wondering about the wandering

[David Ridge]

I have been experiencing sub .70rms guiding fairly consistently but on occassion, my mount just refuses to settle down, zigging and zagging for 30-60 seconds it seems. recent calbration and guiding assistant runs successful but for no apparent reason, it just goes into what appears from the guiding graph to be a dithering but during the middle of an exposure, several exposures in a row! ARRRGH!

Thoughts?
https://openphdguiding.org/logs/dl/PHD2_logs_UpgJ.zip

[Bruce Waddington]

This is really starting to feel like Groundhog Day.  All of your current problems occur when the mount has to be bumped and is apparently responding poorly to guide commands.  Your good stretches of guiding happen when the AO is able to control everything without bumping.  A typical guide session looks like this:

In all cases where the wild oscillations are present, the mount was trying to handle bump corrections.  So let's roll the clock back to June when I said this:

"It doesn't seem to me that you're making any progress with this.  I suspect the underlying performance of the mount is so bad that using an AO isn't a viable option.  An AO is not a solution for a bad mount because it has a limited range of motion.  The only way to test this hypothesis is to do extended testing of the mount without the AO.  Continue to use the OAG but use it without the AO.  If you follow the attached procedures for getting a measurement baseline, it should provide more insight into the problems.  You need to do long-enough guiding sessions on both sides of the pier to demonstrate whatever problems the mount has.  After you've created the new profile using the new-profile-wizard, do NOT change any of the default guiding parameters.  Be sure you specify the correct focal length of the system in the profile wizard in order to avoid wasting a lot of time. "  

Did you ever do this?  I can't find any follow-up from you showing the results of the baseline measurements without the AO involved.  I've seen messages about improving balance and other assorted "ah-ha" moments, but I still question whether you've been able to get this 30-year-old Temma mount working properly.  Has it been cleaned and lubricated regularly?  Has it been used frequently during the 30-year period or is it likely the bearings are corroded and not working right?

Regards,

Bruce

[Bruce Waddington]

This is the response from David, it was sent to my private e-mail by mistake:

Here's 9+ hours of "unassisted" (sans AO) guiding from the moonless night last night. New profile, Calibrated, GA - recommendations applied. Purchased the rig a year ago. Maintenance history unknown.
 

https://openphdguiding.org/logs/dl/PHD2_logs_XiTZ.zip

Thanks

 

David

[Bruce Waddington]

Hi David.  Did you get the e-mail I sent you with some follow-up questions?  I've spent some time looking at your guiding sessions without the AO and they are interesting.  But I would like to know if you've serviced the mount in the time you've owned it - by that I mean at least cleaning and re-lubricating the RA and Dec drive systems.  Also, can you confirm that the mount controller doesn't have non-zero settings for RA and Dec backlash.  If I have more information, I can probably give you more useful feedback.

Bruce

[David Ridge]

Questions from Bruce W: I’ve (Bruce) re-posted your response on the forum so I’m not the only one who can look at it.  Can you send us (on the forum) a couple of pics of your imaging rig, telescope+mount, in addition to a description of your main imaging system, telescope+main camera.  In the time that you’ve had the mount, have the drive systems been cleaned and re-greased?  Was the original owner an imager or don’t you know?

 

Thanks

Bruce

Bruce: I bought this scope from my brother who used it some for imaging but, I think, mostly for observing. I doubt he had any servicing done on the mount. It was too large for him to move around and did not have a a pier mount so it sat mostly unused for 10 years or so. I have not done any servicing on the mount in the 12 months that I've had it.

The imaging system is a Tak 130NFB with a Player One Poseidon OSC, Player One OAG with filter tray and a QHY5III178 guide camera. I have a Tak 645 field flattener and the Starlight SX-AO attached (disabled for this baseline guiding run). As for the mount countroller, I've never used it so I don't know. The mount driver does not.

[David Ridge]

[Bruce Waddington]

Hi David.  This looks like the telescope and mount is permanently mounted on a wooden deck, exposed to the elements?  Is that right?  And is the deck elevated above ground level by a considerable amount?  And the mount is attached to a pier by a tall wooden structure of some kind?  Do you disassemble and re-assemble everything for each imaging session or do you simply use the scope in this location?

I'm still working on your log files but I think it's going to be very hard to make a clear diagnosis, especially considering all these other variables.  I can say there is quite a bit of evidence for substantial stiction at least on on the Dec axis.  Given what you've told us, it seems equally likely that the mount is in need of service - removal and replacement of dried grease, possibly servicing of bearings, etc.  When I asked you before about a backlash compensation feature in the mount controller, I didn't get a clear answer.  That would be set through the hand-controller, not the driver.  So I think you should read through the user manual for the hand-controller, plug it back into the mount, and work through the menus to be sure any backlash compensation settings are zero.  If your brother was using it mostly for viewing, it's quite likely he could have used those settings, that's really what they're intended for.

Regards,

Bruce

On Sunday, September 21, 2025 at 11:25:25 PM UTC-7 dridg...@gmail.com wrote:

[David Ridge]

The scope is permannently mounted on a 10" concrete pier, fortified with 1/2" rebar poured and formed to broad base underground. the pier is about 8' plus 3'x4'x(8-10") base with another 6" spur beneath the pier itself. the wooden extension is anchored to the pier with 6" bolts into the pier. the pier is, of course, isolated from the deck structure by a 1" gap and the deck is rarely traveled during imaging sessions -- it is handled from my office via wifi.

I'm not finding any documentation,either in my manual or online, on setting backlash comp on the Temma2M mount.

THANKS

David

[Bruce Waddington]

I’m sorry, I don’t think I can do much more with your situation, the instability of the mount seems to vary dramatically from one night to another.  I’m fairly convinced that you have a massive amount of stiction on the Dec axis that goes well beyond the ability for guiding software to control.  And as I said in an earlier message, I don’t think the AO has any real hope for handling this well. Here’s a look at one of your recent backlash tests:

 

 North moves are on the left (red) and south moves are on the right (green).  The south section tagged as “1” shows an initial, reasonably quick response to the first few south guide commands.  But this is followed by a long section tagged as “2” where there is no rotation of the Dec axis at all.  This pattern is what we would expect if there was backlash compensation enabled in the mount which is why I kept asking about it.  But the “2” section is clear evidence that there is a significant mechanical problem, probably stiction.  Based on everything you’ve said, I’m inclined to think the mount is in need of service but I can’t say for sure that even that will fix all the problems.

With regard to the photos you sent, I have to say I haven’t ever seen anything like this.  I can imagine you had substantial constraints on where you could locate the scope and you did what you could to get the job done. But it looks to me like a very unstable arrangement and I don’t think the mount can ever perform up to its design goals in this context.  No doubt it all feels very solid and with normal-life construction standards, perhaps it is.  But you need to internalize the measurement scales we’re working with in guiding.  Take a look at the first picture you sent, the one that shows the large refractor sitting on top of your wooden mounting arrangement.  If that camera at the right side of the telescope moves by 10 microns, you will see an immediate 2 arc-sec guiding error. That’s 20% of the thickness of a human hair.  If you aren’t familiar with micron-level distances, take a look at a metric ruler – look at adjacent 1 millimeter marks and imagine further sub-dividing that by 1000.  That would get you to 1 micron intervals.   I have a hard time imagining that the wooden structure, located where it is, can prevent movements that small.  Obviously, that’s just my opinion and I mean no disrespect, I just don’t see how we can expect much stability with this arrangement, especially on top of a mount that has mechanical issues of its own.

I really hope you can get some other help to sort this out, but I think I've said as much as I can.

Good luck,

Bruce

[David Ridge]

thanks for your analysis. 10,000+ users, 9,433 unique setups. That must be fun trying to corral that many cats :)

David