eqmod PEC problem

[alacant]

EQ6 + EQMOD

Hi everyone
I'm forwarding this message from the PHD2 and EQ6 lists, hoping someone has been here before. Apologies if this duplicates it for any of us.

I recorded an 8 cycle PEC curve under EQMOD. It autoloads fine upon starting tracking. It seems however that it is not always being applied. When it is applied it works well, even when being thrashed at regular intervals by the dither:
https://drive.google.com/open?id=0B_uUt0LyuuEdcGI4YmZuVmtFTVk
At other times it has little or no effect; as if it's not being applied at all. PHD2 still has to correct the PEC peaks:
https://drive.google.com/open?id=0B_uUt0LyuuEdcmJOX3lLTDBGWGs

I can see the 'sidereal + PEC' activated throughout and the mount issuing corrections, but it seems to apply the PEC only intermittently.

Any pointers as to what to look out for to get the PEC applied correctly and consistently most gratefully received.
TIA

[chrisshillito]

If EQMOD is showing a rate correction (as it is in your screen shots) then that correction is being applied to the mount - so PEC is active. The fact that it may not be effective could well be down to the residual error. A single cycle PEC playback curve can only ever applies an average correction and  there will always be a residual error - If you load your recorded data into PECPrep you can see a graph of how well the PEC curve can correct your actual data - and what the residual error is.

Your PEC curve shows some very significant oscillations (an ideal mount would produce a pure sine wave) and if these are not harmonics of the worm itself they will slip in phase over time. A single sycle PEC curve cannot correct for this and, as the PEC curve slips in and out of phase with the errors this could explain why you get apparently intermittent results. Again, by using PECPrep to analylse your raw capture data you should be able to identify the period of these oscillations and if they are not true worm harmonics then it would be best to use PECPrep to generate your PEC curve and use its filters to remove those oscillations from the PEC curve.

Chris.

 

[alacant]

Thanks Chris. I'll have a look a pecprep-

Any idea what this playback thing's for? Where did the setting come from? I never changed it. In fact apart from the capture cycles and auto apply pec bits, the rest is a mystery...

[chrisshillito]

The playback settings affect how the PEC curve is applied to the mount. Setting the gain control to <1 will reduce the level of correction proportionally , the phase shift changes the phase of the correction (you wouldn't usually change this unless you were pretty sure your playback was out of phase).

The filter settings control how your capture data is processed to produce the PEC curve. The low pass filter is in seconds and means that error signals with a period of 30 seconds or less will be filtered out of the correction curve. The magnitude filter is filtering out any error components in the frequency spectrum that have a magnitude of less than 10%.

Chris.