5 replies to this topic

[ChrisMoses]

Hi,

My mount doesn't have PEC, but I would like to measure its PE.

What I am thinking is to point the mount at the meridian and dec=0 and shoot an unguided image for a few worm cycles. My expectation is that the PE will show up as oscillations in star images.  

 

Does that sound reasonable?

 

From there, I would like to be more quantitative. By recording the above in Phd2, in Guiding Assistant Mode, I should be able to see the PE in the Phd2 graph.  Right?

 

From there I can open the data in PhdLogViewer and see what is, essentially, an FFT of the PE.  RIght?

 

Any tips or thought?  Any other programs that would be good to use?

 

Thanks,

Chris

[einarin]

https://www.cloudyni...tracking-error/

[DuncanM]

Check out EQMod's Pecprep as well.

[ChrisMoses]

Thanks for the tips.

 

The link above suggests that I should intentionally introduce some RA error in order to be able to see the PE curves on the star trail.  That sounds reasonable, right?

 

Maybe I'm just tired or being slow, but can anyone think of how to quantify the drift from the images. In other words, suppose the peak to peak is 10 pixels.  At my pixel scale of 1.2"/pixel that would be 12". But doesn't that need to be corrected? Or am I just overthinking this?

[WadeH237]

You don't need any software to measure periodic error.

 

Just polar align so that you are intentionally a few degrees from the pole.  That will induce declination drift.  Now if you take a long exposure, your stars produce trails.  You will notice that the trails are not straight - they are squiggly.  Those squiggles are the periodic error.  If you know your image scale in arc seconds per pixel (easily calculated), you can determine the amount of periodic error by counting the width of the squiggles in pixels.  You can also tell the shape of the periodic error curve from the shape of the squiggles.

[DuncanM]

This might help:

 

http://www.sfu.ca/~dmunro/petest.html

 

It's best to use Dec = 0.