Charts

One of the most daunting tasks new observers encounter is actually locating variable stars in the sky. Although finding the region of the sky where the variable resides may be straightforward, the actual identification of the variable is a learned skill involving patience, persistence, and a good chart. You will find that several types of charts exist.

First, you'll need finding charts. The purpose of a finding chart is to get you to the approximate vicinity of the variable star that you want to observe. A good star atlas can serve as a finding chart. Learn the bright stars, visible to your naked eye.

To aid observers with the identification of the variable star, many organizations such as AAVSO, BAA VSS, VSNET and ASSA provide charts. Figure 11.1

Figure 11.1. Star chart of FG Sagittae Chart provided by the British Astronomical Association Vorioble Star Sectio. Used *>lli permission

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is a chart for FG Sge provided by the BAA VSS. You will notice that north and south are inverted and that several stars are indicated by a number. At the bottom of the chart you will see the magnitude listed for the comparison stars. Visit several Web sites from the different organizations. Take a look at what they offer. Right now, I'm going to use the AAVSO as an example.

Like many organizations, AAVSO provides finding charts at their Web site. These charts display the field of the variable, along with other pertinent information that may be useful when observing. Shown on the charts are stars of known constant magnitude, referred to as comparison stars, that are used to make brightness estimates of the variable. With over 3000 charts, the AAVSO is one of the major sources of charts of variable stars. All of these charts are currently available online and may be downloaded for free, or may also be purchased for a fee through AAVSO.

The AAVSO provides different types of charts tailored to meet the needs, experience, and programs of their observers. When making variable star estimates for the AAVSO, they require observers to use these charts in order to avoid the conflict that can arise when magnitudes for the same comparison star are derived from different sets of charts. This could result in two different degrees of variation being recorded for the same star.

Constellation finder charts present wide-field plots that encompass an entire constellation with the location of bright stars and selected variables charted (Figure 11.2). Originally produced for their Hands-on-Astrophysics educational project, these charts may also be of use to the beginner trying to find their way around the sky.

Standard charts are for variable stars that have been in the AAVSO visual observing program for decades, and have comparison star sequences that are established and not subject to change. Always use a standard chart whenever possible. Any new observer should begin by using standard charts.

Preliminary charts are for variable stars that have comparison star sequences that may not be well established, and thus, are subject to change (Figure 11.3). These charts are typically for the more experienced observer.

Also available are reversed charts. These charts are provided in both standard and preliminary format and have been reversed north to south for use with

Figure 11.2. Finder chart for the constellation Celus. Chart provided by the AAVSO. Used with permission.

Figure 11.2. Finder chart for the constellation Celus. Chart provided by the AAVSO. Used with permission.

telescopes with an odd number of reflections, such as Schmidt-Cassegrains or refractors with diagonal mirrors. At present, reversed charts do not exist for all variables charted in the AAVSO visual observing program. A chart reversal project is underway.

Special-purpose charts such as those used for observing eclipsing binaries or RR Lyrae stars or for observing with photoelectric photometers or CCD cameras are also available. You'll also need supernovae charts if you're going to be hunting these interesting events. Supernovae charts are really charts of galaxies with the field stars indicated so that you will recognize a supernova when it appears (Figure U.4). It will be the star not shown on the chart. If you detect a supernova, you must determine its location with a good amount of precision. Usually, the chart will not be a good source of accurate position. Also, remember to get a confirmation from someone else. The Internet is a good place to announce your discovery and ask for a confirmation observation from another astronomer.

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(1900) IS* 54* 41* -22° 20*3 (2000) 16" 00m 20* •22*37:3

PRELIMINARY ARV50 CHART

SUiJiCT TO COWCCTIOM

S Sco (Scorpii)

(1900) IS* 54* 41* -22° 20*3 (2000) 16" 00m 20* •22*37:3

PRELIMINARY ARV50 CHART

SUiJiCT TO COWCCTIOM

Figure 11.3.

Preliminary chart (or i Sco. Chart provided by the AAVSO. Used with permission.

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Drawn by: CES 8/00 from: AAVSO VorloDlo Star Atla» S»qu«»c«: PCP(V>. Hippatco» & fycho Cqtolopu«»

Figure 11.3.

Preliminary chart (or i Sco. Chart provided by the AAVSO. Used with permission.

The charts range in scale from 5 arc minutes per millimeter, "a" scale charts, to 2.5 arc seconds, "g" scale charts. The scales needed for your observing program will depend on the equipment that you are using. Table 11.1 summarizes this information.

Again, many organizations provide charts. It will be well worth your time and effort to take a look at what they offer. You will find the Web address for several organizations later in the book.

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