VARIABLE STARS : Part 7

ARTICLE PAGES

Measuring Variable Stars by Visual Photometry

The most reliable method of measuring the magnitude of stars is the photoelectric photometer. But this is an expensive heavy instrument needing fairly large telescopes to carry its weight, with an accurate drive. This involves the use of valuable electronic refrigerated equipment mounted on the large telescope. Photoelectric photometers are beyond the means of most amateurs.

However, there are visual devices known as visual photometers, and these include (among many other derivatives.);

1. The Bright Field Extinction Photometer
2. The Extinction Photometer
3. Zöllner Photometer
4. Ball Point Photometer

The extinction photometer described below is suitable. Make four measurements on the variable and average the readings. Then do the same to a comparison star, and check how the results compare with the graph. Any variation are then applied to the magnitude reading for the variable. This procedure covers the effects caused by variations due to seeing conditions, altitude, lunar interference etc. as compared with those when the graph was compiled. Another method is the introduction into the field of view of an adjustable artificial star, such as in the Zöllner photometer or the catseye photometer etc. This star is tuned in to equality with the variable and also the comparison star, and compared graphically in a similar manner to that described with the extinction photometer.

Some of these instruments can be readily made by anyone accustomed to fine work with tools.

The Bright Field Extinction Photometer

One method is to install an electric touch globe in front connected to a battery and a potentiometer, and flood the field with measured quantities of light

Ball Point Photometer

Here the touch globe illuminates a pinhole whose image is refocused by a lens at a second pinhole at its conjugated focus. The image of this small hole is picked up by a small ball from the end of a ball point pen, and reflected up by this to the focus of the eyepiece. Meanwhile the variable star can be seen past the ball and compared to an artificial star. When the second hole is at the maximum conjugate focus of the first, its image is at maximum Intensity. But if the unit contains the globe — the first hole and the lens is moved slightly inward, so that the cone of the light becomes truncated and weakened. The intensity of the image is determined by the amount of movement. The values of this must be determined by observation.

Determining the Time of Observation

In variable star observations, it is important to express the time of observation correctly. The method usually employed uses the Julian Date (J.D.) that was first proposed by Joseph Scaiger in the 18th century.

Dates are calculated, based on the zero point that was assigned as the 1st January, 4713 B.C. This date was judged to be far enough back for any known astronomical records, though more recently, records have been found that precede this date. To account for the earlier records, it is possible to have negative dates. These are probably not necessary, as the dating techniques are highly inaccurate, so they are not generally required.

At the time that Julian Dates were introduced, the date was around 2 million days. On the 1st January,1996 the Julian Date is 2 450 082. Julian Dates begin at 12 hours at noon, Universal Time (U.T.), and not 12 hours midnight. The reason for the differences against the other more common time systems, is to align with nightly observations. Otherwise the date will change at midnight, just adding to the confusion of correct time of the observations.

Below is listed a table that can assist the observer to calculate the proper Julian Date.

To find the Julian Date in EAST (Eastern Australian Standard Time), the following calculation should be made.

1) Take the required figure, by month and year from the Monthly Table. (Table 1)
2) Add the Day of the particular month to the result in 1.)
3) To this figure add 0.08333, which gives the correction for the longitude difference from Greenwich.
4) Add the fraction of the day from Table 2, to the time of the observation.
5) Add, if required, the result in fraction of minutes, from Table 3.
6) Add, if required, the result in smaller units of time, by interpolating from Table 3.
7) If the result is more than 24 hours, subtract 12 hours from the result
8) Take the Table 1 reading, if more than 24 hours, and add 0.50000 to the final result.

Table 1.
Monthly Table of Julian Day
(Add 2 450 000 to the Date.)

Table 2.
Day Table for Julian Date

Table 3.
Minute Table for Julian Date

Example:
If an variable observation occurs at
21 hours 50 minutes EAST (11h 50m UT)
on 02nd October, 2007.
What is the Julian Date?
Answer : 2454406.99306

END COMMENTS

This text is designed to;

1) Introduce variable stars to observers for observational contributions by serious amateurs
2) Understand the natures and structure of variable stars
3) Promote observations by amateurs.

I would like to acknowledge the late Mr. Edward Lumley of the Astronomical Society of New South Wales Inc. (A.S.N.S.W.) He was a long-time member of the Society (49 years) and the Leader of its “Variable Star Section Leader” for most of it. He provided much of the information on visual photometer and the Selected Variable Star List. He was an inspiration to me to many on the fellow member of the ASNSW Inc. Ted will surely be missed and will never be forgotten.

Used with Permission.

Last Update : 13th November 2012

Southern Astronomical Delights © (2012)

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