Compound East Bias for German Equatorial Mounts

In the conventional manner I used to properly balance and apply East Biasing to my AP1200, following that I ran PemPro to PEC characterize the RA Worm. Yet I continued experiencing poor RA guiding when in the Eastern Hemisphere.

PemPro requires the mount be centered on a star near the Equator and close to the Meridian on the West side. Thinking about the gear contacts I realized PemPro characterized one side of the Worm to Main gear surfaces, when in the Eastern Hemisphere the opposite gear surfaces are in contact. This was causing my problem of good guiding in the West and poor guiding in the East.

To solve this dilemma I came up with what I call my “Compound” East Biasing technique. I have been using this method of imparting an East Bias on ‘both’ sides of the Merdian for over a year now with good results. Guiding on either side of the Meridian is very accurate and nearly identical with minimal effort.

East Biasing is a method of adding weight to the counterweight shaft in the East side to keep the Worm to Main Gear surfaces in contact, the Worm is “lifting” the Main Gear. Without this additional pressure corrections to the Worm can cause the Main Gear to loose contact with the driving Worm, this is known as Bounce. Bouncing can require additional corrections possibly leading correction oscillations.

In the conventional East Biasing method when on the West side the counterweights pull the Main Gear downwards. This causes the Worm to Main Gear surfaces to change sides and the Main gear “follows” the Worm gear instead of being driven by the Worm. This condition can also introduce Bouncing leading to oscillations.

My Compound method does not require moving counterweights up and down on the counterweight shaft in the dark, it does require the use of hanging Biasing weights as the end of the counterweight shaft in the Eastern Hemisphere to maintain proper Worm to Main Gear contact and pressure. When changing to the West side simply removing the hanging Biasing weights automatically compensates keeping the ‘same’ pressure and the ‘same’ gear surfaces in contact as in the East side. It’s all in the manner in which you plan and apply your Biasing.

First figure out how much weight you require to normally East Bias with the counterweight shaft in the East. I use Lead 8oz Egg Sinkers which have built-in holes and are readily available in most fishing departments. Simply run a string through the holes and loop one end over the counterweight stop on the end of your counterweight shaft. You will need "double" the amount of lead sinkers you needed to impart your normal East Bias. Half will be used in Step-2, half in Step-9 below.

Here’s the tricky part. You are going to East Bias on “both” sides of the mount. Your mount should already be balanced before performing this procedure.

  1. Place your counterweight shaft on the **East** side.
  2. Hang the Biasing weights at the end of the counterweight shaft until you determine your required amount and the shaft falls normally. Note the amount of weight you used. You should have this amount spare to use in Step-9 below.
  3. Do not remove the 'hanging' Biasing weights and swing the counterweight shaft to the **West** side. The counterweight shaft will also fall on this side too.
  4. Now adjust the placement of your main counterweights until the counterweight shaft remains horizontal. Do not allow any downward or upward movement of the counterweight shaft. You just imparted the West side East Biasing, the "Compound" component.
  5. “Carefully” remove the hanging Biasing weights minding the OTA which is on the East. The “OTA” will fall to the East because it is top heavy. This is what you want.
  6. Reattach the lead sinkers and check there’s no downward or upward movement.
  7. Rotate the counterweight shaft placing it on the East side again.
  8. The counterweight shaft will remain motionless if you properly performed Step-4 above. To check this once again "carefully" remove the hanging Biasing weights, the OTA will now fall to the West side.
  9. After replacing the hanging Biasing weights, now add the extra Egg Sinkers to the hanging Biasing weights doubling the amount already there. This imparts the East Bias to your mount which you determined in Strp-2 above.
  10. When working in the Western hemisphere remove the hanging Bising weights.

This technique effectively makes your mount drive from the same side of the 'Worm and Main Gear' surfaces with the same pressure regardless which side of the Meridian you are in. All you have to do is add the weights when imaging in the East and remove them when moving to the West.

There's an additional benefit when using the “Compound” East Biasing technique. Regardless how you used to PEC characterize your Worm now do so in the West near the Equator. Afterwards you should not need to retrain your PEC when you move to the Eastern Hemisphere. “Compound” Biasing maintains the same gear surface contacts and gear pressures so retraining upon a Meridian Flip is not necessary.

It took me awhile experimenting to develop this concept. If you wish to ask questions please feel free to eMail me at I’ll be glad to help.

“May You Go Among The Imperishable Stars”, Smoother