Skip to main content

The Photoelectric Photometry (PEP) Observing Program of the AAVSO: An Introduction

By Jim Fox, former PEP Section Chair

In 1983, the AAVSO developed a photoelectric photometry (PEP) observing program, targeting variable stars of all types that exhibit a small amplitude of variation, often less than one magnitude. In order to participate, you need an equatorially driven telescope of at least 6-inch aperture and a photoelectric photometer. Current program participants use photometers ranging from the solid state SSP-3 to precision PMT instruments. Using your equipment, you can produce data as accurate as that generated in professional observatories, and you will be contributing new and valuable knowledge to the astronomical community.

Since each type photometer may operate differently, we assume that you know the basic operation of your instrument and have at least a rudimentary knowledge of photometry. Most AAVSO program observations are made in the Johnson (V) band using a standardized V filter and the technique known as differential photometry. This method compares the brightness of the variable to the brightness of a star of known magnitude. See one of the references listed below for a general description of photometers and variable star photoelectric photometry. If your interest lies in infrared PEP, see the IR program description.

Making an Observation

A "PEP Observation" for the AAVSO program consists of a sequence of individual observations, called deflections, of the target variable star, a designated comparison star, a check star, and the sky background near each star. (The term deflection is a carry-over from the early days of photometry when the photometer output deflected a needle of a meter or a pen on a strip-chart. Today, many photometers display a digital readout related to measured brightness.)  The check star is used to "check" that the comparison star itself is not an unsuspected variable!  A list of PEP program target, comparison, and check stars is available here.

Always use the specified comparison star on the chart. If a comparison, or comp, star is later found to be variable and a new comp star is designated, historical data can be corrected to the new comparison as long as we know what comp star was used in the past. Be sure to look at the on-line finder chart at the start of each observing season for a given star to be sure that the designated comp and check stars have not changed.

The observing sequence for each target variable is: c, s, v, s, c, s, v, s, c, s, v, s, c, s, k, s, c, and s, where c represents a deflection for the comparison star, v represents a deflection for the target variable, k represents a deflection for the check star, s represents a deflection for the background sky (preferably starless!). The sky deflection should be taken near the immediately preceding target star. How near? Most photometers have a reticule that shows the aperture of the photometer's sensing element. It is usually sufficient (and convenient) to move about one such aperture-width away from the star.

Each reported deflection should be the mean of 2 or 3 readings for each target object. This averaging tends to reduce the errors due to electronic noise in the photometer head and seeing fluctuations. The complete sequence of 18 individual (averaged) deflections constitutes an observation.

Standardizing Data

In order to compare observations from observers using different equipment, the data reduction transforms individual observations to a standard system. Transformations take into account different altitudes and different colors of variable and comparison stars. Stars in the AAVSO program have comp stars chosen that are reasonably close to the target variable. This minimizes the effects of altitude, known as extinction, but the observations must still be corrected for color differences between the two stars. In order to perform this correction, you must determine the transformation coefficient, epsilon-v, for your system.

There are several ways to determine epsilon-v: observing a set of "standard stars" (stars whose brightness and color characteristics are precisely known), observing standard stars within an open star cluster (the "cluster method"), observing two stars that are of widely separated colors. The easiest method for most observers is the latter.

In the two-star method, a red and a blue star, close together in the sky, are observed. By comparing the measured instrumental magnitudes with the known actual magnitudes, the transformation coefficient (correction factor) can be determined. See "Blue-Red star pairs for determining transformation coefficients" by Douglas S. Hall (1983, IAPPP Communication No. 11, p.3) for complete instructions. You must determine your system's value of epsilon-v and tell us your timezone, latitude and longitude before you can submit observations to the AAVSO database.

Getting Started

It is easy to become discouraged by all the requirements of the program. But, as is often the case, the requirements sound more onerous when explained than they are in practice. Integrity of our database requires that observations be accurate to within about 0.02 magnitudes. Many observers routinely submit observations with standard errors of 0.005 magnitudes, or better. With a little practice, you can too.

The following stars in the AAVSO PEP program are good ones to start on. They have no significant extinction or gain setting problems, and there are no known problems with the comparison stars. They are relatively bright, but they do require a color correction, so the observer will need to determine the value of epsilon-v before submitting data to the database. However, go ahead and try your hand at observing these stars, but hold off submitting your data until you know your transformation coefficient. In the meantime, the Chair of the PEP Section will be happy to examine your observational data, answer questions and make suggestions.

Star HD SAO Type Range Period R.A. Dec.
Name       V-mag (days) hh mm ss dd mm ss
CE Tau 36389 94628 SRc 4.2 - 4.5 165 05 32 13 +18 35 42
RS Cnc 78712 61306 SRc 6.2 - 7.7 120 09 10 41 +30 58 22
IN Hya 80567 117605 SRb 6.27 - 6.87 65 09 20 37 +00 10 54
W Boo 129712 83488 SRb 4.7 - 5.4 35 14 43 25 +26 31 42
rho Cas 224014 35879 SRd 4.1 - 6.2 320? 23 54 23 +57 29 52


Modern "GOTO" telescope mountings usually have a stored catalog of bright SAO stars. These five stars should be in that catalog, making them easy to find. Be sure to get current copies of the finder charts before attempting to observe these (or any other) stars. See the on-line address, below.

For More Information

  • PEP Section Chair: Jim Kay (
  • Astronomical Photometry, A.A. Henden and R.H. Kaitchuck, 1998, Willmann-Bell
  • Photoelectric Photometry of Variable Stars, D.S. Hall and R.M. Genet, 1982, IAPPP
  • Astronomical Photoelectric Photometry (Using UBV Photomultiplier Tube Photon Counting), J. Hopkins, 2004, Hopkins Phoenix Observatory, Phoenix, AZ
  • Software for Photoelectric Photometry, S. Ghedini, 1982, Willmann-Bell

Back to the PEP Section Home Page


AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484