GEM45 issue

[Chris Gittins]

My GEM45 has not been guiding properly since the spring.  Last fall it worked fine with an RC-6 plus OAG w/ASI220.  More recently, I tried with an AT115 (and same OAG+camera).  The result was the same:  It wouldn't calibrate.  RA and DEC axis were nowhere near orthogonal and the RA-DEC angle varied wildly from one calibration attempt to the next.  This past weekend I created a new profile for the AT115, again attempted to calibrate, and after that failed ran a Star-Cross test.  I've attached two representative images - one showing representative crosses and another with a zoom-in on one of the crosses showing the excursion in RA.  

Guide logs from 8/22 and 8/23/2025 - https://openphdguiding.org/logs/dl/PHD2_logs_4Whf.zip

Thanks,

Chris

[Bruce Waddington]

I think the actual calibration shows a clearer picture of what's wrong:

RA is in red, Dec is in green.  The most serious problem is in RA where you see this "clumping" of data points which tells us the axis isn't responding smoothly to guide commands.  The first guess is that the RA drive system is substantially over-tightened, but it could be some other mechanical problem with that axis. In the other log you sent, the RA axis actually started moving in the wrong direction, further suggesting over-tightening.   In declination, a reversal of guide corrections from north to south produced almost no movement at all (arrows to the right).  This could be caused by a similar problem on the Dec axis but it could also be caused by a gross weight imbalance of the payload. If you show this graph to the iOptron techs, it may help them track down the problem.

Good luck,

Bruce

[Brian Valente]

Chris

Just to add to Bruce's comments, these kinds of mount issues can be reported as an orthogonality problem, but really it's the axes are not responding correctly. Once you sort them out, the ortho error reported during calibration will likely be fixed as well

Brian

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Brian 

Brian Valente

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[Chris Gittins]

Thanks for digging into this.  I looked at the calibration data but didn't know what to make of it beyond, "This looks bad."

For what it's worth, I've attached an image of calibration data from last summer.   The stats for 45 minutes of guiding immediately afterwards were 0.56" rms in RA and 0.37" rms in DEC.

[Chris Gittins]

Update:   iOptron serviced the mount but the issue persists.   One apparent difference between pre- and post-service is that the failure mode is now reproducible, i.e., calibration graphs look awful the same way on each attempted cal.  

• Link to guide logs - https://openphdguiding.org/logs/dl/PHD2_logs_Vt2Y.zip
• Star-Cross test images still have same characteristics - deviation in DEC while scanning RA.  Pattern was the same looking west as well as looking north.

Service done by iOptron:

• DEC axle: loose, tightened.
• USB- elated boards (USB hub, CMS board) upgraded/modified
• USB 5-wire internal connection cable replaced with shielded one (from CMS board on rear RA to USB hub under dovetail saddle).
• RA axle: very loose, adjusted.
• RA worm rotates uneven, two RA worm bearings rusty, replaced.
• Firmware upgraded

The iOptron techs were excellent about communication and worked on the mount immediately in their initial attempt at repair.  Fingers crossed they're able to resolve the issue when I bring it back.

Chris

[Chris Gittins]

Update 9/7/2025:

I've corresponded with iOptron but haven't brought the mount back for additional service.  They suggested that I try using the ST-4 port.  I plan to do so next time the weather is agreeable.  

[Bruce Waddington]

Well, if you do that, you need to make adjustments in PHD2 in order to not get all fouled up.  Create a new profile specifying the mount connection as "On-Camera' and then use the Aux-mount option to connect to the iOptron mount driver, the one you've presumably been using all along.  You also need to do the calibration for the new profile using the Calibration Assistant.

Bruce

[Chris Gittins]

Hi Bruce,

I will create a new profile.  I've been reading PHD2 documentation and related forums and am feeling reasonably confident that I can get an ST-4-based configuration running.

My thoughts on the situation:

1.  PHD2 works fine and I've been using it correctly.  (Three cheers for Calibration Assistant and the Star-Cross test!) 

2.  It's a mount issue.

3.  It's not clear that switching to ST-4 will fix the problem or provide useful diagnostic information.  (Go-to and tracking work fine.  That suggests the mount is receiving messages correctly.  That those functions work but it won't guide suggests a mechanical problem, e.g., such as what you described above.  If ST-4 works that will suggest it's a command/message issue but observed behavior to date makes that seem unlikely.)

4.  I don't know how to fix the mount.  iOptron service seems my best bet for getting it fixed.  With that in mind, they asked me to try ST-4, so I will.   

5.  This is frustrating.

[Bruce Waddington]

Hi Chris.  Keeping in mind what "free advice" is often worth, I'll make a couple of additional observations on this.  Unless you've taken some extra steps I don't know about, I don't think I can know how to distinguish between the Dec and RA traces of the star in your main camera.  Based on personal experience, I'm going to guess that the problem occurs when the Dec guide direction is reversed and you are getting a displacement in RA as a result.  I had that specific problem with my mount after I got it from a previous owner and it needed to go back to the manufacturer to identify and fix the problem.  It was a Dec bearing that had an insufficient pre-load and was allowing the axis to "rock" when the Dec direction reversed.  It ended up being nothing more than an adjustment.  Looking again at what iOptron did:

• DEC axle: loose, tightened.
• USB- elated boards (USB hub, CMS board) upgraded/modified
• USB 5-wire internal connection cable replaced with shielded one (from CMS board on rear RA to USB hub under dovetail saddle).
• RA axle: very loose, adjusted.
• RA worm rotates uneven, two RA worm bearings rusty, replaced.
• Firmware upgraded

So they replaced two rusty RA bearings but didn't do anything with Dec other than tightening.  Tightening, by the way, could tend to make the problem worse if the bearing has a problem.  If the RA bearings were rusty or corroded, why wouldn't the Dec bearings have the same problem?  I don't know, I'm just asking.  

Anyway, I agree with you that the ST-4 test is worth doing for them but will probably be a swing and a miss.  I think it's good they are taking your problem seriously and are working with you, so it seems there is reason for optimism.

Good luck,

Bruce

[Chris Gittins]

Hi Bruce,

Thanks for your observations.  Looking closer at the star-cross image and associated still, I think you're spot on that the problem occurs when the Dec direction is reversed.   

I looked up the coordinates of the brightest stars in Stellarium to establish RA & Dec.  (I centered on the Blaze Star/HIP 78322 for the test so easy to look up stars in the neighborhood.)   I annotated the attached image accordingly.  There's some drift in RA when scanning north-to-south but the switch from south to north is where it takes off.   I'll mention that to iOptron and send them the image too.  (If I can find time tonight I'll fix it so that north-south is up-down and east-west is left-right!)  

I'll also ask about the DEC bearing and potential overtightening.

Regards,

Chris

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