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Still some DEC issues

 

Hello folks,
I have the latest GM811.? A few weeks back, I posted about adjusting the screw to get the backlash as low as possible, mechanically.? When using the mount for long-exposure imaging, there are still some DEC issues that ruin a frame here and there.? I have the clutch pressure set very low.
0) First, I should mention that sometimes when the seeing is good, the guiding is good too at < 0.6 rms and the DEC axis just gives gentle nudges back and forth to the axis and everything is fine.
1) Sometimes a large DEC pulse (when changing direction) will cause a corresponding spike in RA also.? See attached picture.? Here only the northward pulses (approx 1 second worth, if I remember + TVC 10) cause a spike in RA.? (BTW, I also had these spikes with TVC 0) I noticed when running the backlash test in Guiding assistant that the guide star also moves a little in RA when changing DEC direction from N to S.
2) a dither move in DEC seems to "unsettle" the axis, and it seems to misbehave for maybe the next 30 seconds or more.? It's like the axis builds up "sticktion" and then releases it at some random time later.? In the picture, the sticktion amount is about 3 arc-sec.? After several minutes, this sticktion drops down to about 2 arc-s, and maybe if the seeing is good for a long time and I don't do any dithers in DEC, it gets even lower.? It's like the worm needs to move back and forth a few times before settling down for best performance.
So at this moment dither in DEC is unreliable because the later movement will mess up a frame.? I've resorted to dither RA only and then "manual DEC dither" after a sequence of images.
 

A good puzzle, Alan.

Your mount has spring loaded RA and DEC.? The RA is the G11 type drive and the DEC is the GM8 type.

The RA and DEC are orthogonal.? The only way a vibration from.DEC could affect RA is if a jolt from DEC somehow shook the whole mount.

So: a few questions...

1. What is your tripod type?? If it is lightweight perhaps it could vibrate?

2. Last few emails, it seemed the view was to lock down the worm blocks (as they used to be before the spring loaded mechanism was introduced).? Did you bolt down the RA and DEC worm blocks?? If you didn't I'd suggest you bolt them down tightly on both axes, and skip the looser spring loaded operation to see the effect.??

3. When we spoke, you wanted to keep the same worm to ring gear lube as the factory installed.? It was a semitransparent and lightly red or pink color....I think that's what I saw.? ?I think this is the?
? ? ? Jet-Lube Arctic low temp grease
? that Brian has suggested.? You did not want to take a chance on the CRC brake and caliper grease that I offered.? I wonder if you were to try the thicker CRC lube if it could eliminate the vibration.??

Anyway, that's my suggestion for experiments to try.? ?

Best of luck,
Michael








On Tue, May 4, 2021, 3:27 AM alan137 <acfang137@...> wrote:
Hello folks,
I have the latest GM811.? A few weeks back, I posted about adjusting the screw to get the backlash as low as possible, mechanically.? When using the mount for long-exposure imaging, there are still some DEC issues that ruin a frame here and there.? I have the clutch pressure set very low.
0) First, I should mention that sometimes when the seeing is good, the guiding is good too at < 0.6 rms and the DEC axis just gives gentle nudges back and forth to the axis and everything is fine.
1) Sometimes a large DEC pulse (when changing direction) will cause a corresponding spike in RA also.? See attached picture.? Here only the northward pulses (approx 1 second worth, if I remember + TVC 10) cause a spike in RA.? (BTW, I also had these spikes with TVC 0) I noticed when running the backlash test in Guiding assistant that the guide star also moves a little in RA when changing DEC direction from N to S.
2) a dither move in DEC seems to "unsettle" the axis, and it seems to misbehave for maybe the next 30 seconds or more.? It's like the axis builds up "sticktion" and then releases it at some random time later.? In the picture, the sticktion amount is about 3 arc-sec.? After several minutes, this sticktion drops down to about 2 arc-s, and maybe if the seeing is good for a long time and I don't do any dithers in DEC, it gets even lower.? It's like the worm needs to move back and forth a few times before settling down for best performance.
So at this moment dither in DEC is unreliable because the later movement will mess up a frame.? I've resorted to dither RA only and then "manual DEC dither" after a sequence of images.

 

Alan,

Did you mention what capture software you are using? Please see attached screenshot for the dither settings for SGPro. Here you can change the size of the dither, the number of frames before a dither, and the settling time before imaging resumes. Have you tried manipulating these variables? More dithers result in higher errors, especially if you use after each frame, and your frames are short. It is pretty common for a dither to increase your guiding error for a short period of time, but it should smooth out over time. Have you checked off Fast Recenter after Calibration or Dither in the PHD2 brain guiding settings??

Perhaps your issue is within the various settings, not a stiction effect.

John
 

Tripod is Losmandy HD.
I have not tried locking the blocks yet.
The grease on the worm looks to be the same as the grease on the axis shaft that lubes the needle bearing - a light brown color.
I don't want to change the grease unless I can completely remove the old grease.? Can the axis be taken apart to clean the ring gear all the way around?
The sticktion is either worm-to-ring gear, or the bearings of the axis.
I am using NINA for the capture program, which also have settings for dither size, timeout, etc.? I need the dither to be at least several pixels because that seems to be the splotchiness or fixed point noise in my sensor.? Sometimes this bad behavior happens even 30 seconds after a dither move, so it's not really feasible to set a dither timeout that long.
I was able to reduce the RA spikes a little by lowering the TVC value down to 7.? I don't know how fast the Gemini moves the gears for TVC, but maybe even at a setting of 10 there is occasional "gear banging".? In my first post, I said that I got these spikes even with TVC =0, but I was using a higher PHD backlash compensation setting then.? I dunno, maybe I have to lower the guide speed to below 0.5x to make the gears move slower?
 

TVC does not affect RA in any way

If your DEC movements seem to be impacting RA, it sounds to me like a calibration issue in PHD

If you can post your guidelogs that include a clean calibration, GA run, and then 15-20 minutes of uninterrupted guiding, that would be super helpful
 

Alan,

To get to the ring gear you will need to remove the motor and entire worm, blocks, and gear assemblies. Also the clutch knob with underlying bearings and washers need to be taken off and the main shaft pulled through to separate the clutch plates. This procedure is involved and takes much tweaking and patience to reassemble. But since you have a spring loaded worm type mount, it shouldn't be that old that the lubricant is somehow messed or slowing operation.?Brian can make lots of diagnoses with the PHD2 logs, so that would be a first approach. After that you might want to try some worm adjustments as in the Losmandy video:



Possibly you have some backlash issues, but I would exhaust your software variables before working on the mechanical stuff.
 

Check your cables. ?The only way the dec could effect the RA is if something is tugging, as far as I can tell. ?it does not take much to cause a blip.?

JMD
 

Is there a way to trim the guide log?? Do I post it here or some other place?? Because SOME people who complain about getting a second message from editing probably don't want to get a several hundred kilobyte file that they aren't going to look at anyway.
 

On Tue, May 11, 2021 at 05:54 AM, alan137 wrote:
Is there a way to trim the guide log?? Do I post it here or some other place?? Because SOME people who complain about getting a second message from editing probably don't want to get a several hundred kilobyte file that they aren't going to look at anyway.
Use a Zip utility to compress the log file and upload it to the Files section here:?/g/Losmandy_users/files?and then post a link to it in a message.
Don't send it out as an attachment to your message, unless it's a small file.

Regards,

? ? ?-Paul
 

It's better to use the built-in PHD log uploader and paste the link

Also trimmed logs are not helpful - you can't auto load them into the PHD Log Viewer


Brian

On Tue, May 11, 2021 at 2:54 AM alan137 <acfang137@...> wrote:
Is there a way to trim the guide log?? Do I post it here or some other place?? Because SOME people who complain about getting a second message from editing probably don't want to get a several hundred kilobyte file that they aren't going to look at anyway.



--
Brian?



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