Data on Precision Error Correction for Two LX200 Telescopes
Here is a bit of data on worm drive errors found in two LX200 telescopes. The top plot shows the error curves for my own 12" LX200 measured in August 1997 without PEC training and also with the PEC training after a very good training session. The middle plot shows the same telescope but with a very poor training session. The lower plot shows the error curve for my 10" LX200 without PEC and with PEC after a very good training session. (The 10" was measured sometime in Summer of 1998)
The erratic curve of the RA worm irregularities without PEC are shown for 200 bins (8 minutes). The deviations are about 50 arc seconds peak to peak for the 12" and about 25 arc seconds peak to peak for the 10". With careful PEC training, as shown for both the 12" and the 10", these deviations can be reduced greatly. The improvement factor of 5 to 10 as shown above is obtainable with this careful training. This sort of improvement has been verified by many reports from mapug members.
It is clear that the irregularities are not smooth and nicely
sinusoidal, but are very irregular. This is due to the fact that
the worm surface is ragged on a microscopic level. Remember
that an arc second of pointing error is caused by an irregularity of only
a wavelength of light or so. We can safely assume that all worms
show similar errors as has been reported by many on mapug members over
the years.
It should be perfectly clear from this data and the many reports from experienced astrophotographers just how important PEC training is. We are most fortunate to have this feature on our LX200 telescopes. After careful training, it becomes much easier to either manually guide or to guide with a CCD guider.
Data of this sort is very easy to compile. One simply watches through a calibrated eyepiece, for example the Meade 9 mm or Celestron 12.5 mm, and calls out the error to a person who writes down the value and calls back the time for the next reading. This can easily be done at 2.4 second intervals or any convenient interval for the 8 minutes required to get one cycle of the worm. The exact interval is not important just so the values are taken at consistent intervals. If one has a cassette recorder this can be done by one person calling out the errors and then writing them down on playback.
I suggest you take a sample of this very important information which can be used to judge the quality of the RA drive on your telescope. With the telescope well, polar aligned and without PEC, take about 4 cycles of data like that shown above. Plot the four runs on top of one another. This gives you an idea of how consistent the errors are four four teeth on the worm wheel. If the curves are very similar you can have some confidence that a training on one tooth or two will be good for more positions of the worm wheel. The more cycles you examine, the more you know about the consistency of your worm wheel.
The same data can be taken after the PEC is trained to determine if the PEC training accomplished is good enough or should be refined.
The derivative of these curves gives you critical information about
the frequency of the corrections required to stay within a given error
criterion. With the 216XT which I use for guiding, I pick a star
that is bright enough so that it reads 15 to 20 on the brightness readout
with an exposure of about 1/4 second, These settings give corrections
every second or so. This is often enough to give good guiding with
a well trained PEC.