[PRejto (Peter)]

After reading about this for a while - and procrastinating even longer - I finally decided to test SharpLock with MaxIM. My imaging system consists of the smaller version ONAG with the new helical focuser, an ATIK 414EX as a guide camera and a Moravian G2-8300 on the imaging port. The focus hub is an Optec design running a Starlight Instrument custom built focuser for the large TEC focuser.

After some time stumbling around with MaxIM (I use TSX all the time) trying to figure out how to get it to guide through TSX I was up and running pretty fast. The first step is that both the imaging camera and guide camera must both be at best focus at the same time. I used FocusMax to first focus the luminance filter of my G2-8300 and then quickly tried to focus the guide camera. In rapidly falling temperatures this can be not so easy as you can get the guide camera in focus only to find that the imaging camera has moved from focus. In any case, as a first step they don't have to be super close. After starting MaxIM one picks a guide star/exposure combination that yields a good s/n and around 30,000 adu. Faster exposures for calibration allow for quicker averaging of the roundness calculation and angle. At perfect focus SharpLock sees a cross with equal length arms. As focus shifts in or out the arms change size with respect to one another. The idea in calibration is to increase the star roundness factor by 30% by progressively moving the focuser. Once the program sees enough images (averaged together) giving that out of round condition calibration is over. The out of round is linear up to 30-40% so once you know how far to move the focuser to get 30% out of round it can be done quite rapidly. In any case it only needs to be done once unless you change the camera orientations. Following calibration, since the temperature dropped even further, I reconnected to Focus Max and refocused. Then I adjusted the helical focuser on the ONAG to get as close to perfect roundness as possible while watching the average roundness as displayed in the software. Once that figure was achieved it is set into the software as the "Relative Roundness Reference." This roundness setting is now used to monitor for any changes during Sharplock operation. The focuser simply moves to restore that reference shape. So, once calibration and RRR are finished the "Start/Stop" buttons of SharpLock are activated.

I decided to monitor SL in action by notating the focuser position and temperature shifts followed by 20 sec exposures through the G2-8300 imaging camera. I opened the photos in CCDInspector to see the effect on FWHM values. I followed focus for about 1.5 hours during which time there were 7 focus shifts. After 47 min I was tracking too low to the west and slewed to a new star overhead. Due to a couple of errors I made I lost 17 min before resuming guiding. During that time the temperature dropped .2 degrees and SL made a large 28 step move (30.8 microns). During the 1.5 hours the temperature dropped from 11.2 -10.1 degrees and the focuser made 7 moves. Excluding the large 28 step move, the average size move was ca 5-6 steps (5.5 - 6.6 microns) and the FWHM ranged between 2.88 - 2.02. The 2.88 and 2.60 values were a little surprising so I took another test exposure whilst the focuser was at the same position. The 2nd photos in both cases showed much lower values so I believe these were just random seeing or tracking variations. If I eliminate the two high values, during the 1.5 hours the FWHM average value was: 2.33. My Focus Max HFD values for the night were 2.49, 2.56, and 2.93 with the last value taken shortly before starting the test when the temperature was 11.6. I think it is safe to conclude that a focus shift on my TEC180 would be useful for every .2-.4 degree temperature drop. If one attempted to do this by traditional methods far too much imaging time would be lost.

In conclusion I have to say I am very very excited about this software! To think of not needing to focus night after night almost seems too good to be true. My next plan is to automate both of my scopes. I will be a beta tester for OPTEC with their similar program "FocusLock" which will run with TSX and several other programs.

Peter

The photo of SharpLock shows the long plot of the focusing. The interruption was when I calibrated.