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The AAVSO Photoelectric Photometry (PEP) Section

Welcome to the AAVSO Photoelectric Photometry (PEP) Section!

Watch the Photoelectric Photometry Observing Section webinar (recorded August 15, 2020).

Where it all began

The photoelectric effect, by which light is converted into electricity, has been used by astronomers since the 1890s. First practiced with clumsy selenium photocells, the technique enjoyed a progression of better sensors during its first fifty years, then blossomed after World War II when the photomultiplier tube (PMT) came into widespread service. These sensitive amplifiers ushered in the modern era of photometry. Johnson, Bessell, Cousins, Landolt, and others used them to lay the foundations of the photometric systems we use today.

Unlike electronic cameras, photoelectric photometers have only a single sensor element - one large "pixel." Instead of taking a picture of a variable star with its comparison, the PEP observer aims separately at each, repeatedly sampling both stars.

CCD (and now CMOS) imagers have largely supplanted these "single channel" photometers, but PEP still lives and AAVSO probably has a larger group of PEP observers than any other astronomical organization. AAVSO PEP took off in the 1980s, when the Optec Corporation introduced the SSP-3, a photometer with a photodiode sensor. Compact, portable, and easy to use, the SSP-3 was an ideal tool for measuring bright variable stars - stars that remain problematic targets for imaging systems.

With an 8-10 inch telescope on a GOTO mount, PEP observers can make scientifically useful observations, even in environments with significant light pollution and light trespass. It is far easier to get high quality data with PEP than with imaging systems, and the equipment (bought used) costs less.

Granted, PEP observing is not "sexy" (some would even call it tedious) but it's quick to learn and doesn't require fancy software for data reduction. We still see the stars through an eyepiece and retain a kind of intimacy with the sky that computerized observers lose.

Most of our observers still employ the SSP-3, but some use Optec's PMT photometer, the SSP-5, and we are trying to push the infra-red SSP-4 units back into regular service.

Tom Calderwood (contact), the AAVSO's PEP Section Leader, 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.

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