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.
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.
Click Here for Quick Balance Reference
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.
However, the computerized Schmidt-Cassegrain telescope which are very 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 (CCD or film), 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 Schmidt- and
Maksutov-Cassegrains, including the popular goto models.
The biggest difference between balancing for visual use and for CCD imaging
is that most fork-mounted scopes are mounted in an
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
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
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 of 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
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.
Next, Polar Alignment....