Pempro V2 - PEC Software and More

Ron Stanley

For those of you who are unfamiliar with Pempro, ( it is a software which through the use of a webcam or dedicated astro camera, measures the periodic error of the worm gear on your mount and then if your mount has a periodic error correction it will either program that correction directly to the mount or it will replay the correction to the autoguider port which can then be recorded by the mount. I have been a licensed user of Pempro now for a few years but after I originally bought the license I found that it did not cure the periodic error on my Mountain Instruments 250 mount. The problem was that it would record the periodic error correctly and then program the mount however there was a bug in the Gemini software, which runs the MI-250, in which when the mount was shut down the periodic error corrections which had been recorded by the mount would be index shifted so that when the mount was started again the periodic error correction was not played back correctly and actually made the errors worse! I almost sold my license to Pempro on Astromart because it seemed to work with other mounts, but I'm glad I didn't.

Eventually the problem in the Gemini firmware was corrected and now I find Pempro an indispensable tool for my imaging. The uncorrected periodic error for my MI-250 was around 5 arc seconds peak to peak. Using Pempro it is now half of that! This seems to be true for other mounts as well such as Losmandy, AP, Meade and Celestron. This can make a huge difference in the trailing of stars in unguided images and can be a great help with guided images as well by preventing error before it needs to be guided out.

The software works by using wizards to determine image scale and camera orientation then, imaging a star close to the celestial equator and the meridian. Through the setup the software knows the length of the worm cycle for most of the popular mounts and it measures the movement of the star through a number of these cycles. After the measurements it takes the data and corrects for drift and then fits the data to a curve. The curve is a graphic representation of the displacement of the star in arc seconds due to the error in the worm gear. Once the curve is fitted it can be adjusted or simply programmed back to the mount. This procedure is relatively easy and straightforward and the results have been worth the effort.

This was where Pempro V1 left off, however Pempro V2 was released last year and has added some new features that increase its functionality. The new feature which will be of use to most is a new Polar Alignment Wizard which measures the drift of a star to give you an accurate direction to move your mount in azimuth and declination to achieve polar alignment. After going through the setup wizard, Pempro will either slew to the proper position in the sky or the user can slew to the position. A star finder feature will now help you find a suitably bright star by doing a spiral search and taking overlapping images until one is found. Once a star is found Pempro begins taking images and measuring its movement. After five minutes Pempro will analyze the drift and direct the movement of the mount in azimuth with an arrow going from the bright star in the direction it needs to be moved. The image can be constantly updated to show when the star has moved to the correct position. Once the azimuth is set the measurement can be repeated for a more refined position or Pempro will move the mount to the east to repeat the process for the declination. This procedure is a toss up to me. My Gemini controller has a polar alignment feature that is probably faster and easier for me to use but that may only be because of the particular nature of my set up. The Gemini and Pempro give similar results in the end, they only disagreed by 2-3 arc minutes.

The final feature of the new Pempro is a backlash adjustment feature. Backlash is the problem which arises from the spacing between the gears in the declination axis. When a movement is directed in the declination axis and then a move in the opposite direction is directed, backlash is the amount of movement needed to move from one side of the gear tooth to the other. It can be thought of "play " in the gears. Most popular mounts these days have a feature in which a movement can be programmed into a reverse movement of the declination axis which will take up this "play." The problem has been how to adjust this movement. Until now the only method has been trial and error, but Pempro now gives you a visual tool to tell you when the backlash has been taken out.

It takes an image of a bright star with tracking off. During this star trailing it directs the mount to move in Dec, first in one direction and then the reverse. Over the length of the image, 30 seconds, the result is a saw tooth pattern. The image can be analyzed for backlash as follows, a flattening of the points in the saw tooth image means that the backlash is under corrected and the correction needs to be increased. A gap at the points of the saw tooth means the backlash is over corrected and the correction needs to be decreased. I found this process to be very intuitive and easy to do and it has finally given me a way to quantify a measurement I have been unable to do in the past.

Published in the March 2008 issue of the NightTimes