4 replies to this topic

[waylander]

Hi everyone,

 

Now i seem to have more time to image i bought myself a ZWO OAG to use instead of my 50mm Guide scope. Main scope is 80mm altair ED doublet, ZWO 290mm guide cam. My mount is on the heavy duty pier.

 

Unfortunately i'm not able to get a successful PHD2 calibration no matter what i try.

 

My guiding has never been great -  usually 1.3" RMS with the guide scope no matter how good my PA was, so i thought perhaps i could get some improvement using an OAG - and also not have to set up the guide scope, dew heater, balance it etc. OAG set up was quite easy, focusing accomplished - "awesome" i thought, that's the hard bit done

 

I started with a new profile, put the main scope focal length in (454) and hoped for the best. Now it must be said i rarely do any form of polar alignment - I generally just align north and be done (slack i know) because i have found it not to make that much difference.

 

I've been getting orthogonality errors for almost every cal run (i tried altering guide rates to see if that helped) - you can see in the attached log. 

 

1. From reading the PHD2 manuals this might be backlash? And if so should i just make sure i run the max guide rate to minimise this? Anything else i can do?

2. Why am i seeing errors now, it was "OK" with the guide scope? Why cant i even calibrate at all?

3. Will PA align make more difference with the longer focal length?

4. Will my mount need "fixing"?

 

Many thanks to everyone in advance,

Clear skies, stay healthy!

Attached Files

•  PHD2_GuideLog_2020-04-02_191021.txt   76.59KB   5 downloads

Edited by waylander, 03 April 2020 - 01:54 AM.

[oshimitsu]

This is taken directly from the PHD2 user guide, I'd imagine you need to work on your polar alignment. Good PA, no excellent PA is crucial to imaging.

 

 

 Orthogonality error - the camera axes are normally computed independently even though they should be perpendicular.  The angle calculations do not require great precision, but if they are signfiicantly non-orthogonal, you should repeat the calibration.  If you see repetitive alerts of this type and the axes are significantly non-orthogonal, you'll need to identify the problem and fix it.  Common causes are bad polar alignment, large declination backlash, or large periodic error in RA.  Any of these problems can cause the guide star to move significantly on one axis while PHD2 is trying to measure its motion on the other axis. If you suspect these problems, go ahead and accept the calibration, then run the Guiding Assistant to measure your polar alignment error, declination backlash, and RA tracking error.  In other cases, the mount may not be moving at all, and the measured displacements of the star are just caused by seeing effects.  This sort of problem should be obvious in the calibration graph at the left of the dialog.  If the axis error is relatively small and you are convinced the hardware is working properly, you can avoid further alerts of this type by setting the option to 'Assume Dec orthogonal to RA' in the 'Guiding' tab of the Advanced Setup dialog. But you should do this only if the error is fairly small - otherwise, you are simply ignoring a serious problem .

Edited by oshimitsu, 03 April 2020 - 03:28 AM.

[waylander]

This is taken directly from the PHD2 user guide, I'd imagine you need to work on your polar alignment. Good PA, no excellent PA is crucial to imaging.

 

 

 Orthogonality error - the camera axes are normally computed independently even though they should be perpendicular.  The angle calculations do not require great precision, but if they are signfiicantly non-orthogonal, you should repeat the calibration.  If you see repetitive alerts of this type and the axes are significantly non-orthogonal, you'll need to identify the problem and fix it.  Common causes are bad polar alignment, large declination backlash, or large periodic error in RA.  Any of these problems can cause the guide star to move significantly on one axis while PHD2 is trying to measure its motion on the other axis. If you suspect these problems, go ahead and accept the calibration, then run the Guiding Assistant to measure your polar alignment error, declination backlash, and RA tracking error.  In other cases, the mount may not be moving at all, and the measured displacements of the star are just caused by seeing effects.  This sort of problem should be obvious in the calibration graph at the left of the dialog.  If the axis error is relatively small and you are convinced the hardware is working properly, you can avoid further alerts of this type by setting the option to 'Assume Dec orthogonal to RA' in the 'Guiding' tab of the Advanced Setup dialog. But you should do this only if the error is fairly small - otherwise, you are simply ignoring a serious problem .

Yeah i read that - i'm surprised though at just how much of a difference its now making now i'm using an OAG vs when i was using a guide scope? Is that normal?

 

I also tried metaguide but that failed calibration too...

Edited by waylander, 03 April 2020 - 06:00 AM.

[TelescopeGreg]

Yeah i read that - i'm surprised though at just how much of a difference its now making now i'm using an OAG vs when i was using a guide scope? Is that normal?

 

I also tried metaguide but that failed calibration too...

It's the same issue why a longer telescope is harder to guide.  Consider that the OAG is equivalent to using a longer focal length guide scope.  Longer focal lengths magnify the sky, and therefore any movement, or in this case, any incorrect movement, is also magnified.  What you were getting away with before is now visible and needs to be dealt with.

[waylander]

Right. Just been out. Managed a 1 minute pa error. Set guide rates to 0.9x. Awful seeing tonight with a bit of high mist and 85% moon. Managed to calibrate properly! Guide at 1.3”rms. not many stars visible though so that’s ok for the conditions I think. Didn’t bother imaging due to conditions. But it seems to be sorted now! Thanks all!!