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The AAVSO Photoelectric Photometry (PEP) Program

Welcome to the website of the AAVSO Photoelectric Photometry (PEP) Program!

Photoelectric photometry or "photon counting" is an observational technique used to obtain highly precise, calibrated observations of small amplitude variables. The AAVSO PEP Program was formally established in 1983, but our archive has PEP observations dating back to the 1950's. Currently, the AAVSO has over 52,000 PEP observations of over 550 different variables. Photometric V-filter data from observers following the standard AAVSO observation sequence and submitted via our pipeline are calibrated and transformed to the standard Johnson system. These are the highest-precision data available from the AAVSO, and are often accurate to millimagnitudes. All of these observations are available for free, online, on demand through our data download form.

The AAVSO PEP Committee

What We Do

Simulation of BetelgeuseYou have seen all the Messier objects, at least once. Maybe you have found many of the objects on the Herschel or NGC lists. Now, you are looking for a new challenge, but not one that requires driving for hours to find a dark sky site. You live near a large city or other area that precludes the steady seeing required for planetary work, and you are not sure you want to lay out the cost for a premium CCD camera. What can you do?

Consider photoelectric photometry! If you have a good 6 or 8 inch (or larger) telescope with a reasonable drive on an equatorial or GO-TO alt-azimuth mount, please read on.

In recent years, advances in electronics and the availability of moderately priced photometers have brought PEP into the repertoire of many amateur astronomers. Used photometers can be found on eBay for about the price of a premium eyepiece. With such an instrument mounted on your telescope, you can obtain scientifically useful astronomical data. PEP measurements can even be done in bright, light-polluted areas and during bright phases of the moon. What you would be doing is photoelectric photometry of variable stars.

The AAVSO has developed a PEP observing program, targeting variable stars of all types that exhibit small amplitude variations, often less than one magnitude. Using your equipment, you can produce data as accurate as those generated in professional observatories. Professional astronomers use data produced by amateurs in papers published in the professional journals. The amateurs who provided the data are often given co-author status in the papers. But most important, you will be contributing new and valuable knowledge to the astronomical community.


Why PEP?

Photoelectric photometry is a time-consuming and meticulous process involving multiple measurements of the variable star, a comparison star, a check star, and the sky background for every single data point submitted, as well as a preliminary calibration process to determine how to transform the observer's data. This may seem like a lot of work for a single data point, but calibrated photoelectric photometry provides very precise, accurate data for many small amplitude variables, and is greatly valued by astrophysicists. Furthermore, nearly all PEP targets are bright variables, most brighter than 7th magnitude. Many of these stars are nearly impossible to observe with large telescopes, and many bright but astrophysically important variables (like Betelgeuse) have been neglected by all but the amateur community. Photoelectric data by AAVSO and other amateur photometrists thus provide an important record for many important variables in our sky. And although professional observers rarely perform photometry of bright stars, these objects aretargets for other kinds of professional observing programs like ultrahigh resolution spectroscopy and optical and near-infrared interferometry. Calibrated PEP data from the AAVSO can then be used to correlate the optical photometric behavior with other observables.

Who We Are

The AAVSO PEP program has been formally operating since 1983 and the data archive contains over 52,000 observations. At any given time, we have 15-20 active observers, and they are scattered world-wide. We have observers in Greece, Italy, South Africa, Australia, New Zealand, and throughout the United States. We would like to have more observers in the program, no matter where you are.

Jim Fox (, the Chairman of the AAVSO's PEP Committee will help you get started. He will advise you in any technical matters concerning photometers, telescopes and observing techniques. Finder charts for all stars in the program are maintained by AAVSO Headquarters and are available on the AAVSO web site. You will submit your observations directly to the AAVSO database using tools that are available on the web site.

Technical and scientific assistance for the AAVSO PEP Program is currently provided at AAVSO Headquarters by Dr. Matthew Templeton (

For more information

Please see The Photoelectric Photometry Observing Program of the AAVSO: An Introduction by Committee Chair Jim Fox.

For more suggestions and help with PEP Observing, Jim Fox may also be contacted at

Current PEP Alert and Special Notices, campaigns, and news items

Important links

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