Unexpected guide rate by PHD2 and more problems

[Ishan Mair]

Hi.

I've been using PHD2 for a couple of years now. But I've been noticing two strange things:

(1) PHD 2 calibration calculates the rate of guiding as too high on both axes. Almost about 1.8 times higher than what I set it in EQMOD. It always takes quite a less number of calibration steps than expected. E.g. When I set guide rate in EQMOD to 0.5x sidereal rate, PHD2 calculates a guide rate of 2.098 for DEC. While given the guiding scale of 6.62 arc-sec/px it should be about 1.13.

(2) In RA, most of the corrections are made towards west. While this may be co-incidence, I also notice that the mount is less responsive to a westward correction than an eastward one. It needs multiple westward corrections to put guide star back to its position. But an eastward correction almost always overshoots and is followed by a westward one.

 

Please excuse the DEC graph in the first log. I carelessly set the DEC guide mode to South without confirming the direction of drift.

 

Mount: HEQ5 Pro, Guide Camera: Orion SSAG, Guide scope: Orion 50mm

I'm attaching a couple of guide-logs for analysis and screenshots for showing what I mean .

Thanks for helping.

 

 

Ishan.

PHD2_GuideLog_2016-02-02_214819.txt

PHD2_GuideLog_2016-02-29_205305.txt

[kens]

Interesting problem. I've not really paid much attention to the rate that PHD calculates as long as it ig guiding well. In the end it uses whatever it calculates as px/sec from the calibration.

Possible causes for the discrepancy that I can think of:

• Bug in the software (you'd think this was picked up by now)
• Focal length incorrect (162mm sounds right for a finder guider)
• Something wrong in EQMOD - has your tracking rate been set to something other than Sidereal?

I'm just making wild guesses here

[Ishan Mair]

6 hours ago, kens said:

Interesting problem. I've not really paid much attention to the rate that PHD calculates as long as it ig guiding well. In the end it uses whatever it calculates as px/sec from the calibration.

Possible causes for the discrepancy that I can think of:

• Bug in the software (you'd think this was picked up by now)
• Focal length incorrect (162mm sounds right for a finder guider)
• Something wrong in EQMOD - has your tracking rate been set to something other than Sidereal?

I'm just making wild guesses here

Its strange. Once the guide rate "exceeds" the sidereal rate, backlash in RA creates issues. If something is wrong in EQMOD tracking rate, it should not affect DEC. Something wrong with EQMOD guide rate? I use pulse-guiding.

[kens]

Guiding at more than sidereal would indeed lead to RA backlash. When guiding at less than sidereal, east corrections simply stop the RA motor and let the earth's rotation take care of the correction. So the gears stay engaged and there is no backlash in either direction. At more than sidereal, east corrections need the motor to reverse which causes backlash.

What that tells me is that PHD is calculating the actual guiding rate (2x) correctly, while EQMOD is reporting something else (0.5x)

Is EQMOD set up with the right pulse rate for your mount? It does allow custom rates.

[Ishan Mair]

I have now installed the latest version of PHD2 and EQMOD. Next, I sent manual guide commands by PHD2 with RA & DEC guiding rate set to 0.5x sidereal. Things look ok in daylight. Under the stars might be a totally different story.