Drift Graphs and PEC

[Fir Chlis]

I’m probably obsessing too much but am wondering if these drift graphs from Live Stacking are showing good/bad/expected tracking from my mount (Skywatcher HEQ5 Pro). I am getting good clear images from SharpCap Live Stacking, so I suspect that everything is ‘good enough’ for EAA.

I always get the SharpCap Polar alignment error to less than 1min, Good, and occasionally Excellent.

The two images are made from replays of raw frames, both around 10 minutes total exposure, one at 4s, one at 8s. My camera orientation is such that RA is approximately up and down, Dec is left and right. As I watch the frames replay, the latest position seems to move cyclically – up to the top, then down to the bottom, then moves up again. There's a clear matching pattern in the two drift graphs, with each of them starting at a different position in the cycle. I’m guessing that this is due to periodic errors.

As I understand it, the scale of the drift graph is in pixels, and it looks like I’m getting about 45 pixels drift in the Y direction, which corresponds approximately to RA. I think that the angle must be because my camera isn’t quite aligned with the OTA axis – plate solve reports ‘Orientation : up is 262.1 degrees E of N’.

With an ASI585MC on a Skywatcher 200P, the FoV is approx. 0.64deg x 0.36deg, so at 3840x2160 pixels that seems to be 0.6arcsec per pixel, a total drift of 27arcsec, or near enough half an arc minute. I’m wondering if that is expected for this mount.

Which then leads on to…if this is due to periodic errors, does SharpCap support PEC, or do I need to do it outside of SharpCap using EQMOD for the PEC training? And does anyone else here use /recommend PEC for EAA Live Stacking, or is it just chasing improvements for minimal return?

Cheers

Geoff

Drift 1.jpg (73.57 KiB) Viewed 1592 times

Drift 2.jpg (60.33 KiB) Viewed 1592 times

[admin]

Hi Geoff,

I think that your analysis is spot on - this looks exactly like periodic error in RA movement (and the direction matches up with the orientation of the camera).

The lack of any significant drift in the dec direction and no signifiacant rotation indicates that your polar alignment is pretty much spot on.

The maginitude of periodic error seems reasonable - sources online suggest that periodic errors of between 30 and 120 arc seconds are typical for consumer mounts.

The big question is, do you need to do anything about it, and if so, how. SharpCap does not do periodic error correction, but that can be done via your ASCOM driver typicallay (ie EQMOD). You only really need to do it if the drift *within* a frame is causing the stars to noticeably trail in the frame. From the density of the points on your graph I think that's unlikely at the exposures you are using, but you might need to reconsider if you go to longer exposures. Anyway, the stars in your sub-exposures are the guide as to whether to do anything about it ... if they are round, OK to leave well alone, if they trail out then try PEC or guiding.

cheers,

Robin

[Fir Chlis]

Thank you, Robin. As you say, I think I'd answered my own question, but it's good to get a confirmation.

My subs for EAA don't generally go above 30s, but a bit of experimentation may be worthwhile to get a feel for the maximum possible - though I'm reluctant to spend too much time on testing when clear nights are infrequent. May have to wait for a prolonged spell of good weather.

Cheers

Geoff