Balancing a Fork Mounted Telescopeby: P. Clay Sherrod
This is an issue that has been dealt with in many places but never stressed enough as to importance. To properly balance a fork mounting you must use the
3-dimension Losmandy or equivalent counterweight set, have temporary alternate various weight combinations and follow the steps below.
If you are using your telescope for piggyback imaging, or if you have heavy tail end equipment (robotic focuser, camera, reducers, etc.), there is extra need for the most caution in maintaining balance.
The rule for balance is simple:
"If you unlock either axis and the telescope MOVES....you are out of balance." In such conditions, the telescope will suffer from performance, tracking, GO TOs and even will rapidly wear down over time.
Balancing is simple. BALANCE THE DECLINATION AXIS FIRST......once balanced, then balance the Right Ascension axis.
There are two major RULES regarding balance:
a) for balancing the DEC axis (do this first) you ONLY use weights along the OTA and/or on the rear cell
b) for balancing in the RA axis (only AFTER the DEC is balanced) you must use ONLY the fork arms....you never balance RA adding or moving weights along the OTA.
c) no exceptions to this.
Balancing the DEC axis: (NOTE that if you have a guide scope or piggyback smaller scope riding on top of the telescope, you MUST have an equal offsetting weight 180 degrees around, on the "belly" of the OTA!)
NEVER use weights under the telescope optical tube if you do not have something offsetting that balance above it! You will see why shortly.
1) aim the telescope due south in the equatorial (polar) mode and tilted about 45 degrees
up from the south horizon; if you have weight on both top and bottom of OTA, have the 3-D weight screwed as close to the surface of the OTA
2) unlock the declination;
3) adjust the tube counterweight in this position by sliding up and down the length of the OTA
until the out of balance situation stops;
4) turn the telescope straight up (zenith) and carefully unlock...the telescope will want
to tilt either north or south;
5) at this point, the Meade and similar weights are useless because they cannot account
for the perpendicularity of the torque in this position; here is where the 3-D weight
system shines.....the telescope in most cases will want to tilt northward from the
zenith...if so, the solution is simple....unscrew the counterweight AWAY from
(perpendicular to) the OTA until balance is achieved;
6) if the scope attempts to move southward, then the counterweight is too great and you
must go to a smaller size and start over.
NOW....if this does not work, then it is likely because you have heavy equipment on the
FRONT end of the scope that cannot be balanced because of the center of gravity so close
to the back of the OTA....so:
7) add weight to the rear cell; for most applications other than a very heavy dew shield,
the Peterson rear cell balance is an ideal solution....for others it may not be enough;
8) thus, use the REAR HANDLE of the OTA to add weights in any way that you can to achieve
front-to-back balance in the south-facing (#1 above) position and repeat the other steps
in sequence once that balance is achieved;
Balacing in Right Ascension
Once the Declination axis does not move when unclamped, lock the DEC. and unlock it in RA and turn to the SE sky.....if the scope
moves eastward then add some type of temporary weight via a wire tie to the RIGHT fork arm
handle; if to the west, then opposite.
Remember THREE Rules:
1) Always balance DEC first and RA last
2) ONLY use weights along the OTA tube to balance in DEC, never add weights to the mount
3) ONLY use weights on the fork arms, elbows, etc. to balance for perfect RA balance, NEVER use the OTA or weights on that.
Do it in this order and you will be on your way to perfect balance.
Best of luck!