Balancing for Imaging
Properly balancing your telescope is one of the most important steps in setting up to take CCD images. Improper balance is also one of the biggest sources of problems for imagers. Taking some extra time to understand the concepts behind balancing and to correctly balance your telescope will definitely pay off in the end. Balance is very important - don't let anyone tell you otherwise! No matter how "strong" your telescope's drive motors are (or the manufacturer claims them to be), you will always have more accurate tracking and exert less wear on the mechanical components of your scope if you properly balance it.
Note: It has been our experience that almost all tracking problems are due to improper balance. Many of the balance systems sold for telescopes are not even designed to allow proper balance to be achieved! Read this section carefully and you will avoid a lot of problems. This is the only Note in this entire website written in 14-point font. It's that important!
First, we will look at how to balance a telescope, then discuss the ideas behind balance so you can better understand it.
For more specific details, read the section below, then print out the above Quick Reference to have with you in the field.
If you are using a telescope on a German equatorial mount you should already be somewhat familiar with the basic concepts of balancing a telescope if you have already used it visually. However, the fork-mounted Schmidt-Cassegrain telescopes which are popular among CCD imagers do not typically need to be balanced (as they are set up from the factory) for visual use. Anytime you place a significant amount of additional weight on your telescope you should rebalance. If you add 2" eyepieces and diagonal, a Crayford-style focuser, a dewshield, or a camera, you should rebalance the telescope so the motor drives do not have to work any harder than necessary and so the tracking is as accurate as possible.
When you set up for CCD imaging, you might do nothing more than mount a CCD camera in place of the telescope's eyepiece and diagonal. Since many CCDs do not weight much more than a typical eyepiece and diagonal, not much rebalancing is necessary. But more than likely you will attach other accessories such as dewshields, motorized focusers, focal reducers, etc. Now is when proper balance becomes so critical.
Balancing a Fork-Mounted Telescope
This section covers all fork-mounted scopes such as the popular goto Schmidt-Cassegrain telescopes.
The biggest difference between balancing for visual use and for CCD imaging is that most fork-mounted scopes are mounted in an altitude-azimuth configuration visually but must be on an equatorial wedge for imaging. This adds one more variable for balancing.
It is important to balance the scope in a vertical position first, then horizontally. See the section at the bottom of the page for details why.
Begin by aiming the telescope straight up. If the telescope is top-heavy (from the finderscope, etc.), you will need to add weights to the bottom, usually by use of a counterweight slide bar as seen above. Some telescopes come with a slide bar, but for most scopes this will be an additional accessory.
Above: If the scope is top-heavy (toward the finderscope, or right in this picture), add weights to the opposite side until balance is achieved. If the telescope is bottom-heavy (toward the counterweight bar) remove weights until balance is achieved. When you are close to balance you may need to give the scope a gentle push in each direction to tell whether more adjustment is necessary or not.
After the telescope is balanced vertically, point the scope horizontally. You can now move the sliding weights to balance front to back.
Above: If the telescope is front-heavy slide the counterweights toward the back (eyepiece-end) until the scope is balanced. If the scope is back-heavy slide the counterweights toward the front (away from the eyepiece) until balance is achieved.
Balancing a German-Equatorial-Mounted Telescope
If your telescope is mounted on a German Equatorial mount, you will likely be familiar with balancing from visual observing. However, balance is much more critical for CCD imaging, and there is a right way and a wrong way to balance your scope, so be sure to look over the following section to make sure you are using the correct method.
It is important to balance the telescope tube (declination axis) first! See the section at the bottom of this page for details why. In some cases (but not most) you may need to place additional counterweights on the telescope dovetail bar if the telescope is very back-heavy (if you are using a large CCD camera, and focuser, focal reducer, etc.).
Above: Push the telescope gently up and down to determine if it is front or back heavy.
Above: If the telescope is front-heavy, slide the tube (or additional dovetail counterweights) rearward on the mount. If the scope is rear-heavy, slide the tube forward on the mount.
Note: Remember to return the telescope to its upright position (with the tube pointing north) before moving the optical tube so there is no chance of the scope falling off the mount!
Above: Return the telescope to the horizontal position once balanced in declination. If the telescope is heavy toward the optical tube, slide the counterweight down the shaft away from the scope. If the telescope is heavy toward the counterweight, slide the weight up the shaft toward the scope.
Why do you have to balance a fork-mounted telescope vertically first, or balance a German-mounted scope's optical tube first? The reason lies in the position of the center of gravity. If you do not balance a fork-mounted scope vertically first, the center of mass of the tube is offset, meaning the position of the counterweights front-to-back on the scope will be incorrect. Even if the telescope seems balanced in one position, when pointed to another area of the sky it may not be balanced.
German Equatorial Mount
If the telescope tube is not balanced first, then the center of gravity (CG) of the system is not along the counterweight shaft axis. This will cause the counterweight to be placed too close to the telescope.
Balancing the optical tube first places the center of gravity in the correct position. Now the counterweight will be in the right place when the telescope is balanced in right ascension and the entire system has been properly dynamically balanced.
If the telescope is not balanced vertically first, the center of gravity (CG) will be displaced. In this example, the telescope is too heavy toward the counterweight and the center of gravity is low. This will cause the counterweight, when balanced horizontally, to be placed too far forward.
If the telescope is balanced vertically first, the center of gravity is in the correct position. The counterweight, when balanced horizontally, ends up in the right place and the telescope is properly dynamically balanced.