balancing an equatorially mounted telescope paul mcgale
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Balancing an equatorially mounted telescope
Paul McGale
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A typical equatorial mount
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Why balance?
• Dangerous if clutches become free
• If driven, mount can labour when the scope is in certain orientations
• Most tracking errors are the result of an out of balance scope → bad autoguiding!
• Correctly balanced scope should not move in any orientation when clutches are free
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Why balance?
• Balanced when the centre of mass of all the moving parts lies on the polar/RA axis
• Large majority of books, manuals, and websites do not tell you how to do this correctly!!
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Procedure
• Hopefully manufacturer has constructed the mount such that the polar axis is at right angles to the declination axis!
• Idea is to start to balance around the declination axis (2 positions) and then balance around the polar axis (2 positions)
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Positions
1. Meridian, tube horizontal
2. Meridian, tube vertical
3. Meridian, tube vertical
4. Six hours away from meridian, tube vertical (ideally pointing north)
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• Balancing around the declination axis
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Dec a
xisPolar axis
C
• Want to get the centre of gravity, C, of the tube onto the declination axis.
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Starting position
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With scope in horizontal position, swing back and forth to test balance
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Scope back heavy, so will need some weight toward to the front to balance
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D
C
• Centre of gravity C of all the parts rotating around the declination axis not necessarily on that axis, D, but only in C somewhere above or below it
• Difference between position of D to C most obvious when scope is rotated 90 degrees to current position
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With scope back in meridian position, it will fall left or right if unbalanced
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Scope will fall left or right if there isn’t symmetry in the weight distribution. Above would require a weight at the 7 o’clock position to balance “radially”
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DC
To get C onto D need to move weights horizontally!
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Counter-weight has been moved horizontally to achieve radial balance
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• Balancing around the polar axis
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Dec a
xisPolar axis
C
• Balancing around declination axis has brought the centre of gravity, C, of the tube onto the declination axis.
• Need to balance on the polar axis to get C to coincide with the intersection of the mount axes.
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Starting position
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Swing scope to test balance
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• Centre of gravity of whole system, R, is now in the vertical plane of the polar axis
• Don’t know if R coincides with A though – the intersection of the polar and declination axes
• R not on A may be due to e.g. build of the mount, placing of the motors etc not symmetrical w.r.t the plane of the two axes
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• Difference between position of R to A most obvious when polar axis is rotated 90 degrees to current position
• Will notice an inequality in ease of motion of the scope if R not on A
• Can add a weight at X to balance (or any point above axis e.g. X’ laterally to counterweights)
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N
W T
P
A
X
R
.X’
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X’
Yerkes’ solution to placing a weight at X’
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Dec a
xisPolar axis
C
Done!!
Telescope should now be in balance for all positions in the sky
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Closing thoughts
• May want some bias to mesh gears– Polar axis, a slight increase in weight on east side of mount– Dec axis, a slight increase in weight at end of scope
• Small counter-weight far from intersect or big counter-weight near intersect
• For small scopes can test balance by trial and error• Balance can be calculated in advance from the
moments
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Reference:
• Title: On the Balancing of Equatorial TelescopesAuthors: van Biesbroeck, Georges.Journal: Popular Astronomy, Vol. 50, p.253Bibliographic Code: 1942PA.....50..253V
• http://adsabs.harvard.edu/full/1942PA.....50..253V