How To Use Variable Star Charts

What Scale to Use?

On this photographic plate of the Andromeda Nebula (the image is a negative with stars appearing as black dots against the background of white space), Edwin Hubble found and marked several suspected novae with the letter "N". Later he discovered one of these was actually a cepheid variable - crossing out the "N" he wrote "Var!" in the upper right hand corner. Image Credit: Mount Wilson Observatory, Historical Archive

Using the method described below, new observers should ascertain the approximate size of the field of view of their telescopes with the different eyepieces. Point the telescope at a region not far from the celestial equator and without moving the instrument allow a bright star to trail through the field. The star will move at a rate of one degree in four minutes, near the equator. For example, if two minutes are required for the star to pass across the center of the field, from edge to edge, the diameter of the field is one-half of one degree.

Once the instrument's field is determined, a circle with the proper diameter may be drawn on the chart, with the variable at the center, as an aid in identifying a new field. Or, it may be useful to represent the field on the chart by using a piece of cardboard with the proper-size hole in it, or by making a wire ring to lay over the chart, etc.

Orientation of Charts

In order to use the charts successfully, you must learn how to orient them properly to the sky. On AAVSO chart scales "a", "aa", and "ab", north is up and east is to the left. These charts are appropriate for use with the unaided eye or with binoculars.

The drawing above shows how a group of stars appear in binoculars, in a typical Newtonian reflector with an even number of reflections, and in a typical refractor or Cassagrain telescope with a right angle prism (odd number of reflections). Below each instrument is drawn a chart typed commonly used with that instrument. The charts are also oriented correctly.

For chart scales "b" and larger, south is up and west is to the left. These charts are appropriate for use with reflecting telescopes where there is an even number of reflections, resulting in a field that is seen upside-down. For refracting and Schmidt-Cassegrain telescopes, a right-angle prism (diagonal) is normally used, resulting in an odd number of reflections. This produces an image which is right-side up, but east and west are flipped (i.e. a mirror image). In this case, whenever possible, you would be well advised to use AAVSO reversed charts on which north is up and west is to the left. If you are in need of a reversed chart and one does not yet exist, it may be possible to reverse a chart yourself by either flipping the chart over and redrawing it through the back side, or using computer imaging software to do it for you.

Comparison Stars

The scale of magnitudes may seem confusing at first, because the larger the number, the fainter the star. The average limit of naked-eye visibility is 6th magnitude. Stars like Antares, Spica, and Pollux are 1st magnitude, and Arcturus and Vega are 0 magnitude. The very bright star, Canopus, is -1 (minus one), and the brightest star in the sky, Sirius, is -1.5.

On AAVSO charts, the comparison stars are designated with numbers which indicate their magnitude to tenths. The decimal point is omitted to avoid confusion with the dots which represent stars. Thus 84 and 90 indicate two stars whose magnitudes are 8.4 and 9.0, respectively.

The magnitudes of the comparison stars used on AAVSO charts have been determined carefully and are considered as a sort of measuring rod in estimating the magnitude of the variable. It is important for the observer to keep a record of which comparison stars are used when making an estimate of a variable's brightness.

Because the magnitude scale is actually logarithmic, a star "twice as faint" as another would not be represented by the magnitude number simply doubling in value. For this reason, the observer must always be careful to use comparison stars that are not too far apart in brightness-not more than 0.5 or 0.6 of a magnitude apart-when making estimates of brightness.

Knowing Your Limit

It is best to use only just enough optical aid to enable the variable to be seen with ease. In general, if the variable is brighter than 5th magnitude, the "unaided" eye is best; if between the 5th and 7th, the finder or a good pair of field glasses is advised; and if below 7th magnitude, high-power binoculars or a telescope of three inches aperture or more, according to the magnitude of the variable, should be used. Estimates of brightness are easiest and most accurate when they are 2 to 4 magnitudes above the limit of the instrument.

The table below is an approximate guide to limiting magnitudes versus telescope size. What you are actually able to observe with your own equipment may be quite different from this, due to varying seeing conditions and quality of the telescope. You may wish to create your own table of limiting magnitudes by using a star atlas or chart with magnitudes given for easy-to-find non-variable stars.

When a faint companion star is found near a variable, be sure that the two stars are not confused with each other. If the variable is near the limit of visibility and some doubt exists as to positive identity, indicate this in your report.

The experienced observer does not spend time on variables below his/her telescope limit.

Identification of the Variable

Click image to view a chart that illustrates a typical star hop from the bright key star, beta Cep, to the variable star, T Cep. Note that the observer's telescopic field-of-view has been drawn in and that a bright asterism is being used to help find the way from beta to T Cep.

Check the immediate region carefully to be sure that the star you identify as the variable is not another faint star very close to the variable's position.

An eyepiece of higher power will be necessary when the variable is faint or in a very crowded field of stars. Also, it will probably be necessary to use the "d" or "e" type charts in order to obtain positive identification of the variable.

When you are observing, relax. Don't waste time on variables you cannot locate. If you cannot find a variable star after a reasonable effort, make a note and move on to your next variable. After your observing session, re-examine the atlas and charts and see if you can determine why you could not find the variable. Next time you are observing, try again!

Estimating the Variable's Brightness

Any optical instrument's resolving power is greatest at its center of field. Thus, when the comparison star and the variable are widely separated, they should not be viewed simultaneously but they should be brought successively into the center of the field.

If the variable and the comparison star are close together, they should be placed at equal distance from the center, and the line between the two stars should be as parallel as possible to the connecting line between your eyes to prevent what is known as "position angle error." If this is not the case, turn your head or the erecting prism (if used). The position angle effect can produce errors of up to 0.5 magnitude.

It must be stressed that all observing must be done near the center of the instrument's field. Most telescopes do not have 100% illumination over the field of all eyepieces, and there is greater aberration of the image, the closer it is positioned toward the edge of the objective in refractors or of the mirror in reflectors.

Use at least two comparison stars, and if possible, more. If the interval between comparison stars is very large, say 0.5 or greater, use extreme care in estimating how the interval between the brighter comparison star and the variable compares with that between the variable and the fainter comparison star.

Record exactly what you see, regardless of seeming discrepancies in your observations. You should go into each observing session with a clear head; do not let your estimate be prejudiced by your previous estimates or by what you THINK the star should be doing.

If the variable is not seen because of extreme faintness, clouds, or moonlight, then note the faintest star visible in the region. If that star should be 11.5, record your observation of the variable as <11.5, which means that the variable is invisible and must have been below, or fainter, than, magnitude 11.5. The left-pointing bracket is a symbol for "fainter than".

When observing variables which have a decidedly red color, it is recommended that the estimate be made by the so-called "quick glance" method rather than by prolonged "stares". Due to the Purkinje effect, red stars tend to excite the retina of the eye when watched for any length of time; accordingly, red stars would appear to become unduly bright in comparison to blue stars, thus producing an erroneous impression of the relative magnitudes.

When a variable shows its color even when using averted vision, it is strongly recommended that the estimates be made by the out-of-focus method. That is, the eyepiece must be drawn out of focus so far that the stars become visible as colorless disks. In this way a systematic error due to the Purkinje effect is avoided.

If the color of the variable is visible even when the stars are out of focus, you may need to use a smaller telescope or an aperture mask.

Reporting Observations