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

Dec a

xisPolar 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”

DC

To get C onto D need to move weights horizontally!

Counter-weight has been moved horizontally to achieve radial balance

• Balancing around the polar axis

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.

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

W T

P

A

X

R

.X’

X’

Yerkes’ solution to placing a weight at X’

Dec a

xisPolar axis

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 TelescopesAuthors: van Biesbroeck, Georges.Journal: Popular Astronomy, Vol. 50, p.253Bibliographic Code: 1942PA.....50..253V

• http://adsabs.harvard.edu/full/1942PA.....50..253V