Preparing Your Own Charts

Some of the astronomical associations are not going to be happy with this section of the book (they really like you to use their charts) but for you to truly understand how a star chart works you must make and use a few of your own. Besides, in some cases, you'll find no charts

^preparations

Figure 11.4.

Supernova chart for NGC 1365. Chart provided by the AAVSO. Used with permission.

0329-36B NGC 1365 (For)

{19C0J 03"29-36°28'7 «"•«««"« (2000) 03h33m26s -SB^OS'i

0329-353 SN 2001DU FOV: 20YW

Drav*iby KM8/0: SM 200 1dj is located aoout 90' westand

Pom Digitized Sky Survey 10" south of the nucleus of NGC1365, and Sequence. iJSHO A2.0 Catalog atcut 20" west ot a !5tti mag toregroud star 'he approximate oosibon is showi c<o»Tqrt©200l AAVSC

available for a particular star or field and so you must prepare your own.

A great place to begin to produce your own finder charts is at the USNO Finder Chart Service (httpj/ftp. nofs.navy.mil/data/FchPixf). This facility allows you to extract catalog data from the USNQ-A2.0 and/or ACT

Table 11.1.

Scale

Arc/min

Area

Recommended use

0

5 min

15 degrees

binoculars/finder

ob

2.5 min

7.5 degrees

binoculars/finder

b

1 min

8 degrees

small telescope

c

40 sec

2 degree

3-4 inch telescope

d

20 sec

1 degree

> 4 inch telescope

e

10 sec

30 minutes

large telescope

1

5 sec

15 minutes

large telescope

9

2.5 sec

7.5 minutes

large telescope

catalogs and plot finder charts from these lists. In addition, you can find the raw images from scanning the major photographic surveys. You can overplot the catalog data onto the images, as well as overplot your own additional markers.

When you first go to this Web site, you will be provided with a split screen. The top part will provide instructions for using the Finder Chart Service. The lower screen allows you to query the service. This is a remarkable service and the folks at the US Naval Observatory provide a great resource; however, because the Finder Chart Service is a comprehensive service, you're not going to simply push a button and get a nice finder chart. Plan on spending an hour or so investigating this facility. Don't go to this service minutes before you need a finder chart and expect to zip through the process and get your chart.

Another great place to begin to put a chart together is SIMBAD at the Centre de Données Astronomiques de Strasbourg (CDS) (http://simbad.u-strasbg.fr/sim-fid.pl). The SIMBAD astronomical database provides basic data, cross-identifications and a bibliography for astronomical objects outside the solar system. The "SIMBAD: Query by identifier, coordinates or reference code" page allows you to enter a variable star by name or coordinates. It then provides you with a data page. From the basic data page, you can open the "ALADIN Java: Sky Atlas." ALADIN is an interactive software sky atlas that allows you to visualize digitized images of any part of the sky, to superimpose entries from astronomical catalogs, and to interactively access related data and information.

Sound complicated? The CDS provides a tutorial for their main services, including the SIMBAD astronomical database and ALADIN sky atlas, at (http://cdsweb.u-strasbg.fr/Tutorial/index.gml).

ESBSGSSS^^H

A star sequence is a list of stars used to judge the brightness of a variable star. They are the comparison stars and they must be selected very carefully. There are several things to consider when selecting comparison stars.

First, they cannot be variable. Finding stars that are not variable is not as simple as it sounds. In fact, it's not unheard of to discover that a comparison star that has been used for some time is actually variable when it's examined closely. Also, when observing the brightest stars, it's fairly difficult to find close bright stars that are not variable.

Then, you want a number of constant stars that span the amplitude range of the variable. You want a sequence of stars that extend a little bit brighter and fainter than the variable is known to shine. This way, you can interpolate by placing your variable between a brighter and fainter star when estimating its brightness.

In an ideal sequence, all of the stars are of the same spectral type as the variable. Unfortunately, this will rarely happen. If possible, select stars close to the same spectral type as the variable. Even this may be difficult to do. For example, if the variable star is an A-type star, try to find other A-type stars to use as comparison stars. When this is impossible, try to use B-type or F-type stars. Stay as close to the variable star's spectral type as possible.

And finally, you want your comparison stars to be close enough to the variable so that you can make quick comparisons without moving long distances across the sky. Not only is this time-consuming but if you are comparing your variable with a comparison star more than a degree removed from the variable, the effects of the atmosphere begin to become noticeable.

As you can see, much effort goes into selecting the star sequence that provides the comparison stars. The importance of comparing your variable star to acceptable comparison stars should also be obvious now. If hundreds, even thousands, of variable star observers all compared their variable stars to different comparison stars, with no concern for the factors just discussed, it would be impossible to compare estimates, check for errors, or standardize the observations.

When possible, use approved charts from the BAA VSS, AAVSO or VSNET. Other organizations provide charts too. When you're required to make your own charts, check the literature for stars that have been used as comparison stars in the past. If you find yourself needing comparison stars and none exist, carefully develop your own sequence with the assistance of other variable-star observers. The odds are, for even the smallest portion of the sky, someone, somewhere has a good start on a star sequence that will provide comparison stars.

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