Transcript Balancing an equatorially mounted telescope

Balancing an equatorially
mounted telescope
Paul McGale
A typical equatorial mount
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
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!!
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)
Positions
1. Meridian, tube horizontal
2. Meridian, tube vertical
3. Meridian, tube vertical
4. Six hours away from meridian, tube
vertical (ideally pointing north)
• Balancing around the declination axis
C
• Want to get the centre of gravity, C, of the tube onto the declination axis.
Starting position
With scope in
horizontal position,
swing back and
forth to test balance
Scope back heavy, so will need some
weight toward to the front to balance
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
With scope back in meridian
position, it will fall left or right if
unbalanced
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”
To get C onto D need to move weights horizontally!
C
D
Counter-weight has been
moved horizontally to
achieve radial balance
• Balancing around the polar 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.
Starting position
Swing scope to test
balance
• 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
• 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)
N
.X’
W
X
A
R
P
T
X’
Yerkes’ solution
to placing a
weight at X’
C
Done!!
Telescope should now be in balance for all positions in the sky
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
Reference:
• Title: On the Balancing of Equatorial
Telescopes
Authors: van Biesbroeck, Georges.
Journal: Popular Astronomy, Vol. 50, p.253
Bibliographic Code: 1942PA.....50..253V
• http://adsabs.harvard.edu/full/1942PA.....50..253V