count the number of exposures back to the most in-focus, then run the microfocuser back for the correct amount of time. This is not an optimal solution and it won't give perfect focus, but right now I just don't know how to make the Series IV work.