Jump to content


Photo

Scope balancing


  • Please log in to reply
9 replies to this topic

#1 GeorgeCarey

GeorgeCarey

    Member

  • Beta Tester
  • 345 posts

Posted 06 May 2015 - 04:17 PM

We all know how important it is to get the scope properly balanced with a DDM mount.

Radial balance is difficult to achieve, so while my mount is back with ASA I have made a rig to assist in balancing.

The whole system, scope, camera, focuser, telescope plate etc, is suspended on a thin steel cable.

Obviously great care is taken to make sure the cable will not come loose or snap!

 

There is a white card placed behind the cable with a straight line drawn on it.

I took care to ensure that the line was perpendicular to the telescope plate.

 

This is the setup:

 

balance01.jpg

 

 

The first trial showed promise. The cable did not snap!

(The scope was only lifted a centimetre.)

 

The cable is not quite parallel to the marked line, suggesting that the there is a small counterweight needed near the camera.

 

balance02.jpg

 

The success of this experiment relies on the cable being accurately positioned over the centre of the telescope plate.

An error of a millimetre will produce an inaccurate result.

 

I will spend the next few days thinking up a way to accurately assess the cable position.

 

What do people think? Is this a sensible way to measure balance?

 

I am also considering taking the focuser/camera assembly off the scope tube and making a separate balance check with it alone.

 

 

George

 

 


ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#2 nakbrooks

nakbrooks

    Member

  • Beta Tester
  • 268 posts
  • LocationWiltshire (England) & Hautes-Pyrenees (France)

Posted 06 May 2015 - 07:04 PM

The results will be interesting.

Theoretically if all the mass is distributed equally on both axes, and if there are no off-axis masses that aren't compensated for, the balancing tool in Autoslew should give identical results whatever the orientation. But it doesn't, at least not for me. If I adjust things to get perfect balance in one orientation (eg pointing at polar North with the mount in the West) then move to another orientation (eg pointing at the zenith with the mount East) I find I am no longer balanced - the difference isn't much, not enough to worry about, but the fact it exists at all is interesting.

If you achieve a perfect physical balance as you describe in all 3 axes it will be interesting to see what Autoslew's balance tool makes of it when you get your mount back and try it in various orientations.

These displacement activities while we wait for hardware to be fixed or long periods of bad weather to end can get a bit anal I find. I've spent several weeks re-architecting my perfectly functional observatory web site to be multi-lingual and use an n-tier architecture. Total waste of time really, but it keeps me out of mischief and I now know a lot more about object-oriented PHP programming and client/server interaction using JQuery and AJAX than I ever thought I would need to know! Particularly at my age.

But I'm back on site at the observatory this weekend for 3 weeks and the weather forecast is excellent so hopefully I can get back to astronomy.

Nigel
Nigel Brooks
Stratis Observatory, Hautes Pyrénées, France.
http://www.facebook.com/stratisobservatory

#3 GeorgeCarey

GeorgeCarey

    Member

  • Beta Tester
  • 345 posts

Posted 06 May 2015 - 07:29 PM

I think the Autoslew balancing tool is great, but I am not convinced it will allow you to determine radial balance.
My DEC axis always feels harder to rotate in one direction than the other.
I tried an experiment by putting the counterweight shaft to the East and pointing the scope vertically.
I then used a counterweight near the camera to give perfect balance according to the Autoslew tool.
With the scope pointing upwards the main component would be radial balance.
When I moved the counterweight shaft to the West, I got different results.
The pre tensioned bearings seem very tight, and friction possibly overwhelms fine balance attempts.
What would settle the issue is if I could get hold of a perfectly symmetrical tube with no cameras/focusers.
That would be the best way to check the balancing tool.

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#4 GeorgeCarey

GeorgeCarey

    Member

  • Beta Tester
  • 345 posts

Posted 13 May 2015 - 11:13 AM

First try at balancing the system. I bolted an aluminium bar to the side of the camera as seen here:

 

bar1.jpg

 

 

It seems to be about the right weight to balance the scope.

 

bar2.jpg

 

 

Obviously this is an awkward solution. However, it is easy to calculate what weight is needed at the end of a shorter bar.


ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#5 RamaSpaceShip

RamaSpaceShip

    Member

  • Members
  • 167 posts
  • LocationIn the forest near Bordeaux, France

Posted 13 May 2015 - 12:43 PM

Hi George,

 

Instead of adding weight, why don't you rotate the tube inside the rings so that the line and the cable are parallel?

 

This would be a much simpler solution, providing of course, that you can do the rotation.

 

 

Bernard



#6 GeorgeCarey

GeorgeCarey

    Member

  • Beta Tester
  • 345 posts

Posted 13 May 2015 - 03:17 PM

Hi Bernard,

That would work, but I like to have the vanes of the secondary support oriented N,S,E and W.

I could rotate the camera to make North upwards in the images, but then the diffraction spikes would be at a strange angle.

 

 

George


Edited by GeorgeCarey, 13 May 2015 - 03:18 PM.

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#7 GeorgeCarey

GeorgeCarey

    Member

  • Beta Tester
  • 345 posts

Posted 21 May 2015 - 01:18 PM

Second phase of the balancing.

I made a template to suspend the camera/focuser, and added a lead counterweight to the balance arm.

The balance was checked by seeing if the steel cable was at 90 degrees to horizontal lines on the camera.

 

hang.jpg

 

 

 

This gif shows the perpendicularity:

 

90degrees.gif

 

 

The only thing that could upset the radial balance now is asymmetry in the tube rings, and the placement of camera and focuser cables.


ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#8 Adamo

Adamo

    Member

  • Members
  • 19 posts
  • LocationLublin, Poland

Posted 25 May 2015 - 10:41 AM

This is madness :)



#9 GeorgeCarey

GeorgeCarey

    Member

  • Beta Tester
  • 345 posts

Posted 23 June 2015 - 02:21 PM

Camera now back on telescope. I decided the suspension cradle was inadequate,

because it was not easy to check that the cable was perfectly centred on the telescope plate.

This version uses a rectangular bar:

 

balance04.jpg

 

 

 

Result seems very good, The scope is well balanced:

 

balance03.jpg


Edited by GeorgeCarey, 23 June 2015 - 02:21 PM.

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#10 GeorgeCarey

GeorgeCarey

    Member

  • Beta Tester
  • 345 posts

Posted 01 July 2015 - 12:05 PM

The mount is now back from ASA and on the pier, so it is time to get the final stage of balancing the telescope finished.

This is to check the lateral balance:

 

balance05.jpg

 

 

The scope tube has been marked to show the position that the tube rings should be in.

It should be possible to reproduce this position if the scope has to be detached from the mount and replaced.

 

balance06.jpg


Edited by GeorgeCarey, 01 July 2015 - 12:16 PM.

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk





0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users