The night of June 17/18 was forecast to be on-and-off cloudy, so I decided to use a few hours after sunset to improve the polar alignment of the telescope. By picking relatively bright stars, I could work through light clouds.
The main setup was:
Notes from the night:
Refer to the notes of UT May 19, 2016 for a description of the adjustments to the 12-inch mount's polar alignment. I have been making gradual adjustments while acquiring ASASSN-18ey measurements over the past few weeks, so the alignment wasn't terrible; but tonight, I wanted to concentrate on it without worrying about the effect on science.
First, I considered the ALTITUDE axis. I pointed at stars within a degree of the celestial equator and very low in the west; so low that the sources several times slipped behind trees. This should be a good test of the altitude axis, with very little influence from the azimuth axis.
My technique was to take a set of unfiltered images of exposure time 5 seconds, with 30-second pauses in between each image. I wrote down the centroid of a bright star in each image, as reported by MaximDL on raw images, then waited 14 images (about 9 minutes) later, and measured the centroid of the same star. The only important feature was the position in the Dec direction.
Before making any changes, the star drifted 21 pixels North after 14 frames.
I turned the altitude knob 1 notch in the CW direction -- and then realized that I should have gone CCW. Whoops.
So, then I turned the altitude knob 3 notches in the CCW direction. The drift became 3 pixels South after 14 images. Too far!
Turn knob 0.5 notch CW, get drift of 4 pixels South.
Turn knob 0.5 notch CW, get drift of 1 pixels South.
STOP at this point, and lock the altitude bolts on the mount. A drift of about 1-2 arcseconds in 9 minutes is pretty darn small.
Next, I considered the AZIMUTH axis. I pointed at stars within a degree of the celestial equator and within one hour of the meridian. This should be a good test of the azimuth axis, with very little influence from the altitude axis. Again, all the matters is any drift in the Dec direction.
Before making any changes, the star drifted 6 pixels North after 14 frames.
I turned the azimuth knob 0.5 notch in the CW direction. The drift remained at 6 pixels North after 14 images.
I turned the azimuth knob 0.5 notch in the CW direction. The drift became at 2 pixels North after 14 images.
STOP at this point, and I locked the azimuth bolts on the mount.
Note that the star moved quite a bit in the East-West direction during all these tests. That's a "feature" of the RA motor drive, which has quite a bit of irregularity. It may be possible to use a guider to make corrections only in the East-West direction to remove those variations. We'll see ....
Last modified 6/18/2018 by MWR.