Help Needed with PHD2 Log Data
Bob Mimiaga
bw_msgboard
Hi Bob. Thanks for taking a look at the data yourself and being specific about your concerns. You’re running up against limitations with the mount on both axes, so I’ll try to identify those for you.
Declination
It looks like you have tried to do the Dec backlash measurement in the Guiding Assistant but the test never completes – is that right? Your mount appears to have a many-second delay in reversing the direction of the Dec axis rotation compounded by static resistance on that axis (stiction). Here’s a particularly clear example (Dec in green):
You can see that a long stream of guide pulses are sent to the mount before the axis starts to move - but then the axis over-shoots. All those earlier guide pulses that didn’t accomplish anything were pumping energy into the Dec drive system and once the resistance was overcome, that energy was released and the axis rotated too far. This behavior shows up most clearly when the initial guide star deflection is large. These correction curves don’t look symmetric to me so they might be caused by a significant weight imbalance on the Dec axis – there’s no way to know. The start of the problem at the far-left in this graph was probably caused by something tugging on the mount or guide camera – remember that the Dec motor doesn’t run at all during imaging unless it’s told to. If you can increase the guide speeds in the mount above the current 0.5x sidereal, you should get an immediate improvement in these Dec reversal problems.
RA
Your mount has a considerable amount of periodic error, probably typical for these mounts, and that is limiting your overall guiding results. If you run the Guiding Assistant for about 20 minutes, you’ll get a pretty clear view of the periodic error. But we can get an approximate view from the guide log (RA in red):
So you can see the native RA tracking is wandering around within an envelope of about +/- 15 arc-sec. But it doesn’t exhibit the nice smooth sine wave you’d like to see. If we look at the frequency components, we see why:
The biggest contributor, the arrow at the far right, occurs on intervals of about 600 seconds, which is the native worm period of your mount. The next two arrows to the left show faster harmonics at 300 sec and 60 sec. These are coming from the various reduction gears in the RA drive system. All of these periodic error contributors could probably be improved if you program a high-quality periodic error correction into the mount. The last contributor at the far left occurs every 11 seconds and this is evidently because of a wobble in the pinion gear on the motor. There’s an interesting discussion of all this stuff here:
https://stargazerslounge.com/topic/307873-avx-motor-disassembly/
That 11-second wobble isn’t amenable to guiding, it’s something that requires a mechanical fix. After you’ve done a periodic error correction, you can try using the PHD2 PPEC guiding algorithm in RA. Set the period length to 600 seconds and see if it helps. In fact, you can try this in any case.
This periodic error is what’s causing the peculiar calibration result you were talking about:
The oscillation in the Dec points (green) is just a reflection of the higher-frequency periodic error you have in RA.
It looks to me like you’ve done a good job with PHD2, you seem to have a reasonable setup and you’ve avoided the operational mistakes that are so common. I think it’s really a question now of how much you want to or are able to improve the behavior of your mount. Or, if you’re serious about the hobby, maybe you will want to upgrade to a better mount – that may come down to whether these guiding issues are really the limiting factor in the final images you’re getting.
Good luck,
Bruce
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Bob Mimiaga
Bob Mimiaga
Brian Valente
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Brian Valente
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Bob Mimiaga
Bryan
Bob Mimiaga
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bw_msgboard
Hi Bob, see below.
From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Bob Mimiaga
Sent: Sunday, March 14, 2021 7:17
PM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding]
Help Needed with PHD2 Log Data
Bruce,
First of all I'd like to thank you for the detailed analysis and explanation of what seems to be going on with my mount. This part of AP can be quite technical to a neophyte such as myself. Interpreting the PHD2 logs is a case in point. After a few years into this hobby I've come to realize that the AVX mount isn't the best mount to use for serious AP imaging. My primary imaging camera is a Canon 60D but with my SCT's FL of 1280mm tracking can be quite a challenge. At some future point I may switch mounts but for now because of costs I'll stick with the AVX, but would like to improve it's tracking if possible. I recently purchased an ASI1600MM so I'll have to wait awhile on another mount. So I've got a few more questions resulting from your analysis and really appreciate your feedback;
1. Yes I attempted to perform the DEC backlash measurement but the guiding assistant never completed it. I wish now I had recorded the error message. But in the past I've able to complete the DEC Backlash but haven't taken a close look at it. What is a good to average Backlash measurement? Does PHD2 provide a Backlash correction value I can input into my AVX hand controller?
You can always go back and look at results from earlier Guiding Assistant runs. There’s a ‘Restore previous’ drop-down at the bottom of the GA window and there’s a history of the last three GA runs. You can also look at the graph of the backlash test results even if the test doesn’t complete. You probably got a message saying that the mount never established consistent movement to the south. The whole point of this test is to support use of the PHD2 Dec Backlash Compensation feature which is very different from anything that’s implemented in your mount. Do NOT use backlash compensation in your mount or hand-controller at this point. If you’re getting reversal delays longer than several seconds, you will probably have trouble getting good results. If it takes that long to make a Dec correction, the RA tracking is likely to wander too far off track. For large Dec backlash problems, we recommend learning to use uni-directional Dec guiding – that’s described in the manual.
2. Regarding the DEC Graph you mentioned that "You can see that a long stream of guide pulses are sent to the mount before the axis starts to move - but then the axis over-shoots." That evening I tried to add some East Heavy weight which may have caused part of the problem as you suggested. It wasn't overly heavy in the eastern direction but would that explain the results you see? In addition to getting an accurate DEC Backlash programed into my mount, I'll try increasing the guide speed above the current 0.5 sidereal as you suggested.
I think maybe you’re confused here. If you were tinkering around with east/west imbalance, that will affect RA, not Dec. However, you should check to be sure the OTA is reasonably balanced in Dec. Then increase the mount guide speeds and see what happens. As I said above, don’t try to use any Dec backlash correction in the mount.
3. Regarding Your RA results I think I can make an improvement here by calculating a PPEC value that I can enter into my mount software. Your analysis of mount's RA shows extreme RA fault on the 600 second intervals. I noticed that but didn't know why. Your explanation of the reason was quite helpful. I just don't know the best software to use to calculate the PPEC value. Any suggestions? Celestron has a tool called CPWI but it doesn't seem to be a simple process. Once I get a good PPEC value uploaded into my mount I'll run the PHD2 PPEC guiding algorithm in RA to see what results I get.
4. What software are you using to view the DEC and RA analysis images that you attached with your email? It seems to be quite helpful.
To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/35e840ce-7043-45a3-98b6-8c0e2493da31n%40googlegroups.com.