PEC was made available for Gemini-II not so long ago and I was finally able to get the tools I needed to make it happen. The current version of PEMpro which is THE reference for PE analysis is not compatible with DSLR and many CCD’s for that matter. Having a SSAG, a Lodestar and a DSLR and no expensive capture software I was out of luck.
Ray (the author of PEMpro) made me realized PEMpro is actually compatible with webcams. I found a 2005 Creative Labs webcam which I decided to break down to see if it would work. I Basically striped down the feet and the optics and glued a 1.25″ tube to the front that will act as a nose piece. That’s it, about 15 minutes of work and I now have PEMpro compatible hardware.
I spent about 3 nights in front my garage to learn using PEMpro. The software is pretty intuitive and Ray is available for support directly from his forums, I’ve had many questions and he was able to help me every time.
I started with PEMpro’s Polar Alignment Wizard which helps you adjust Azimuth and Altitude by measuring the drift. Because PEMpro actually measures the camera angle it will even tell you where it’s drifting where you need to apply a correction.
The tutorial says to wait 2 minutes before making any adjustment and if you are getting close you will want to wait these 2 minutes. However if you are pretty far and see a bif drift (like I usually am from my garage since I can’t see polaris) you can usually start making corrections after 20 seconds.
On my best attempt it took me about 15 minutes to get the Azimuth really close. I usually stop making correction if PEMpro says that my correction is less than 1 arcmin. It’s not perfect but it’s good enough.
After you are done with the Azimuth adjustment, PEMpro will guide through the same steps for the Altitude. The drift for Altitude (for me at least) is usually not as smooth as it is for Azimuth. This may be related to the PE, I am not sure. This tends to make me increase the time before I do a correction a little more to make sure the suggested correction isn’t too affected by the random spikes.
After this is done, you can always refine your PA by redoing some of the previous steps.
As far as I know it is not required to have a perfect polar alignment in order to acquire PE data but I usually try to do it anyway.
Before acquiring data it is recommend to slew to a star that is around the intersection of the meridian and the equator. I usually chose a star on the west side to make sure a flip won’t be necessary.
PEMpro recommends to acquire at least 6 worm cycles to have sufficient data to have decent statistics. On the G11, a cycle is about 4 minutes, this number depends on the number of teeth the RA gear has (360 teeth for the G11).
Once the data is acquired you will want to analyze the data. On the next screen you can select the fundamentals that you want to be taken into account for the correction. Ray recommended that I only choose 1st, 2nd and 4th. Once this is done the curve will be generated and you will be asked if you want it to be uploaded to the Gemini-II. I personally had to click NO and invert the curve on the next screen before uploading to the mount. Otherwise the error would be doubled.
With PEC OFF my periodic error with my stock G11 was about +8 / -7 or about 15 arcsec peak-to-peak. After the curve is uploaded I would usually re-acquire a couple of worm cycles to check by how much the PE has been reduced.
With PEC ON, my periodic error went down to about 8 arcsec peak-to-peak which is pretty good to me!