Saturday, January 27, 2007

Measuring periodic error

Last night we had the first clear night on a weekend in about 6 weeks. The only bad thing about this time of year is that really clear nights usually mean really cold nights. This was no exception - quickly dropping to the 30's not long after sunset. Seeing was average, and transparency was above average. The waxing moon was a little over 50% illuminated. These are the times I like to use for experimentation. This night I decided that I needed to get some good data on the periodic error of my EQ6 mount. This brings up a couple of questions:
1. What the heck is periodic error?
2. Why do I want to spend time measuring it on a clear night?
For the first question, about all I can say is that periodic error has to do with the inaccuracy of the gears that move the mount in right ascension and declination. Gears made for mounts in my price range (i.e. cheap) just aren't made perfect. Actually, no gears are made perfect, but the more money you can spend on a mount, the more precision there is. The shape of the gear has a pattern to it which repeats on each revolution of the gear. Each revolution makes up one period. We measure the movement of the mount over several periods to get a picture of the error in the gear. Modern electronics can adapt to this error and help minimize it.
For the second question, I am measuring the periodic error because I will be sending my mount away in the near future to be treated with a new "dry-film" lubricant that is used by people like NASA to improve the mesh between load-bearing gears. Sounds like high-tech stuff, huh? It's a retired NASA engineer that has developed this service for amateur astronomers, and I have been selected as a beta tester. I'll never know if it improves anything unless I have data showing the performance before the treatment.
So, with a clear night at hand I lugged all my gear outside and tried to remember how everything hooks up. Funny how much you forget after 6 weeks of watching it rain every weekend. Once I got everything running, I pointed the mount at a star near the intersection of the celestial equator and my meridian, and fired up K3CCDTools. It was the first time for me using this software, and I had heard good things about it. After getting things lined up, I started a data log and walked away for a little bit. My mount's gear makes one revolution, or period, in 8 minutes. So, I wanted at least 3 periods to get good data. After that, I pointed the mount over to M42 and started some imaging. This time I used Metaguide since that is what I am comfortable with. K3CCDTools will also guide, but sitting outside in the cold is not the best time to learn something new, especially when you haven't even read the manual. With Metaguide ready to do some autoguiding, I started imaging M42 and went back inside to defrost my toes. Meanwhile, Metaguide was logging data while ImagesPlus was happily snapping away 3-minute exposures of M42.
By midnight I was ready for bed and carried my ice-cold equipment back in the house. This morning I imported the date into Excel and examined the results. Seems like my periodic error is 20 arcseconds from peak to peak. Pretty average for a mount in this price range, and about what I expected. The log file from Metaguide showed the error pattern over a longer time. It's interesting how every third wave in the error plot has a weird shape. It's not yet totally clear what all this means, but at least I have some baseline data upon which to make a comparison. Hopefully after my mount is modified, I can repeat this testing and see some improvement. Let's keep our fingers crossed!

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