All-Sky Use

If you want to find objects anywhere in the sky by dialing their coordinates, you should understand the many precise adjustments required to your telescope.

Suppose your lowest-power, widest-field eyepiece gives a 1° true field of view, typical of amateur instruments. If the telescope is pointed ½° wrong, your object will be on the edge of the field where it will go unnoticed. Merely to place it closer to the center than to the edge, you have to aim with ¼° accuracy.

What are the adjustments? The axis of the telescope's optical system should be made truly perpendicular to the mount's declination axis. This in turn should be perpendicular to the mount's polar axis. The polar axis must be accurately aligned on the celestial pole. The circles themselves must be positioned just right. Last, you must read the circles accurately — usually to a small fraction of their finest gradation.

Some of these adjustments have two degrees of freedom, such as in altitude and azimuth when aligning on the celestial pole. So all told, there are eight variables where error can creep in.

Based on the way simple random errors add up, each of these eight adjustments must be good to 0°.09 accuracy to achieve an average total error of 0°.25 in where the telescope is pointed. Half the time the errors will add up to be better than this, half the time worse. To make them fall consistently on the better side, you should strive for even finer accuracy — say 0°.05 — in each adjustment.

No wonder setting circles have a reputation for never working.

We'll deal with each adjustment in turn.

First, make sure that the optics of the telescope are collimated (aligned) as best you can. Collimation on a reflector is usually just a matter of turning the adjustment screws behind the primary mirror to make a slightly out-of-focus-star image perfectly round when centered. On a Schmidt-Cassegrain telescope, you make tiny adjustments to the screws on the secondary mirror mount. Refractors rarely need collimation. Instructions for collimating a telescope usually come with it.

If you use a star diagonal, such as on a Schmidt-Cassegrain, be sure it too is collimated if it has adjustment screws on its back. Using high power, center the scope on an object while viewing "straight through" without the diagonal. Then insert the diagonal and see if the object is still centered. If it's not, turn the diagonal's adjustment screws until it is.

The reason for getting collimation all squared away first is that when you collimate a telescope, you change its aim point — that is, the direction of its optical axis with respect to the tube. After you collimate you will have to realign the finderscope to match the main telescope's new aim.

Now swing the tube to about 90° declination. While looking through your lowest power eyepiece, swing the mount back and forth in right ascension by turning the polar axis. You will see the field slowly turning. Make slight adjustments to the declination so the motion of the field is minimized when you turn the scope.

Ideally, you will find a declination position where the stars rotate around the exact center of the field. This happy state of affairs means you have gotten the optical axis truly parallel to the mount's polar axis.

Don't expect it to happen. Instead, you will only be able to find a place where the field motion is minimized, not reduced to zero. The point of sky around which the field appears to rotate will be off to one side, perhaps out of view entirely.

You want to shim the telescope tube in its cradle, or adjust the fork arms if the scope has a fork mount, to bring this point to the center of view. While turning the scope in right ascension, form a mental image of where the field's center of rotation lies. Nudge the scope that way to judge which side of the cradle needs to be shimmed, or which fork arm raised.

You can use strips of brass or plastic or folded-up aluminum foil for shimming. Adjust a fork arm on a Schmidt-Cassegrain scope by loosening the bolts that hold it to the drive base and sliding the arm slightly up or down. (This may be limited by the size of the bolt holes) The adjustment may take quite a bit of trial and error, but it's a job you'll only have to do once.

If your telescope tube can rotate in its cradle (a convenience on many reflectors), you may find you can get closer to the ideal after rotating the tube by some amount. Try this first, then do the shimming. Just remember that in actual use, you may need to rotate the tube back to the position it's in right now before the setting circles will work well. Mark the tube so you can do this if the circles later give problems.

Once you've done the best you can, loosen the declination circle, turn it to read precisely 90°, and retighten it permanently.