Intermittent ‘shaky’ star images and spikes on PHD2 graph

[Bonnie Ryder]

I’ve had my AVX mount since 03.13.2020.  The mount has been running ok but when I recently switched my rig from a Nikon lens to the heavier ZS81 telescope I noticed some problems with star shape that needs solving.

The AVX’s PA & DEC axises have always been very stiff and an astrophotographer friend of mine (who built his own mount & is a machinist by trade) found out that I have been waiting over 6 months to have an AVX hypertune job done, with still no word when to send it in, he offered to take a look at it. He took it apart, cleaned, re-greased and reset the DEC gears that were off by over 1mm. The RA & DEC axises move much easier and I can balance it more accurately (slightly heavier to the east).  Motor seems to run much smoother also.

This helped with all of the problems EXCEPT for series of intermittent times on various targets when I get fairly regularly spaced images with “shaky” stars.  

Previously, PHD2 calibration had shown that the DEC measurements were terrible and suggested I use the DEC North guide mode.  I did this and the result was more extremely wild swings in the DEC so I went back to ‘auto’ Dec guiding mode. I put PHD2 into multi star guiding mode and when guiding was smooth it was very good, but still had the areas of ‘spikes’ that correlated with my ‘shaky’ star images. 

The PHD2 logs (included below) show the RA & DEC spikes which pretty much correlate with my images.  I also looked at the logs from Sept, before the mount was serviced and the spikes were there back then so that’s not the issue. 

Obviously not a problem with PHD2 but any suggestions or ideas what is causing these spikes (mount issue)?   I have more log files if anybody need more info.

Equipment: AVX, polar align with iPolar scope, Nikon D7100, Hotech field flattener, ZenithStar81 scope, ZWO 120mm guide camera with a ZWO 30F4 mini scope and use PHD2 and Plate Solve with Sequence Generator Pro running on Minix mini computer with Window 10 Pro.

Cross posted to CN (PHD2 forum) 

Here are the targets I did and times I started them:

8:00p Slewed to M2 started PHD2 calibrations for 8 minutes, ran it twice, ended at 8:35p

8:37p Slewed to Vega 2x60s

8:50 Slewed to M31 40x180s (2hrs)

11:03p Slewed to M45 40x180s (2hrs)

01:18a Slewed to M42 60x180s (3hrs) 

[bw_msgboard]

Hi Bonnie.  Are these the guiding spikes you're talking about? (RA in red)

 

 

I really don't think this has anything to do with the mount.  The biggest spikes are in Dec (green) and the Dec motor is mostly not running at these times.  The consistent intervals suggest there's something happening every 10 minutes or so that's causing unwanted movement in the guiding assembly.  Are these filter changes or something else correlated to your imaging?  Of course, all of this is amplified by the weak guiding assembly you're using and the huge guider image scale of 6.5 arc-sec/px.  These things have unfortunately become popular and they are really a headache for guiding.  Usually, they employ some kind of flimsy stalk-mounted finder-scope, Delrin-tipped thumb-screws, and an unsupported guide camera hanging off the back.  So we can quickly do the math here.  These "big" Dec excursions - say 15 arc-sec - correspond to a mechanical shift of the guide camera sensor of just over 2 pixels.  On your system, that translates to a movement of just under 8 microns, about 15% of the thickness of a human hair.  I think it's unlikely such a flimsy arrangement can prevent movements that small.

 

When you do get these large Dec excursions, the backlash in your mount makes it slow to recover. - over 1 minute in the worst cases.  You talked about having the mount serviced - have you measured the Dec backlash again with the Guiding Assistant?  If you increase the mount guide speeds up close to 1x sidereal, you will reduce the amount of reversal delay in Dec by nearly 1/2.  When you get a recommendation from the GA about uni-directional Dec guiding that is either 'north' or 'south', that appllies to the current pointing position of the scope.  The direction of drift will change at some point in the sky so this can't be a once-and-forever setting.

 

All of that said, I think the place to start is to get a better arrangement for a guiding assembly, one that has a guider image scale of  3-4 arc-sec/px with very rigid mechanical fittings.

 

Good luck,

Bruce

 

 

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Bonnie Ryder
Sent: Monday, November 08, 2021 6:56 PM
To: Open PHD Guiding
Subject: [open-phd-guiding] Intermittent ‘shaky’ star images and spikes on PHD2 graph

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