Jul 21, 2016 UT: Tests of the off-axis guider

Michael Richmond
Jul 21, 2016

On the night of Jul 20/21, 2016, Jen Connelly, incoming RIT student B Hanel, and I tested the new camera + filterwheel + off-axis guider.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ATIK 11000 CCD camera with EFW-2 filterwheel, 1.32 arcsec/pixel (binned 2x2)
• off-axis guider and Orion StarShoot Autoguider

Notes from the night

• sky was clear
• full Moon didn't affect our tests
• twilight flats were difficult to acquire, due to the long-ish readout time of the ATIK CCD. Taking 10 images in BVRI won't work with a fixed exposure time.
• focus positions at 23 C
  • B: 0.540 yields FWHM 2.7 pix
  • V: 0.549 yields FWHM 2.7 pix
  • R: 0.575 yields FWHM 2.5 pix (but particular finicky)
  • I: 0.582 yields FWHM 2.4 pix

We spent most of the night trying to figure out how to get the guider working properly. From previous experience, we knew that it is necessary to use the guide camera in 1x1 format, with no binning.

The settings with MaximDL that seemed to yield the best results were as follows; I'll comment after listing them.

• autoguider output -> LX200 Protocol
• handpaddle speed set to 1 = Guide
• X speed = Y speed = 50 (units are 'pixels per second')
• aggressiveness: X = 5, Y = 2
• angle = -30 (units are 'degrees')

One of the big mysteries was "which autoguider output choice to use?" We finally figured out that "LX200 Protocol" was the right choice. However, it took us a while for two reasons: first, the speed setting of 50 (pixels per second) is considerably larger than the corresponding value for the SBIG ST-9 camera's built-in guide chip, which I've been using for months. Second, this "speed" depends on the "Speed" setting of the LX200's handpaddle control: the faster the handpaddle setting, the larger the motions of the telescope. I usually leave the handpaddle setting at "4 = 16x", which is convenient for small pointing adjustments, but at that setting, the guiding correction motions are way, way too large.

The "angle" setting is just a guess. We think we have the sign correct, as the telescope would move in generally the proper direction when we placed the star off-center. However, the exact value of the angle has not been measured properly. We should do some tests in the future, taking images with the guide camera while moving the telescope manually in East and North directions.

Last modified 07/21/2016 by MWR.