Jul 19, 2023: Training the 12-inch telescope's drive for backlash

Michael Richmond
Jul 19, 2023

I continued my investigation of backlash in the Dec motion of the 12-inch telescope tonight. Tracy Davis unexpectedly visited during the evening and helped with the work.

Setup included

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ASI 6200MM CMOS camera
• R filter
• images binned 4x4
• no guiding
• focus position in R:
  • 0.369 yields FWHM 2.9 at T = 69/20

Notes from the night:

• clear and almost cool
• no Moon
• we smelled a skunk in the back yard, but never saw one
• it appears that the guide camera is connected to port COM1, but this needs confirmation on the next clear night.

Following some helpful suggestions on the Cloudynights forum, I ran the "Train Drive" menu option, selecting Dec/alt axis only. I initially attempted to do this during the day, by focusing the telescope on a nearby telephone pole. In order to focus on an object at such a short distance --- only about 100 m from the telescope --- I had to place the eyepiece quite a distance behind the mirror:

However, I discovered that, because our telescope is permanently mounted in polar mode, we had to run the test at night, on a star. So, I waited for the sky to grow dark.

The telescope prompted me to slew to a star near the celestial equator and meridian. I chose σ Ser = HD 147449, which was bright enough to appear in very short CCD exposures.

• first, the telescope slewed about 1.5-2 degrees North, and prompted me to move the star back using the South button.
• I did so.
• the telescope then slewed about 1.5-2 degrees South and prompted me to move the star back using the North button.
• I did so

And that was that.

After finishing this "training", I tried repeating the backlash tests from yesterday. When the "Anti-Backlash" setting was at its default "001%" value, there was no change.

But, now, when I increased the setting, I DID see a difference: the delay between my finger pressing the controller's button and the telescope moving was smaller. For slew speeds of 3x or 4x, the delay was eliminated with settings of 150% or 199%. That's the good news.

The bad news is that, at these large settings, motion at very slow speeds involved a sudden jump, followed by a slower, smooth motion. If the plan is to use the autoguider to make small corrections in Dec, these jumps could overshoot. It's not clear which value of the Anti-Backlash will be best; there may not be a single value which works for guiding and for ordinary, manual slewing, too.

I then spent a short time playing with the guide camera. Things I discovered:

• current setting is for "Rotate 90 degrees CCW" ...
• which yields orientation of images on the computer screen with West up and North at right
• error messages from MaximDL about "can't communicate with guide camera" appear for port settings of COM4, COM5, COM6, but not for COM1. Is COM1 really the right port to use? Must check another time.