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PMC-Eight EXOS 2 PHD Guide Log ASCOM pulse-guiding mode


hubbell_jerry@...
 

I have been working with fellow PMC-Eight owner, Mike Leemhuis, to get his auto-guiding working and that reminded me that I meant to post a PHD Guide Log to the group to show the performance, my partner in the MSRO, is getting. I took this log using the EXOS 2 PMC-Eight system with ASCOM pulse-guiding. The scope was a 250mm guide scope with the QHY5-IIL guide camera. 

Here is the link for the guide log file. You need to download and use the program PHDLogViewer to view the data:


Here is the guide log I posted in the files section

https://groups.yahoo.com/neo/groups/ESPMC-Eight/files/Mount%20Related%20Files/PHD2_GuideLog_2018-03-04_204205%20-%20Copy.txt


Jerry Hubbell

Director Electrical Engineering

Explore Scientific, LLC.




Robert Hoskin <r_hoskin@...>
 

Jerry,

This is pretty interesting. :-)

I'm nowhere near ready to try guiding, and this is the first phd2 log I've really paid attention to - but it does look like the mount is staying well within the arcsec limit of the guide camera most of the time.   Just a wild guess, but I think you're likely pretty pleased with this :-)

Couple of questions:
- Can you post a sample image or two from this session?
- What do you think is going on with the RA vs DEC corrections?  They're quite different.
- How was the scope balanced for this?  Evenly, or one-side-heavy?
- What do you think the guided performance tells us about probable unguided performance?

Thanks!

- Bob
 



From: "hubbell_jerry@... [ESPMC-Eight]"
To: ESPMC-Eight@...
Sent: Thursday, March 15, 2018 5:32 PM
Subject: [ESPMC-Eight] PMC-Eight EXOS 2 PHD Guide Log ASCOM pulse-guiding mode

 
I have been working with fellow PMC-Eight owner, Mike Leemhuis, to get his auto-guiding working and that reminded me that I meant to post a PHD Guide Log to the group to show the performance, my partner in the MSRO, is getting. I took this log using the EXOS 2 PMC-Eight system with ASCOM pulse-guiding. The scope was a 250mm guide scope with the QHY5-IIL guide camera. 

Here is the link for the guide log file. You need to download and use the program PHDLogViewer to view the data:


Here is the guide log I posted in the files section

Jerry Hubbell
Director Electrical Engineering
Explore Scientific, LLC.





hubbell_jerry@...
 

Hi Bob,

Yes, the DEC trace is very interesting in that it shows a very different behavior than the RA axis. If you zoom up on the time axis and just look at the DEC trace, the corrections start when the error is > 0.2 pixels and they build up over time as the error becomes greater and greater up to about +/- 1 pixel then there is a move back to zero error. It appears that the correcting pulses are not really having an effect until a certain amount of time passes (or maybe a certain amount of error is built up), then the mount finally moves back to the zero position. The overall shape is a saw-tooth. 

I think it is important to remember here that the DEC axis is not supposed to move with a perfect polar alignment, here the star is moving back and forth across the target location on the camera chip (guiding position) which according to the log is at 180.958 px on the y (DEC) axis. This can only really be because of a small amount of lash in the DEC axis.

The target coordinates are: Dec = 21.4 deg, Hour angle = -2.10 hr,  So the DEC, and the RA axis for that matter, is not theoretically in a position where the axis could be perfectly balanced and rock back and forth and oscillate slowly. Such a position might be when the DEC axis is at the same value as the latitude where the scope is pointed along the path where it passes through zenith and gravity is not pulling it one way or another in the DEC axis plane. This would act like a balanced point where sliding the telescope on the mounting plate would not bias this one way or the other.

In this case, the DEC position is at 21.4 degrees (73.1 degrees to the horizon), not 38.3 degrees (90 degrees to the horizon) where it would need to be to cross the zenith and to be in a balanced condition. This 73.1 degrees to the horizon provides a force against the DEC gear equal to about 30% (sin 90-73.1 degrees) of the imbalance amount  felt when the tube is horizontal in the balancing position.

So, there is no doubt that any bias in the balance on the DEC axis is reduced by 70%, it may, or may not be enough to cause the little bit of lash to show itself. In this case, I don't know how much bias was put into the DEC axis, if any. More than likely the DEC axis was virtually balanced, or close to it, where the RA was imbalanced since that is the driving axis So, in this case, any lash in the DEC axis would probably show itself when pointed close to vertical as in this case. 

So, the result of this long-winded explanation, is that what is happening is that the DEC axis has a little bit of lash where the axis does not respond at all to the corrections until it gets to the point (around 1 arc-second of error) where it starts to respond and ramps back down to zero error. At that point it continues to drift down due to the lash now in the opposite direction and then finally responds and moves back up to zero error. I think it is important to understand here also that when the mount isn't responding to the correcting pulses, the axis gears are just free-floating.

The primary indicator that lash is involved is that the mount does not respond when PHD is pulsing the DEC axis. This would probably only show itself when the telescope is pointed near a DEC coordinate of 90-latitude degrees. This would even be the case if the DEC axis balance was biased.

This could be the basis for a lash measurement in the DEC axis. The procedure would be to point the mount to near zenith and then record the guiding graph for the DEC axis and look for this behavior. You could measure the peak to peak guiding error and that would give you an indication of the amount of lash in arc-seconds.

I apologize for the length of this message, but I thought it important to talk out and explain my thinking on this. I could be out in left field, but I think this makes logical sense.

I'd like to hear other opinions and thoughts on this. This type of problem is very interesting to me and it is an opportunity to learn something about our mounts.

Thanks 

Jerry Hubbell
Director Electrical Engineering
Explore Scientific, LLC.