CGE Pro Minimizing DEC backlash

[mark matzner]

Hi Group,
my mount is new, purchased in January of this year. It features the spring loaded DEC and RA drives, and is likely the last iteration by Celestron. I raised a question to their tech group about reducing DEC backlash, specifically with the mechanical adjustments. The following is their partial response and deals strictly with using the softwares to mitigate the backlash. While not what I was looking for, it appears to be a thoughtful response.

QUOTE FROM CELESTRON:
"We consider acceptable backlash to be within 3 seconds of delay when reversing directions at rate 3. Rate 3 is 4x sidereal speed. Of course this will translate into a very long delay when autoguiding at ½ sidereal.

Concerning ½ sidereal speed for autoguiding:
The default guide rate is set to ½ sidereal which in most cases is too slow. This is further compounded by the existing backlash, which explains why PHD2 reports such a large amount of Dec backlash. There are 3 ways to combat this before making any mechanical adjustment:

1. Adjust the autoguide rate in the NexStar+ hand control from 50 (default) to 99 for R.A. and especially Dec. This will double the autoguide speed, making autoguide movements much more responsive. Auto guide aggression rates can always be changed in PHD2 while guiding, so it’s better to start with the maximum responsiveness.
2. Despite the suggested calibration step size in PHD2, increase the calibration step 50% or so. For example, if it was 100ms, increase to 150ms. This will cause larger movements to help complete the calibration process. Otherwise, PHD2 may iterate 20, 30 or more calibration steps in Dec which can make for a messy calibration. If the steps are too large it can be toned down later.
3. Finally, identify which direction the Dec drift is in. This can be done by turning Dec aggression to 0 while guiding, or running the drift align procedure in PHD2. Once you identify which direction Dec is drifting in, disengage Dec guiding in the other direction (in the “brain” advanced settings of PHD2). Once you do this, Dec backlash no longer becomes a factor. Unless polar alignment is absolutely perfect, we will see a trend in Dec drift. This is actually advantageous so that we can keep Dec moving to one side of the gear backlash. ".
END QUOTE

I'm curious what this group's thoughts are regarding the above recommendations.
Also, my apologies if this violates the forum's TOS. Moderators should feel free to delete, if so.

Thanks,
Mark

[bw_msgboard]

 

 

-----Original Message-----
From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of mark matzner
Sent: Saturday, October 14, 2017 4:22 AM
To: Open PHD Guiding
Subject: [open-phd-guiding] CGE Pro Minimizing DEC backlash

 

Hi Group,

my mount is new, purchased in January of this year.  It features the spring loaded DEC and RA drives, and is likely the last iteration by Celestron.  I raised a question to their tech group about reducing DEC backlash, specifically with the mechanical adjustments.  The following is their partial response and deals strictly with using the softwares to mitigate the backlash.  While not what I was looking for, it appears to be a thoughtful response.

 

QUOTE FROM CELESTRON:

"We consider acceptable backlash to be within 3 seconds of delay when reversing directions at rate 3. Rate 3 is 4x sidereal speed. Of course this will translate into a very long delay when autoguiding at ½ sidereal.

 

Wow.  12” to reverse direction with a guide speed of 1x sidereal.

 

Concerning ½ sidereal speed for autoguiding:

The default guide rate is set to ½ sidereal which in most cases is too slow. This is further compounded by the existing backlash, which explains why PHD2 reports such a large amount of Dec backlash. There are 3 ways to combat this before making any mechanical adjustment:

 

1.    Adjust the autoguide rate in the NexStar+ hand control from 50 (default) to 99 for R.A. and especially Dec. This will double the autoguide speed, making autoguide movements much more responsive. Auto guide aggression rates can always be changed in PHD2 while guiding, so it’s better to start with the maximum responsiveness.

 

Yes, this is what we normally recommend for people who are struggling with backlash that can’t be improved mechanically.

 

2.    Despite the suggested calibration step size in PHD2, increase the calibration step 50% or so. For example, if it was 100ms, increase to 150ms. This will cause larger movements to help complete the calibration process. Otherwise, PHD2 may iterate 20, 30 or more calibration steps in Dec which can make for a messy calibration. If the steps are too large it can be toned down later.

 

I guess there’s nothing actually wrong with this but it seems like an unnecessary tap-dance and it still won’t guarantee a successful calibration.  Since the issue is backlash, the simpler solution is just to clear the north backlash before starting the calibration.  This is what we recommend and document in the Help file for dealing with poor calibration due to backlash.  Just make sure the last Dec movement is in the north direction before starting calibration.  You can do that via slewing or by manually moving the mount north for 20 seconds or so at guide speed.  The backlash will then show up only when PHD2 tries to restore the calibration star to its original position – but that is irrelevant to calibration.  The calibration only measures the north guide corrections.  Since re-calibration should be infrequent and there’s no reason to be calibrating “on-target”, pre-moving the mount north should be a pretty easy workaround.

 

3.    Finally, identify which direction the Dec drift is in. This can be done by turning Dec aggression to 0 while guiding, or running the drift align procedure in PHD2. Once you identify which direction Dec is drifting in, disengage Dec guiding in the other direction (in the “brain” advanced settings of PHD2). Once you do this, Dec backlash no longer becomes a factor. Unless polar alignment is absolutely perfect, we will see a trend in Dec drift. This is actually advantageous so that we can keep Dec moving to one side of the gear backlash. ".

END QUOTE

 

Yes, uni-directional Dec guiding is a useful way to sidestep Dec backlash and you can get reasonable imaging results this way.  But it isn’t a free lunch and can pose some operational challenges for beginners.  But if the mount isn’t up to the task of doing full Dec guiding, this is probably the only option.

 

Bruce

 

I'm curious what this group's thoughts are regarding the above recommendations.

Also, my apologies if this violates the forum's TOS.  Moderators should feel free to delete, if so.

 

Thanks,

Mark

 

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[Andy Galasso]

Mark,

Thanks for posting that information.  I added the info to the PHD2 wiki in the mount info section: Celestron info.  There's a link back to this discussion thread so readers can also see Bruce's comments (which I completely agree with.)

Andy

[peter wolsley]

Mark,

That's very disappointing that Celestron considers roughly 180 arc-seconds of backlash "acceptable".  Hopefully your CGE-Pro has far less backlash.  It's been my experience that most of the Celestron customers whom have submitted logs on this forum have much smaller than 180 arc-seconds of DEC backlash.  To me this statement clarifies at what threshold Celestron might consider replacing the mount.

Peter

[mark matzner]

Bruce, Andy, Peter,

Thanks for your responses. I always feel that I get honest, non condescending, answers from this forum and you guys. And great software.

Regarding Celestron support, they seem to be good folks and are usually very helpful. They've been as good as gold helping me thru warrantee issues.
But I believe that they just will not give out adjustment information for the CGE Pro drives. This is true even though they do provide info on their new CGX series even though those appear much more complicated.

So, I'll give this further thought and likely go into the DEC drive. There's a lot of slop there and the adjustment is simple.

BTHW Bruce, if not mistaken, one of your images was featured in the latest Astronomy. Congrats!

Mark

[bw_msgboard]

-----Original Message-----
From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of mark matzner
Sent: Sunday, October 15, 2017 5:42 AM
To: Open PHD Guiding

Subject: [open-phd-guiding] Re: CGE Pro Minimizing DEC backlash

 

Bruce, Andy, Peter,

 

Thanks for your responses.  I always feel that I get honest, non condescending, answers from this forum and you guys.  And great software.

 

Regarding Celestron support, they seem to be good folks and are usually very helpful.  They've been as good as gold helping me thru warrantee issues.

But I believe that they just will not give out adjustment information for the CGE Pro drives.  This is true even though they do provide info on their new CGX series even though those appear much more complicated.

 

So, I'll give this further thought and likely go into the DEC drive.  There's a lot of slop there and the adjustment is simple.

 

Probably a good idea.  If it were me, I’d try to “sneak up on it” a little bit so you don’t get it too tight and create a binding problem.  Of course, my actual technique is to go find an available hardware engineer, buy him a beer, and get him to adjust it. <g>

 

BTHW Bruce, if not mistaken, one of your images was featured in the latest Astronomy.  Congrats!

 

Thanks.  It’s fairly inexplicable. <lol>