Making Observations for Obtaining Epsilon-V Coefficient Using the Two-Star Method

Preliminaries

Assuming you have a suitable photoelectric photometry system (telescope and photometer), you will also need the following:

1. The example calculation from Dr. D.S. Hall, available from the PEP Committee Chair, or from the NASA Abstract Data Service.
2. AAVSO Transformation Star Charts (four are available at ftp://ftp.aavso.org/charts/misc/PEP/) or your own finder charts for the stars listed below. (Use your favorite planetarium-type program.)

Procedure

This information sheet expands on the information in the paper by Dr. Hall.

Select a convenient star pair and observe the selected pair when it is near your meridian on a night of exceptional clarity. Make a series of measurements through your V filter in the following sequence: r, s, b, s, r, s, b, s, r, s, b, s, r, s, b, s, r, s, b, s, r, s, b, s, r, s, b, s, r, s, where r is a red star measurement, b is a blue star measurement, and s is a sky background measurement. Each measurement must be an average of 3 or more individual deflections. You should end up with 7 measurements of the blue star and 8 measurements of the red star. You can make additional readings of the pairs if you wish, as long as you end up with a red measurement bracketing each of the blue measurements.

If you use either a pulse counting system or an SSP-3, your integration time should be 10 seconds or longer.

If you are using a pulse counting system, no gain value is entered, but you must do your own compensation for dead time before recording in the readings. If you use an SSP-3, enter 1, 10, or 100 for the gain, whichever is used. (Depending on your telescope aperture, all stars can be measured with a gain setting of 10.) If you use a DC integrating system, enter the half magnitude gain step used.

Treat the blue star as a "variable" and the red star as a "comparison" and determine the differential magnitude. You can do this by hand, or, since the procedure is rather calculation intensive, use a spreadsheet to do the calculations. If you use a spreadsheet, save it for future determinations. The procedure can be found in any of the standard references, but is outlined here.

1. For each star, calculate the net deflection by (r - s) or (b - s), where s is the sky measurement immediately following the star measurement. The result will be the net deflection for each star measurement.
2. For each blue star measurement, calculate the instrumental differential magnitude Δv using
   Δv = -2.5 log10 (2*b/(r1 + r2))
   where b is the net blue deflection and r1, r2 are the net red deflections on either side of the blue measurement.
3. Average the Δv values so determined to calculate a mean Δv(0). If you can, also calculate a standard deviation of the Δv's, Sv.

Then calculate your transformation coefficient following the method described in Dr. Hall's paper:

4. Calculate your transformation coefficient, epsilon(V) or "ε(V)" using
   ε(v) = (ΔV - Δv(0))/Δ(B - V)
   where ΔV and Δ(B - V) are found from the table below for the pair you selected. Δ is always in the sense of blue star minus red star.
5. Send your calculations to the PEP Committee Chair for review after your first determination, and then report your ε(v), the standard deviation, Sv, and the star-pair used for the determination.

Epsilon V must be determined for your own particular photometer and telescope combination. Due to the possibility of changes in the photometer detector and the telescope mirror coatings over a period of time, transformation calibration should be done periodically, every year or two

Star pairs for measuring transformation coefficients

The first of each pair is the Red star; treat it as the comparison star. Treat the Blue star as the variable star.

** Indicates star pair for which an AAVSO finder chart is available at ftp://ftp.aavso.org/charts/misc/PEP/