Re: AP1600GTOE RA drift


Ray Gralak
 

To check tracking I point the telescope at 0 DEC and HA=-0 10 00 (ie, 10 minutes
east of the meridian).
At this location there should be no RA drift and only a DEC
drift as a result of any remaining polar alignment error.
Sorry, but that's not true. Even with perfect tracking, RA drift can occur because of even the tiniest amount flexure of the scope, focuser, and camera assembly. For instance, I can measure easily it with my AP Traveler on my 1200 mount.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma


-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Friday, April 7, 2017 1:35 PM
To: ap-gto@...
Subject: [ap-gto] AP1600GTOE RA drift



I have polar aligned the mount and without using a pointing model I get good
pointing accuracy, always close to the center of the ccd sensor. I have used
PEMPRO to check the alignment and it also shows the mount is well polar aligned.

To check tracking I point the telescope at 0 DEC and HA=-0 10 00 (ie, 10 minutes
east of the meridian). At this location there should be no RA drift and only a DEC
drift as a result of any remaining polar alignment error. I then take a 10 second
image, wait 5 minutes and take another 10 second image. I plate solve both images
and get this:



Drift:
Rho (arc-sec): 4.3
Rho (pixels ): 7.6
Theta (degrees): 77.3

RA (arc-sec): 4.2
RA (pixels): 7.4

DEC (arc-sec): 0.9
DEC (pixels): 1.7

Duration (seconds): 317

Drift Rate (arcsec/sec): 0.0136 @ 77.3 degrees

Drift Rate RA (arcsec/sec) : 0.0133

Drift Rate DEC (arcsec/sec): 0.0030




Rho is the arc second displacement from the plate solve of image 1 to image 2. The
plate scale is 0.57 arcsec/pixel.




Theta is the direction of the drift.




Notice the drift in RA is 4.2 arcsec and DEC is 0.9 arcsec. This is where I am
puzzled, the DEC drift I would expect because of a slight polar alignment error, but
why the very large RA error when this mount has absolute encoders?





My first thought is the encoders or clock is off, or there is a Southern Hemisphere
correction being applied that is opposite to what it should be.




Craig







Join main@ap-gto.groups.io to automatically receive all group messages.