Offsetting from Stars

Their inherent inaccuracies give less trouble if you use setting circles only to measure your way a few degrees across the sky rather than all around the celestial sphere. This is the offsetting method of finding objects from a known star.

This method works even with the oldest-style setting circles that only read hour angle from the celestial meridian instead of right ascension. (These are identified by their 0 to ±12 hour markings that can't be set to anything but 0 when the scope is pointed at the meridian.)

First check that the telescope is polar-aligned moderately well. The polar axis of the mounting should be aimed at the celestial pole to within a couple of degrees. (Instructions for polar alignment come with most equatorial scopes.)

Look up the coordinates of your target object and any fairly bright star within 10° or so of it. Subtract the right ascension and declination of the star from those of the object. The result tells you how far from the star to swing in declination going north (or south if the value is negative), and how far in right ascension going east (or west if negative).

Most setting circles have rulings every 1° in declination and every 5 minutes of time in right ascension. So express your declination offset in degrees and right ascension in minutes. Try to read the declination dial to a tenth of a degree and the right ascension dial to one minute or better.

Offsetting can be very useful if the normal method of finding objects — star-hopping with the aid of a good map — isn't working. Perhaps you don't have a map that shows enough stars for you to home in on the exact point. Perhaps your finderscope is too small or the light pollution too bad, or you've repeatedly gotten lost in a difficult field and want to try a new tack.

Offsetting is especially efficient when you plan to survey many objects in a small area of sky. Work out your offsets indoors beforehand, and write them in your observing notebook.