Strange Mach2 RA oscillation.


weihaowang
 

Hi

I am at a  remote site for imaging.  This month I had used Mach2 several times without having any problems.  But today, it suddenly behaves abnormally.  The RA axis quickly oscillate for many arcseconds.  The attached image is a screenshot from PHD2.  As you can see, stars become highly elongated along east-west even under an exposure time of 1 second.  What puzzled me is that if I release the RA clutch knobs and only tighten them very very slightly, the problem goes away.  If I further tighten them with hand (not with any tool), the oscillation shows up.  The mount is well balanced.  I looked around and did not find anything (cables etc) that can interfere the RA axis.

And as I am typing now, the problems goes away.  I will try to image now. However, I hope people here can provide some guess on what happened, so I know what to do next time this problem shows up to either solve it or diagnose it.

Many thanks.

Wei-Hao
--

Homepage:

http://www.asiaa.sinica.edu.tw/~whwang/

Astrobin gallery:
http://www.astrobin.com/users/whwang/


weihaowang
 

The problem keeps coming back.  I used two 12V power sources (a deep cycle battery and a power converter).  The result doesn't change.  The oscillation often occurs after a slew with higher speed.  Then it goes away after 2 or 3 minutes, and tracks OK.  Dithering slew with guide speed doesn't seem to trigger this problem.  

Please let me know what I can do.  There are clouds now, so I can't do meaningful imaging.  If possible, I can dedicate the night to solving this problem.
--

Homepage:

http://www.asiaa.sinica.edu.tw/~whwang/

Astrobin gallery:
http://www.astrobin.com/users/whwang/


Roland Christen
 

I will send you some instruction for the RA oscillation.

Roland Christen
Astro-Physics Inc.

-----Original Message-----
From: weihaowang <whwang@...>
To: main@ap-gto.groups.io
Sent: Tue, Jul 13, 2021 9:34 am
Subject: Re: [ap-gto] Strange Mach2 RA oscillation.

The problem keeps coming back.  I used two 12V power sources (a deep cycle battery and a power converter).  The result doesn't change.  The oscillation often occurs after a slew with higher speed.  Then it goes away after 2 or 3 minutes, and tracks OK.  Dithering slew with guide speed doesn't seem to trigger this problem.  

Please let me know what I can do.  There are clouds now, so I can't do meaningful imaging.  If possible, I can dedicate the night to solving this problem.
--
Homepage:

--
Roland Christen
Astro-Physics


George
 

Wei-Hao,

 

As we discussed through private emails, the oscillation issue that you experienced is caused by the absolute encoders desperately trying to correct for the instability caused by substituting a customized and inadequate counterweight shaft in place of the shaft that ships with the mount.   The pendulum-like flexure created by multiple step-down threaded adapters to a very thin counterweight shaft is magnified by the counterweights set at the very bottom of the shaft.

 

 

When you restore the supplied counterweight shaft to the mount the problem will go away.    We take great pride and consideration of every detail when designing our mounts.   Please do not try to modify them.  

 

Regards,

 

George

 

George Whitney

Astro-Physics, Inc.

Phone:  815-222-6538 (direct line)

Phone:  815-282-1513 (office)

Email:  george@...

 

 

From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of weihaowang
Sent: Tuesday, July 13, 2021 09:04
To: main@ap-gto.groups.io
Subject: [ap-gto] Strange Mach2 RA oscillation.

 

Hi

I am at a  remote site for imaging.  This month I had used Mach2 several times without having any problems.  But today, it suddenly behaves abnormally.  The RA axis quickly oscillate for many arcseconds.  The attached image is a screenshot from PHD2.  As you can see, stars become highly elongated along east-west even under an exposure time of 1 second.  What puzzled me is that if I release the RA clutch knobs and only tighten them very very slightly, the problem goes away.  If I further tighten them with hand (not with any tool), the oscillation shows up.  The mount is well balanced.  I looked around and did not find anything (cables etc.) that can interfere the RA axis.

And as I am typing now, the problems goes away.  I will try to image now. However, I hope people here can provide some guess on what happened, so I know what to do next time this problem shows up to either solve it or diagnose it.

Many thanks.

Wei-Hao
--

Homepage:

http://www.asiaa.sinica.edu.tw/~whwang/

Astrobin gallery:
http://www.astrobin.com/users/whwang/

_._,_._,_

 


 

This is so familiar.  Years ago, I wasn't able to balance a new heavier load on my Celestron CGEM with the stock counterweight bar and weights.  I had the facilities, so the "obvious" solution was to turn a 6" extension to the bar and move the weights out more.  The first (and last) time I used it I immediately noticed very wobbly and lengthy settling after any movement.  It needed more weights closer to the mount - not the same ones further away!

Jerome


 

You are quite right Jerome. This is a potential issue for any mount. It’s all physics.

 

Clear Skies,

Marj Christen

Astro-Physics

11250 Forest Hills Road

Machesney Park, IL 61115

Phone: 815-282-1513

www.astro-physics.com

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Jerome Allison
Sent: Tuesday, July 13, 2021 9:09 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Strange Mach2 RA oscillation.

 

This is so familiar.  Years ago, I wasn't able to balance a new heavier load on my Celestron CGEM with the stock counterweight bar and weights.  I had the facilities, so the "obvious" solution was to turn a 6" extension to the bar and move the weights out more.  The first (and last) time I used it I immediately noticed very wobbly and lengthy settling after any movement.  It needed more weights closer to the mount - not the same ones further away!


Jerome