Understanding MaxDec Duration Setting

[mrgary...@gmail.com]

Hi all:

Last night, I made my first attempt at using PHD2 for the season. I took 50 3-minute exposures

of the Pelican Nebula using my StellarVue SVQ-100 astrograph equipped with a Canon T3i

dSLR. The spotting scope has a 210 mm focal length, while the Imaging Source guide camera

has a 5.6 micron x 5.6 micron pitch.

Overall, most every frame looks half-decent, especially since I went to bed for the several

hours that it took to collect the data, checking in on things every 90 minutes. The sky was

mostly clear, with some moisture to it. So, not perfect.

When my runs were over, I noticed that the PHD2 gave an indication of the need to adjust

the Max Dec Duration setting. What exactly does this do, and how do I know how best to

make the adjustment, i.e. decreasing the value of it vs increasing the value.

The Guide log is attached. The Debug file is about 19 MB and can be accessed at:

https://drive.google.com/open?id=1Xuwl6QUefA5MIY27DtO4iGtvO6PFV7R-

Thanks in advance.

Gary

[mrgary...@gmail.com]

I should note that I dithered the images, which contributes to the tracking pattern

in the attachment of the PHD2 display.

[bw_msgboard]

Hi Gary, sorry you ran into problems.  I’ll start by answering your specific question then come back to what’s going on with your mount.  The max-duration setting is one of the least useful parameters and rarely needs to be changed.  It might be helpful in special cases like spectroscopy or where you’re having to use an especially small search region.  But the default setting for max-duration rarely needs to be changed.  In the 2.6.5dev1 release, we replaced this message with something that tries to be more helpful.  Essentially, it’s saying that the tracking and guiding has gotten so bad that the guide star is now too far from the original lock-point and can’t be restored without making multiple very large corrections.  In other words, something has gone seriously wrong.

 

In your case, there were a couple of reasons for this, usually in Declination.  First, there were many places where the Dec just plunged off-target:

 

 

I don’t know if this was because you were banging around with the scope or had a cable snag or something like that.  But this causes the guide star to be yanked far away from the lock-point and it might trigger the alert message you’re seeing.  You’ll have to eliminate all of these kinds of things to get reasonable guiding.  The second type of problem shows up in the last guiding session:

 

 

In this case, the Dec axis is simply not responding to any of the Dec guide commands so the guide star continued to drift off-target.  Something seems to be interfering with the mount’s ability to move south at this point, perhaps a cable snag or some other impediment.  This will eventually also trigger the alert you saw.

 

I think you’re probably at a point where you need to see exactly how your mount is behaving and where you have mechanical issues. You’ll need to be sure everything is fastened very securely, nothing is moving around, and there are no cable routing issues.  I’d suggest running the Guiding Assistant for at least 10 minutes, then let it measure the declination backlash.  I don’t think we know what kind of mount you have.  I see you’re using a GP-USB connection, why is that?  Is there no ASCOM mount driver for your mount?  If PHD2 had an ASCOM connection to the mount, even via the Aux-mount feature, we would have more information to help you sort through the problems.  When you do start running tests, be sure to use the default guiding parameters – trying to adjust them to deal with these kinds of problems is not going to work.

 

Hope this helps,

Bruce

 

 

 

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[G. Meehan]

Hi Bruce,

Thank you for taking the time to inspect my log files.

I am using a Vixen GPD2 mount whose dual-axis hand controller has been modified to take an ST-4 style autoguider port.

The mount interfaces to my laptop through a Shoestring Astronomy GPUSB device. It probably isn't the best mount for deep

sky astrophotography, but it serves my general needs pretty well. Oddly enough, my 3-minute exposures looked pretty

decent, due no doubt to the wide field of view.

I'll further investigate the declination axis for mechanical issues when the weather and mosquitoes cooperate.

Best regards,

Gary

On Sun, Sep 2, 2018 at 11:45 AM, bw_msgboard <bw_m...@earthlink.net> wrote:

Hi Gary, sorry you ran into problems.  I’ll start by answering your specific question then come back to what’s going on with your mount.  The max-duration setting is one of the least useful parameters and rarely needs to be changed.  It might be helpful in special cases like spectroscopy or where you’re having to use an especially small search region.  But the default setting for max-duration rarely needs to be changed.  In the 2.6.5dev1 release, we replaced this message with something that tries to be more helpful.  Essentially, it’s saying that the tracking and guiding has gotten so bad that the guide star is now too far from the original lock-point and can’t be restored without making multiple very large corrections.  In other words, something has gone seriously wrong.

 

In your case, there were a couple of reasons for this, usually in Declination.  First, there were many places where the Dec just plunged off-target:

 

 

I don’t know if this was because you were banging around with the scope or had a cable snag or something like that.  But this causes the guide star to be yanked far away from the lock-point and it might trigger the alert message you’re seeing.  You’ll have to eliminate all of these kinds of things to get reasonable guiding.  The second type of problem shows up in the last guiding session:

 

 

In this case, the Dec axis is simply not responding to any of the Dec guide commands so the guide star continued to drift off-target.  Something seems to be interfering with the mount’s ability to move south at this point, perhaps a cable snag or some other impediment.  This will eventually also trigger the alert you saw.

 

I think you’re probably at a point where you need to see exactly how your mount is behaving and where you have mechanical issues. You’ll need to be sure everything is fastened very securely, nothing is moving around, and there are no cable routing issues.  I’d suggest running the Guiding Assistant for at least 10 minutes, then let it measure the declination backlash.  I don’t think we know what kind of mount you have.  I see you’re using a GP-USB connection, why is that?  Is there no ASCOM mount driver for your mount?  If PHD2 had an ASCOM connection to the mount, even via the Aux-mount feature, we would have more information to help you sort through the problems.  When you do start running tests, be sure to use the default guiding parameters – trying to adjust them to deal with these kinds of problems is not going to work.

 

Hope this helps,

Bruce

 

 

 

From: open-phd-guiding@googlegroups.com [mailto:open-phd-guiding@googlegroups.com] On Behalf Of mrgary...@gmail.com
Sent: Sunday, September 02, 2018 5:44 AM
To: Open PHD Guiding
Subject: [open-phd-guiding] Understanding MaxDec Duration Setting

 

Hi all:

 

Last night, I made my first attempt at using PHD2 for the season. I took 50 3-minute exposures

of the Pelican Nebula using my StellarVue SVQ-100 astrograph equipped with a Canon T3i

dSLR. The spotting scope has a 210 mm focal length, while the Imaging Source guide camera

has a 5.6 micron x 5.6 micron pitch.

 

Overall, most every frame looks half-decent, especially since I went to bed for the several

hours that it took to collect the data, checking in on things every 90 minutes. The sky was

mostly clear, with some moisture to it. So, not perfect.

 

When my runs were over, I noticed that the PHD2 gave an indication of the need to adjust

the Max Dec Duration setting. What exactly does this do, and how do I know how best to

make the adjustment, i.e. decreasing the value of it vs increasing the value.

 

The Guide log is attached. The Debug file is about 19 MB and can be accessed at:

 

https://drive.google.com/open?id=1Xuwl6QUefA5MIY27DtO4iGtvO6PFV7R-

 

Thanks in advance.

 

Gary

 

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