…continuedThe Setting Circles on Your Telescope
Now a confession: we've skipped a step. In the case of a German equatorial mount we haven't checked that the declination axis is perpendicular to the polar axis, and with a fork mount we aren't sure if the optical axis is perpendicular to the declination axis. That's because there is little or nothing you can do about it. Trust the manufacturer and cross your fingers.
The next step is accurate alignment on the celestial pole. Some telescopes come with pole-finding reticles for their finderscopes. Another method that is especially precise is described in the article "Accurate Polar Alignment."
Now, at last, the setting circles are ready for their intended use!
The declination circle need never be touched again. But the right ascension circle does have to be repositioned at the start of each observing session, because the sky is always moving.
Aim at a bright star whose right ascension you know. (It's handy to keep the right ascensions of a dozen bright stars on the inside cover of your observing notebook.) Slide the right ascension circle to read the correct value for that star. On a German equatorial mount, the star should be on the same side of the mount as the objects you'll be looking for.
Now you can dial in the right ascension and declination of any object in the sky. Look in your lowest-power eyepiece, and there it should be.
If your right ascension circle is driven by your telescope's clock drive, as is the case with all Schmidt-Cassegrains we know about and many reflectors, you can dial in object after object all night without touching it again. If the circle is not driven, reposition it to the right ascension of the current object just before swinging to the next.
Technology to the Rescue
New ways have recently been invented to circumvent the problems that make setting circles so error-prone. These methods revolve around the "digital setting circle." In its simplest form, this is nothing more than a readout in little red numbers of what an ordinary setting circle tells you with a dial and pointer. But once this data is electronically encoded, a computer chip can begin to work miracles with it.
In some versions you can simply "initialize" the circles by setting on two or three bright stars at the beginning of a session, and the chip corrects for misalignments of many kinds even failure to polar-align at all.
The next step up in sophistication is automatically correcting for lack of perpendicularity in the mount's axes compensating for imperfect mechanics by smart electronics.
Team up good digital setting circles with a computerized data base of celestial objects, and you gain the astounding finding capabilities of a "computer assisted" or "robotic" telescope. These are currently working a revolution in high-end amateur astronomy, finally fulfilling the promise of what many people thought setting circles were supposed to do all along.