GeorgeCarey Posted May 6, 2015 Report Share Posted May 6, 2015 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: 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. 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 Link to comment Share on other sites More sharing options...
0 nakbrooks Posted May 6, 2015 Report Share Posted May 6, 2015 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 Link to comment Share on other sites More sharing options...
0 GeorgeCarey Posted May 6, 2015 Author Report Share Posted May 6, 2015 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. Link to comment Share on other sites More sharing options...
0 GeorgeCarey Posted May 13, 2015 Author Report Share Posted May 13, 2015 First try at balancing the system. I bolted an aluminium bar to the side of the camera as seen here: It seems to be about the right weight to balance the scope. Obviously this is an awkward solution. However, it is easy to calculate what weight is needed at the end of a shorter bar. Link to comment Share on other sites More sharing options...
0 RamaSpaceShip Posted May 13, 2015 Report Share Posted May 13, 2015 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 Link to comment Share on other sites More sharing options...
0 GeorgeCarey Posted May 13, 2015 Author Report Share Posted May 13, 2015 (edited) 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 May 13, 2015 by GeorgeCarey Link to comment Share on other sites More sharing options...
0 GeorgeCarey Posted May 21, 2015 Author Report Share Posted May 21, 2015 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. This gif shows the perpendicularity: The only thing that could upset the radial balance now is asymmetry in the tube rings, and the placement of camera and focuser cables. Link to comment Share on other sites More sharing options...
0 Adamo Posted May 25, 2015 Report Share Posted May 25, 2015 This is madness Link to comment Share on other sites More sharing options...
0 GeorgeCarey Posted June 23, 2015 Author Report Share Posted June 23, 2015 (edited) 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: Result seems very good, The scope is well balanced: Edited June 23, 2015 by GeorgeCarey Link to comment Share on other sites More sharing options...
0 GeorgeCarey Posted July 1, 2015 Author Report Share Posted July 1, 2015 (edited) 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: 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. Edited July 1, 2015 by GeorgeCarey Link to comment Share on other sites More sharing options...
Question
GeorgeCarey
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:
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.
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
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