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Old 18-09-2014, 09:14 PM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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Join Date: Jul 2013
Location: Kilcoy, QLD
Posts: 2,058
Quote:
Originally Posted by SamD View Post
You could try close to 90 deg declination in Octans, and even turn off tracking on the mount. This should show how much PHD wobble there is for stars which really are stationary in the sky.
Thanks mate :-)

Quote:
Originally Posted by Shiraz View Post
Crikey Lee, you are having big problems.

Your original graph showed that the RA is not correcting properly - it gets into a no-mans-land where phd2 is generating lots of correction signals but the mount is not responding. Provided phd is generating normal corrections signals and your mount autoguide speed is set at 0.5x, that still suggests backlash to me (ie the motor is moving, but the gearing is not transferring that motion immediately to the wheel). If you cannot feel any worm slop, it could be due to the internal transfer gears. As others have said and you have tried, you should be able to manage this by offsetting the weights to make it east or west heavy, but the offset weight must be sufficient that it overcomes stiction (ie the mount moves freely under gravity when the clutch is released). If that doesn't help, there is not much left mechanically, apart from bearing preload. If the bearings are too tight, you could get jerky motion - I recently improved my DEC tracking by backing off the bearing preload a bit.

If you are looking low down east, you will be viewing through lots of turbulent atmosphere. The turbulence could have an RA-worse component due to slowly rising plumes of warm air (they don't move sideways, so have less effect on DEC). Maybe do a test up near the zenith and see what happens when you are looking through less air. Imaging below 45 degrees will be a lost cause most of the time anyway due to turbulence, unless your system is heavily undersampled.
That's very interesting actually. I've definitely been below 45 degrees except when drift aligning az. I did try tonight and all kinds of angles and directions, including near zenith and at -90 dec but I still see stars bouncing around.

Quote:
Originally Posted by Amaranthus View Post
PEC should only effect RA, not DEC! Strange. I assume you were guiding when you recorded it - maybe there is an iterative/circular problem here...
Yeah I thought that was one of the more interesting things!

Quote:
Originally Posted by Amaranthus View Post
I've been mulling over this problem Lee and something Ray mentioned, has led me to offer another suggestion. The fact that you're seeing weirdness even when PHD is not active suggests that maybe your mount is tracking too quickly (or slowly)...

You've tried changing the maximum duration of the guide pulse in RA to no avail, but what about the minimum? That is, how about adjusting the Guide Speed (0.nn x sidereal rate) downwards, to say 0.25x or even lower the sidereal rate in EQMOD. After all, guiding on the RA axis works by either increasing or slowing the standard guide rate, so as to avoid issues with backlash (DEC works differently, which is why an algorithm to correct for directional shift, such as resist switch, is appropriate).

If this doesn't work, try ramping up the sidereal rate to 0.75 or 1 (or more) -- assuming that you've currently got it on the default of 0.5x. Experiment, and see if this helps...
Thanks mate, I'll look into that as well :-)

Here's some more charts captured over the past couple of days. I just saw a thread where people in SE QLD were complaining about bad seeing lately, so maybe that actually is a large contributor to my problems. Hope so!

First graph is when I was recording the PE the other night, with mostly default PHD2 settings.
Second is close to the zenith with guiding/tracking enabled.
Third one is close to the zenith with guiding disabled tracking enabled.... note the scale on this one!
Final one is with the QHY5L-II directly in the focuser, not in the OAG... best one, but still not good.
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