Finished Polar Alignment, First PEC Training

We finally had two clear nights of excellent conditions this past weekend. The sky was clear, the temperature was above freezing for the first time in months, there was no moon, and, being very early spring, there were no bugs. I have been waiting several months for an opportunity like this to finish installation and calibration of the mount.

2015-04-12-Polar-ResultsFirst, I finished refining the polar alignment. This ended up taking four passes, when I thought only two or three would be needed. After the first polar alignment modeling session over a month ago, substantial polar error remained and a rather large correction was recommended by the TPoint software. Last night, I applied that large correction and ran a new modeling run. I didn’t expect the results to be perfect, just closer, and, sure enough, TPoint reported substantial improvement to the polar alignment but with a minor correction still needed. I must have misread the decimal point in the correction instructions because, on pass three, the polar alignment was still off by about the same amount and another minor correction was recommended. I made that correction, paying more attention to getting the decimal point in the right place, then ran a final TPoint model just to check the alignment.

The results were very good. TPoint is reporting my polar alignment to be off by 1/2 arcminute in azimuth and 3/4 arcminutes in altitude. That is certainly good enough for now.

2015-04-12-pec-captureNext, I wanted to do an initial, first cut, of the mount’s periodic error correction. I collected about four worm rotations’ worth of data using the autoguider camera. Even without PEC turned on, the Paramount’s impressive performance showed up. Uncorrected, I recorded a periodic error of ± 1.1 pixel, or about ± 3.8 arcseconds. That is better than the best I ever achieved, with PEC turned on, on my Losmandy G11 amount.

2015-04-12-pec-curveThis generated a periodic error correction curve to be uploaded to the mount which applies a correction of ± 0.4 arcseconds.

2015-04-12-corrected-computedI verified this by doing another PEC data capture, this time with PEC turned on. Sure enough, the corrected PEC is now showing at about ± 0.5 arc seconds. That is, frankly, incredible; especially since I’m not particularly impressed with my PE data capture yet. Seeing was quite poor tonight, probably in the range of 3-4 arcseconds, so my PE data capture was probably seeing the target star scintillating due to bad seeing more than it was seeing mount drift.

2015-04-12-LeoTriplet-2x2-600-singleJust for fun, and as a more practical test, I decided to end the evening with a 10-minute guided exposure of the Leo triplet. I used five-second guide camera exposure intervals, with a low guiding aggressiveness to avoid “chasing seeing”. Although I forgot to take a screenshot of the guiding graph, guiding during this 10-minute exposure showed that it was making very small corrections, oscillating around a 1.5 arc second error. The single resulting image is shown here, without dark or flat fields. Nice pinpoint stars.

 Update: PE worse than I thought

Updated added later:  I have realized I made an error in recording the PE data above – I didn’t have the camera aligned “top-north”, so some of the error was masked as Dec motion.  Error is worse than recorded – in fact, it seems out-of-spec at about +/- 9 arc-seconds uncorrected.  Other posts will record the story of tracking this down and correcting it.

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