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PHD2 guiding issues

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#1 AussieGazer

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Posted 30 March 2020 - 01:17 AM

Hi all

 

So I have been using PHD2 for a little while now, and feel that I have been able to decently guide for a little while now.

 

I use the PDA tool to polar align, and have been able to get pretty good 2 and 3 minute subs of DSOs. Like most I'm sure, I get the odd issue where there are large corrections made by PHD2 - beyond what you'd want to see, but it's usually few and far between.

 

Last night however, I was getting huge corrections almost throughout the entire night. I re-checked my polar alignment, I checked the balance on the mount. The mount was on a concrete driveway. All was fine.

 

At the start of the night, there was a bit of wind, but that dropped off, while the mount correction issues did not subside.

 

Log posted below. Any ideas as to what went wrong?

 

My only thought was I am shooting from a slightly different part of the state of where I live, and maybe I should have done another calibration before starting.

 

AG

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#2 Stelios

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Posted 30 March 2020 - 02:31 AM

I think your guide camera may have shifted. I assume you use pulse guiding (mount connection to laptop--no ST4 port). That doesn't always require recalibration, but it does if your guide camera is rotated even a bit in its holder. 

 

In any case, nothing gross pops up in the PhD2 settings. You *may* have had awful seeing--but the way to diagnose things best is to start with a good polar alignment (don't know what the "PDA tool" is--you should use Sharpcap Pro (10 British Pounds/year, works in Southern Hemisphere, or PhD2's drift align (slower)). Then follow with a calibration in an area *near meridian and equator* (re-usable anywhere, but more accurate there than in regions closer to the pole). 

 

If you have those two done right, and the problem persists, post again. Note that with your mount best guiding would be East-Heavy (slightly) and since you disabled Backlash Comp you may need to guide North or South rather than auto *if* you get bad oscillations in DEC (which was a secondary problem in your logs, dwarfed by the RA swings).


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#3 AussieGazer

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Posted 30 March 2020 - 02:43 AM

I think your guide camera may have shifted. I assume you use pulse guiding (mount connection to laptop--no ST4 port). That doesn't always require recalibration, but it does if your guide camera is rotated even a bit in its holder. 

 

In any case, nothing gross pops up in the PhD2 settings. You *may* have had awful seeing--but the way to diagnose things best is to start with a good polar alignment (don't know what the "PDA tool" is--you should use Sharpcap Pro (10 British Pounds/year, works in Southern Hemisphere, or PhD2's drift align (slower)). Then follow with a calibration in an area *near meridian and equator* (re-usable anywhere, but more accurate there than in regions closer to the pole). 

 

If you have those two done right, and the problem persists, post again. Note that with your mount best guiding would be East-Heavy (slightly) and since you disabled Backlash Comp you may need to guide North or South rather than auto *if* you get bad oscillations in DEC (which was a secondary problem in your logs, dwarfed by the RA swings).

Thank you for looking at the log, really appreciate it.

 

PDA is "Polar Drift Align" in PHD2. I have been meaning to get into Sharpcap, just haven't done it yet.

 

Yes, I use pulse guiding.

 

Any idea what the RA swings were due to?

 

And I made no conscious decision to disable backlash comp. I think I left most of the settings "as is" when setting up EQMod. Should I switch it on? To be honest, I still don't quite understand the backlash issue.

 

P.S. - it's quite possible the guide scope moved slightly in the journey here. But I can confirm what the guide scope was looking at was the same as what the main scope was capturing.


Edited by AussieGazer, 30 March 2020 - 02:45 AM.


#4 Stelios

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Posted 30 March 2020 - 06:12 PM

Thank you for looking at the log, really appreciate it.

 

PDA is "Polar Drift Align" in PHD2. I have been meaning to get into Sharpcap, just haven't done it yet.

 

Yes, I use pulse guiding.

 

Any idea what the RA swings were due to?

 

And I made no conscious decision to disable backlash comp. I think I left most of the settings "as is" when setting up EQMod. Should I switch it on? To be honest, I still don't quite understand the backlash issue.

 

P.S. - it's quite possible the guide scope moved slightly in the journey here. But I can confirm what the guide scope was looking at was the same as what the main scope was capturing.

Calibration is needed in order to make sure that when PhD2 sees the star move, say, at a 30 degree angle, it knows how much of a pulse to send in DEC or in RA to return it back. To do that, it must calibrate--move the mount in RA, see how the star moves, then move it in DEC and see how the star moves *now*.

 

It also computes backlash *in DEC only*--when, upon reversing direction, initially a push results in no motion. In a mount with no backlash, a forward pulse of 50ms followed by a back pulse of 50ms would return to the initial spot--but in mounts with backlash it doesn't, requiring extra pushes (say 50ms forward, and 150ms back to end up in the original spot). Once the backlash has been absorbed, motion in the new direction is normal. This happens because the gears are not perfectly meshed. 

 

The reason backlash is computed in DEC only is that in RA adjustments/corrections are made by stopping and resuming motion, *not* by reversing motion. In DEC (where, ideally, with a perfect PA no motion would be needed) you need to move to make corrections. 

 

PhD2's guiding assistant will also compute backlash for you (and advise, if it's excessive, to guide in one direction only--North or South vs Auto). This has nothing to do with EQMod. The one thing to check with EQMod and PhD2 is the guiding *rate*. The guiding rate in EQASCOM must match the guiding rate in PhD2. See this if you haven't already.

 

The RA swings may simply be due to the wrong calibration--the mount, in effect, was told to make the wrong corrections. Make sure that you have a good calibration and reasonably good polar alignment, also that the guide rates match, and try again. If the RA (as expected) calms down, and you still have DEC issues, enable backlash compensation *or* guide in one direction.


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#5 AussieGazer

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Posted 13 April 2020 - 10:17 PM

Calibration is needed in order to make sure that when PhD2 sees the star move, say, at a 30 degree angle, it knows how much of a pulse to send in DEC or in RA to return it back. To do that, it must calibrate--move the mount in RA, see how the star moves, then move it in DEC and see how the star moves *now*.

 

It also computes backlash *in DEC only*--when, upon reversing direction, initially a push results in no motion. In a mount with no backlash, a forward pulse of 50ms followed by a back pulse of 50ms would return to the initial spot--but in mounts with backlash it doesn't, requiring extra pushes (say 50ms forward, and 150ms back to end up in the original spot). Once the backlash has been absorbed, motion in the new direction is normal. This happens because the gears are not perfectly meshed. 

 

The reason backlash is computed in DEC only is that in RA adjustments/corrections are made by stopping and resuming motion, *not* by reversing motion. In DEC (where, ideally, with a perfect PA no motion would be needed) you need to move to make corrections. 

 

PhD2's guiding assistant will also compute backlash for you (and advise, if it's excessive, to guide in one direction only--North or South vs Auto). This has nothing to do with EQMod. The one thing to check with EQMod and PhD2 is the guiding *rate*. The guiding rate in EQASCOM must match the guiding rate in PhD2. See this if you haven't already.

 

The RA swings may simply be due to the wrong calibration--the mount, in effect, was told to make the wrong corrections. Make sure that you have a good calibration and reasonably good polar alignment, also that the guide rates match, and try again. If the RA (as expected) calms down, and you still have DEC issues, enable backlash compensation *or* guide in one direction.

Just an update on this. I managed to get out for the first time last night (only had a 2 hour window), but it was enough time to re-calibrate and confirm guiding is all ok again.

 

Thinking back, I clearly brought it upon myself, as I adjusted the guide scope to get it more precisely aligned with the main scope, but did not re-calibrate after doing so.

 

A lesson for me which is now learned!

 

Thanks all for your help.


Edited by AussieGazer, 13 April 2020 - 10:18 PM.


#6 mrkhagol

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Posted 19 April 2020 - 12:41 AM

This question is in regards to whoever can answer based on how they understand it...

I am at point with my celestron avx mount that I would do quick align and then do phd2 guiding to do a drift align.

And this has gotten me really close. But I do see that the objects are off center. So from the way celestron avx mount handles locating and tracking to my polar alignment which could still be off by a very few amount....

I am also able to get less than 1 or around 1 to 1.2 total error on my phd2 guiding so I can tell I am close but that could be attributing to having object off center.

But celestron avx has sync option which when you center/align object and then sync, it 'tends' to have the object centered as much as possible.

 

So my question is if sync sending commands to the mount and phd2 guiding sending commands to the mount...does this really get compounded to the point that phd2 maybe over correcting adjustment received from sync option of avx mount.?




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