New Gem28 mount

[Rick Cohen]

I've been imaging with the iOptron GEM28 mount for a couple of months now.  I'm hoping to get better guiding than I have right now.  The RA seems to guide OK, but the DEC seems to be all over the place.  I seem to get better images when I turn guiding off for DEC.  

I'm imaging with a William Optics Z61 scope and a ASI183MC Pro camera.  My guide scope is a 32mm WO guide scope at a focal length of 120mm.  The guide camera is an ASI120 mono mini.

The last of the log files has a 30 minute guided period, followed by a 30 minute Guiding Assistant period, followed by a 30 minute guided period with DEC turned off.

This log was created at home in a Bortle 7 area on a night with transparency and seeing average to above average.

Any help is appreciated.

https://openphdguiding.org/logs/dl/PHD2_logs_DAVN.zip

[bw_msgboard]

Hi Rick.  It looks to me like the biggest problem is that the mount performs poorly whenever the direction of Dec guiding is reversed.  Here is one of many examples:

 

 

You can see that a sizable Dec excursion begins near the left and PHD2 issues 9 consecutive (south) guide pulses trying to correct it.  The mount shows no reaction during this time but on the 10th guide command, it abruply over-shoots and thus triggers another series of guide pulses in the opposite (north) direction.  This pattern happens repeatedly and the Dec guiding never stabilizes for any significant length of time.  This behavior can come from multiple interacting problems: 1) a backlash period when the drive system isn't fully engaged and isn't turning the Dec axis and 2) some form of resistance (stiction) on the axis that prevents it from moving until the resistance is overcome.  Stiction can be present because the axis doesn't rotate completely smoothly, or because there is a significant weight imbalance in Dec or because there are cables or other external components that are interfering - just to name a few reasons.  If the gears are fully engaged but the axis can't rotate, the motor is essentially pumping energy into the system that is then stored elsewhere - often by  deforming or compressing other parts of the mechanical system.  Once the static resistance is overcome, that stored energy is released and the axis then rotates more than the desired amount - and you get over-shoots like this.  These are mechanical problems that need mechanical correction, they're not something that can be "fixed" by guiding.  You can, however, improve the responsiveness of the Dec system by increasing the guide speed setting in the mount.  You're currently guiding at 0.5x sidereal so I'd suggest bumping that up to 1.0x sidereal or slightly less.  That will improve responsiveness for any backlash component and may improve the ability of the system to push through small amounts of static resistance.  You should also be sure the OTA is well-balanced in Dec for all pointing positions and that the cables are well-routed so they can't tug on any of the gear and interfere with its movement.

 

There is one other significant limitation to your overall system - your reliance on a tiny guide scope arrangement and the associated huge guider image scale of 6.45 arc-sec/px.  This puts you in a precarious position at best.  If the guide camera sensor moves by as little as 4 microns, you will see a very large guide star excursion of 6.5 arc-sec and probably a ruined image.  4 microns is less than 10% of the thickness of a human hair - so ask yourself if that little guider assembly is really rigid enough to not move that much as it traverses a 180 degree arc across the sky.  In terms of operational procedures, you should also be using multi-star guiding.  That means setting the option to do so and letting PHD2 select the guide stars based on the min-HFD and saturation values you specify - details are in the manual.

 

Good luck,

Bruce

 

 

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Rick Cohen
Sent: Tuesday, September 28, 2021 1:05 PM
To: Open PHD Guiding
Subject: [open-phd-guiding] New Gem28 mount

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[Rick Cohen]

Thanks, Bruce.  That's excellent information.  I will be sending the guide log to iOptron support to see if they have any answers for the poor DEC performance.  I'm pretty sure I had it well balanced and the cables weren't snagging.

I meant to use the multi-star guiding but missed setting it.  I've got clear skies coming in a couple of days and will try again.

As far as the small guide setup.  I plugged the numbers into astronomy.tools and it looks like replacing the 120mm scope with a 210 using the same camera would bring the image scale down to 3.68 arc-sec/pixel.  Is that more reasonable?  Or should it be even smaller?