Issue on Dec guiding

[Jean Baptiste Marquette]

Dear PHD2 gurus,

I work with a Meade SCT 12", OAG guiding on a Losmandy G11 mount. The calibration shows an issue on DEC I don't understand. The South backlash is constant.

Link to the log: https://openphdguiding.org/logs/dl/PHD2_logs_qG2a.zip

Upgrade to latest version has no effect.

Thanks for your expertise,

Clear skies,

JB

[Bruce Waddington]

There's a major problem related to the Dec axis on the mount that effectively prevents it from being guided in the southern direction.  This shows up in all the operations you performed with the system.  During calibration, the Dec axis moved normally to the north but then never moved at all when the calibration tried to switch to south guiding in order to put the guide star back to its starting position.  The backlash test you did with the Guiding Assistant makes the situation pretty clear:

The red data points show the mount response to 27 consecutive guide commands to the 'north'.  The green data points show how the mount responded when the direction was reversed and another 27 consecutive guide commands were issued for south moves.  As you can see, the mount didn't respond at all.  Each pulse lasted for 0.5 seconds, so the total series of 27 guide commands south represented a total of 13.5 seconds with no movement.  This is almost certainly a mechanical problem that you will need to fix.  It could be due to a very large amount of "slop" in the Dec drive gears which should be easy to determine.  It might also be caused by something else like a loose set screw or some other fixture/fastener in the drive system that has become loose.  In either case, the mechanics needed to be corrected.  That said, before you start tearing into the drive system, you should be absolutely sure that there isn't a cable routing problem of some kind that's interfering with the ability of the Dec axis to move smoothly and freely in both directions.  You also need to be sure that the payload is well-balanced in Dec and the Dec axis clutch is tight.  The Meade SCT is fairly heavy so if it isn't well-balanced in Dec, it can create these kinds of conditions.  Also, be aware that successful slewing of the mount doesn't tell you anything useful about its ability to be guided because the amount of physical energy used for small guide commands is typically hundreds of times smaller than the energy used for slewing.  

Good luck,

Bruce

[Brian Valente]

JB

What vintage is your G11? Does it have the spring-loaded worms?

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Brian 

Brian Valente

astronomy portfolio https://www.astrobin.com/users/bvalente/ 

[Jean Baptiste Marquette]

Hi Bruce,

Thanks for your analysis, I fully agree. I have to explore the suggestions you gave.

Clear skies,

JB

[Jean Baptiste Marquette]

I don't know exactly, as this G11 is... a gift! But I upgraded it to Gemini 2, with the appropriate motors and gears.
It worked like a charm, with my previous Newton Sky-Watcher 250 Quattro, with a piggy back guide scope.

Then, your assumption on cable routing and balancing is the first to check!

Clear skies,

JB

[Brian Valente]

>>> a gift! But I upgraded it to Gemini 2, with the appropriate motors and gears.

if it pre-dates the Gemini2, you upgraded the motors but you still have a regular worm gear (not the spring-loaded one. you would have to have sent in your mount to Losmandy for that update). Sounds like the worm mesh needs to be revisited on your Dec axis

If you have the one piece worm, they have a video for that: https://www.youtube.com/watch?v=IiQlTMaUXkg

if you don't know, you'll probably want to contact losmandy for direction 

Brian

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