Finding Variable Stars with "Go To" Computerised Telescopes

Many modern telescopes are computerised, with a "go to" function, allowing the user to key in the identity of the target, and then to sit back and watch as the telescope swings toward it. If you wish to find variable stars this way, I recommend that you create a chart with a reasonably bright "finder star" near the variable, and that you programme the telescope to go to that bright star first. Unless you have a very expensive setup, your target may not be in the centre of the field of view, and if that is the case, having a bright, easily recognizable target makes life easier.

Once you are near the variable, consult your chart and find the variable by moving the telescope manually. The problem with trying to let the telescope slew to the variable itself is that the field may be crowded with stars, the variable will be of similar magnitude to surrounding stars (i.e., it won't stand out), there may not necessarily be an easily recognizable pattern of stars (asterism) near the variable, and you may waste time trying (sometimes with no success) to reach your target. This advice comes from personal experience.

Finder and Comparison Star Charts

A finder chart allows you to identify a star, or, if it is faint, brighter stars in its vicinity to help you get orientated. For the brightest stars, the finder chart may also be the comparison star chart, which will show the variable and stars near the variable with numbers beside them, such as 59, 70 and 103. The stars so numbered are comparison stars (sometimes called "comp" stars), and the numbers refer to their magnitudes. The magnitudes are shown with the decimal places omitted, so that they cannot be confused with faint stars. Thus, 59 is magnitude 5.9, 70 is magnitude 7.0 and 103 is magnitude 10.3. If the variable is rather faint, more than one chart may be needed to allow easy "star hopping" to find it. In general, finder and comparison star charts for fainter variables will cover a smaller field of view. I usually print my own finder charts from one or other planetarium programme on my home computer, and find it a good idea to show stars on the charts as faint as the limiting magnitude of the instrument through which I observe that field.

If you use binoculars or a Newtonian reflector telescope, use charts in which north is up, and west is to the right (i.e., west is 90 degrees clockwise from north) - the same orientation that you see in those instruments, and with your unaided eyes. Of course, the view in the telescope will be inverted (upside down), but otherwise the orientation is as stated.

If you use a refracting telescope or a Schmidt-Cassegrain telescope, use charts in which north is up, and east is to the right (i.e., east is 90 degrees clockwise from north).

If you use a refracting telescope or a Schmidt-Cassegran telescope, and your eyepiece is inserted into an erecting prism, use charts in which north is up, and west is to the right.

Organizations that foster the observation of variable stars (see below) produce comparison star charts. The use of these charts standardizes the comparison stars that are used to make estimates of the magnitudes of the variables. The use of these charts, with "official" comparison stars, is encouraged.