PHD - Non-orthogonal error

[frugal]

When I calibrate PHD2, I get the following warning at the end of calibration:

"The RA and Declination angles computed in the calibration are questionable.  Normally, these angles will be nearly perpendicular, having an 'orthogonality error' of less than 10 degrees.  In this calibration, your error was 15 degrees. This could mean the calibration is inaccurate, perhaps because of small or erratic star movement during the calibration."

Looking at the graph it shows on the warning window, the RA and DEC steps look like they are in a nice straight line, just not at 90 degrees to each other. I can drift align and get the polar alignment down to <1', but on recalibration I still get the error.

Reading around on t'interwebs there does not appear to be much information about the causes of this warning, just that you can check the "assume they are actually 90 degrees apart" check box (which smacks of taking the car to the garage because the engine management light has come on to be told "this means there is something wrong with the engine, here is a bit of tape to put over the warning light").

Does anyone have any ideas about the causes of this particular error? I am using an AZ-EQ6 mount with the scope and guide scope side by side on an ADM dual saddle. I have checked that everything is tight, and that the cables are not pulling. The guide scope is an ST80, and the guide camera is a Lodestar X2.

[glowingturnip]

i think they've set the nag level too low on that warning, I regularly see errors of about 12-15 degrees and the resulting guiding is fine.

It's designed to trap situations where there is a clear lack of orthogonality, I've had 50 degree errors before, and even one that was close to 90 degrees, so that it thought RA and Dec were in the same direction as each other.  Needless to say, guiding was not fine then.  You get another nag if your orthogonality error is different to the previous orthogonality error too, even if it's now an acceptable one.  Sigh

You can see what it actually looks like if you select the RA/Dec overlay in one of the menus - resulting grid should be rectilinear-ish

[frugal]

Stuart, 

But what causes the lack of orthogonality? If it is raised as a warning, then it must be because there is a problem. I am trying to understand what the underlying problem could be in order to rectify the situation.

[IanL]

If you haven't got it already, grab the PHD 2 log viewer tool here:

http://openphdguiding.org/

One of the things it can do is give you a nice view of the calibration points plotted on a radar graph similar to the one in the guiding application for guide star positions. It might give you a clue as to which of the two axes is causing the problem and how. This will be easier if you have the camera sensor orthogonal to the RA an Dec axes before calibrating. If all is well then the outward steps and inward steps should be in a line on both axes.

I'd suspect polar alignment causing drift and thus 'pulling' or both axes off square, especially if you are taking a lot of steps or quite long guide exposures allowing the drift to build up. In that case a I think you would see the line of outward calibration points at an angle to the inward points on one or both axes.

If not that then maybe flexure in the guide scope mounting or focuser perhaps. In that case you might see a curved line of steps or a straight line at more or less than 90 degrees to the other axis.

I would expect bad seeing to average out over the course of calibration so less likely to be that.

[glowingturnip]

just thinking out loud, as it were, but couldn't RA backlash have something to do with it ?  To complete the RA calibration, PHD is pushing the mount east, and then there'll be a moment where the mount is in backlash and will drift very slightly in RA until the tracking takes over and starts pushing west again.  If phd takes its first north step while it's still in RA backlash, then that step would look non-orthogonal.

[frugal]

Ian, 

Thanks for the pointers. I had not seen the PHD Log Viewer application before, I had only known about the PHD Lab application that does something similar, but does not show the calibration information like this.

Here is a couple of images of a previous guiding attempt from 26th Feb, I do not have any more recent on this machine. The next time I get the problem I will make sure to transfer the log off of the laptop.

Looking at it in the log viewer, I can clearly see the return steps coming back along a different path than the outgoing steps.

The guide trace looks like this:

Certainly not the sub arc second that other people seem to be getting

[Phillyo]

Hi Frugal,

I have been getting this error occasionally and I was wondering if you ever managed to get to the bottom of the issue?

Cheers,

Phil.