Could anyone help me analysis my RA error?

[Yang Hu]

I imaged two objects in one night. When pointing to M101 (DEC +54.5 start time: 21:20 session length: 2h24m), RA RMS error was 1.05". Then I switched to M16 (DEC -13.8 start time: 00:01 session length: 4~5h) which is near the celestial equator, RA RMS error increased to 1.54" and it produced enlonged stars in my main scope.

I inspected the guiding log and I could not identify the problem except that RA errors are pretty large and there are usually two small guide pulses before RA error reverses direction.

Guiding log:

https://openphdguiding.org/logs/dl/PHD2_logs_aRGU.zip

Equipment:

183mm guide scope with 120mini (4.23arc-sec/px)

750mm main scope with 533mcpro (1.03arc-sec/px)

Thanks!

[wave...@talktalk.net]

I think your Zip file has an error so doesn't unpack.

- Jack

[bw_msgboard]

The difference between results for the two pointing positions is explained by the geometry of the sky – a given angular error in RA translates into a measured error on the sensor according to 1/cos(Dec).  So if we take the 1.05 arc-sec error you got at Dec=-13 and forecast how that translates into error at Dec=53, we get 1.6 arc-sec which is pretty close.  This has nothing to do with guiding, it’s just a reflection of RA tracking errors in your mount.  Before guiding, what PHD2 sees is this:

 

 

So this is roughly a 30 arc-sec peak-peak error that PHD2 is trying to guide out with a frequency signature that looks like this:

 

 

The longer-period errors could probably be reduced by getting a high-quality periodic error curve programmed into the mount.  The higher frequency components like the 13-sec error are caused by common problems with the EQ spur gears.  If you look on this forum and others, you will find explanations of how people have improved on these by fiddling around with the meshing of those gears.

 

Hope this helps,

Bruce

 

 

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[bw_msgboard]

Sorry, Yang, I typed the first part of my answer incorrectly.  I should have said the 1.05 a-s error you got at Dec=53 corresponds to an expected tracking error of 1.6 arc-sec at Dec=-13.  I said it backwards…

 

Bruce

 

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[Yang Hu]

Hey Jack,

I'll re-upload my log when I get home tonight.

Thanks.

[Yang Hu]

Bruce,

Thanks for clarifying the RA error problem!

I have a couple of questions:

1. How did you get this analysis graph?

2. I am using EQMOD "AutoPEC" function and have the "sidereal + pec" mode enabled, any other idea about the long PE?

3. Does Rowan Belt Mod correct spur gear meshing error?

Yang.