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Diagnosing tracking errors with IRIS

Posted: Fri Sep 09, 2016 3:13 pm
by rwilkinson
I've just been helping out one of our Members who's been having tracking problems with his EQ mount.
Having carefully aligned his polarscope with the mount's axis (followng the process outlined here: ... finder.pdf), he took care to get Polaris "in the bubble" at the start of the night's imaging, but then was dismayed to find his target drifting steadily across the field of his sensor.

Thankfully he'd saved all his images, so we were able to load them into IRIS and use the TRACK command to generate a STAT.DAT file which measures the displacement of a chosen star through the image sequence. Then we read this into a spreadsheet and plotted the errors along the RA & Declination axes:
trackerror.png (24.45 KiB) Viewed 6056 times
This is exactly the same sort of plot which is produced by autoguiding programs whilst they are running, but in this case we were using IRIS to do a retrospective analyis of tracking from a few nights before.
And this proved to be a classic example of "declination drift" (with a little field-rotation and a small periodic error along the RA axis).

This is symptomatic of polar mis-alignment, but how could it be so far out after he'd used his polar-scope? Well, this time it wasn't another case of using the "wrong pole-star", but a more systematic error - he'd fitted a diagonal mirror to his polarscope, and not accounted for its vertical flip of his view of the reticle. Therefore he'd positioned the "bubble" at the wrong angle before carefully aligning Polaris into it! :roll:

So the moral of this story is: always save your data, even when things are going wrong - they can often help with subsequent diagnosis of the problem. :idea: