Is there a way to measure the length of a star trail?

[j.w.]

Nebulosity has a very nice feature that allows one to measure star trails to quantify tracking error ("Measure Distance").  Is there a way to do that in PI?

Thanks,
John

[pfile]

well, dynamicPSF will model the star trail as an ellipse and then tell you what it's eccentricity is... is that good enough?

if you are interested in grading your subs, you should use the SubFrameSelector script from mike schuster. i think juan is very shortly going to add this to 1.8RC7 and push it as an update. of course, SubFrameSelector uses the same PSF modelling code to come up with a good average FHWM for your image and then you can accept/reject frames based on FHWM or one of many related attributes...

rob

[dayers]

I have been using DynamicPSF for this purpose also, using the Aspect Ratio (a) metric. Is this what you mean, Rob?

My rule of thumb for "good enough" guiding is stars that look round when viewed at 1:1, plus  a > 0.9 for summation of a dozen or so PSF measurements.

Dave

[j.w.]

Thanks Rob and Dave.  The DynamicPSF might fit the bill.  What I'm trying to do is quantify poor tracking (representative attached) as I try to work with the mount manufacturer to isolate and fix the problem.  I'll take a look at both tools though, as Mike's SubFrameSelector would make it easier to crunch through a batch of images collected during a sample run.

John

[pfile]

for that, if you know your image scale then you can just measure the length of the star trail and directly say what the error is in arcseconds.

is that just periodic error? what kind of mount is it?

[j.w.]

Yep, that's exactly what I had been using Nebulosity for.  I do all of my image processing in PI though, so I had hoped I could do the length measurement as well.  In playing with DynamicPSF, the length measurements are shorter than the manual measurements I'm taking in Nebulosity.  I probably just need to read through the documentation for DynamicPSF more thoroughly.

It's not PE (although there undoubtedly is some included, but a much smaller component of the overall error), but it's tracking error of some kind with my Paramount MX.

John

[pfile]

i think the ability to read out coordinates and maybe dx/dy in arcseconds of an image with a proper WCS header was requested by someone but i'm not sure what juan said in response.

[Andres.Pozo]

i think the ability to read out coordinates and maybe dx/dy in arcseconds of an image with a proper WCS header was requested by someone but i'm not sure what juan said in response.

The readout of coordinates can be currently done in the preview window of AnnotateImage.