Guiding tips

[cwoodhou]

I set to understanding the guiding performance on my new mount after the latest firmware update. That update improves DEC slew accuracy and permits a sensible TheSkyX TPoint model. That 50-point model confirmed the polar alignment error measured by SharpCapPro and gave me a pointing accuracy of 26" RMS. Not bad, considering the PE of the mount. With better confidence in the polar alignment, I could work on the guide settings. The telescope was a WO GT102 and I was using an off-axis guider with a QHY178 (2.4u pixels).

I always calibrate PHD2 at low declination for maximum accuracy.
I discovered that the mount's PE can trip up PHD2's calibration. Even with short exposures (0.5 seconds), the drift in RA during the DEC calibration causes some issues, which in turn can make the RA guiding unstable. This occurs if you try calibrating when the PE error is changing rapidly. It is detected by using the PHD2 tool that displays the calibration data. If the red and blue arms are not orthogonal, it has the potential to ruin guiding.

The solution is simply to repeat the PHD2 calibration process a few times until you hit a period in the PE cycle that is stable. The red and blue arms then become orthogonal and you are good to go. I measured the tracking error with PHD2 with its outputs disabled over 10 minutes and noted the peak error and the rate of change. On my mount is was ±30" PE and a maximum rate of change of 0.6"/second.

I do all guiding evaluations at low declination, where it is more sensitive to RA tracking errors. With a guide rate of 0.5x set (still common across both axes) the PPEC algorithm, with default settings and a period of 433 seconds, kept up with the PE error and matched and sometimes outperformed the DEC axes. If I lengthened the exposure to 1 second, the trace in PHD2 showed a more significant and consistent + or - error at different times i.e. it was not keeping up. To use exposures longer than 0.5 seconds, the aggression would need to be increase, at the danger of overcompensating in the presence of seeing. I used the default 0.2 pixel min move setting, equivalent to 0.15"

The DEC guiding is also tricky. The drift was low and it needed a very gentle touch to remove the drift and not react to seeing. With short exposures, I used the hysteresis algorithm with a low aggression of 10 and 0.2 pix min move. If my drift had been significant, I would have tried the other algorithms. No evidence of backlash in the guiding.

The overall guiding RMS over half an hour was about 0.55". The real guiding error is probably smaller, as the RA and DEC RMS values were similar, indicating that with these short exposures, astronomical seeing was significantly contributing at this low altitude to the PHD2 guide trace noise.

After guiding for several hours, I used Andy Galasso's PHDLog viewer app to analyze the outcomes. It has a nice feature that analyses the underlying tracking error and PE period.

I reckon if Pegasus follow through with individual guide rates, I would anticipate getting better results using 0.75x for RA and 0.25x (or lower) for DEC.

hope that helps

[Jockey]

Thanks for the info and taking the time to post it.

Cheers

[alpheratz06]

Thanks for the post. My NYX is just out of the box and I had a dreadful night of impossibility to calibrate the guiding. Very frustrating. I have used Pegasus recommandation about phd parameters.m, made several attempts and it does not work.
I'll have to try again...