Bad backlash in dec?

[jdd...@gmail.com]

I've noticed that after dithering, my dec axis takes a good while to get back to "normal" and I'm guessing it is backlash. But, I have run the guiding assistant and accepted the parameters. Any other things to check?

[Brian Valente]

Hi jddias

You are correct that is dec backlash. You should measure it via the guiding assistant and enable it (as will likely be the recommendation from the GA)

Otherwise things look pretty good

On Wed, Apr 9, 2025 at 9:16 AM jdd...@gmail.com <jdd...@gmail.com> wrote:

I've noticed that after dithering, my dec axis takes a good while to get back to "normal" and I'm guessing it is backlash. But, I have run the guiding assistant and accepted the parameters. Any other things to check?

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Brian 

Brian Valente

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[Brian Valente]

@JDDias

Bruce was looking at this as well and suggested an alternative explanation: you may be experiencing stiction. See the example below where the correction happens, but then there is an equal amount of overshoot.

If your Dec axis is too tight, there may not be much backlash and instead it may be stiction.

Have you attempted dec axis adjustments in the past? It may be worth re-examining the dec axis for proper tightness. 

On Wed, Apr 9, 2025 at 9:16 AM jdd...@gmail.com <jdd...@gmail.com> wrote:

I've noticed that after dithering, my dec axis takes a good while to get back to "normal" and I'm guessing it is backlash. But, I have run the guiding assistant and accepted the parameters. Any other things to check?

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[jdd...@gmail.com]

Thanks Brian - the mount (a Losmandy G811) is at a remote observatory so I haven't touched it personally, but I did as the staff to return the scope to home after it got out of whack one night. I went and looked at a log from before that and the same problem seems to occur (attached log for reference).

So, I guess you're suggesting that the dec clutch could be over tightened?

[jdd...@gmail.com]

OK, they backed off the dec clutch, but didn't seem to make a difference. Seems to over correct dec and then settle back down after a couple of minutes.

[Brian Valente]

Did you also follow my original suggestion regarding testing and enabling backlash?

Brian 

Brian Valente

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

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[John Dias]

Ah, rats no - got excited about the clutch possibility. Will do tonight.

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[jdd...@gmail.com]

OK, so ran guiding assistant with backlash check enabled. Seems to have improved some but not completely.

[Brian Valente]

(not sure your name?)

The guidelog you posted shows backlash compensation was not enabled, so you need to enable it to see any value

Also your polar alignment has gotten out of whack, >10 arcmin. It also shows this in your guiding as nearly all of your non-dither corrections are in one direction

Regarding the stiction, the clutch does not impact that. The Losmandy mounts have the spring-loaded worm, so chances are the spring tension may need to be adjusted.

Brian

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[jdd...@gmail.com]

Brian, this is just a follow up to say thanks and to confirm my issue is much improved (to the point where I'm happy with it).

After a long and arduous journey with the remote observatory staff which included PA, a bad/dirty DEC cable and a string of cloudy nights, I was able to confirm last night that PA is much better at 1.3' and the dither settling in DEC is much faster and less "out of whack"

Logs attached for perusal, thanks again.

John Dias

[Brian Valente]

it looks much better. 

One thing to confirm is the amount of Dec backlash. it was set at 20ms, although I think it has adjusted itself higher. Maybe set it to 800ms and see how it goes.

If settling in Dec ends up being more than you'd like, you can also dither in just RA as another option (setting in PHD)

Brian

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