Photoelectric Photometry (PEP) Observer's Guide

The Hitch-hiker's Guide to Photoelectric Photometry (PEP)

Download Version 2.0 Now (1.4 MB)

by Tom Calderwood

[A translation of Version 1.0 of this Guide into French is available below]

Our vision: High quality photometry of bright, astrophysically interesting stars.

The AAVSO photoelectric section was founded in the late 1970s. We use old-school technology, but we can get superior results on bright stars. Compared to CCD or DSLR systems, our equipment is fundamentally simpler to calibrate and operate, and data reduction is straightforward. What we lack in sensitivity, we make up for in quality. With properly chosen targets and careful technique, we remain a viable research group. Talk to us—we're friendly people, and there is no substitute for conversations with experienced observers. Our web page is at:

This document is a work-in-progress, representing my best understanding of PEP as practiced at AAVSO. It lacks the polish of other AAVSO manuals, but I think you will find it entertaining reading. The content lies somewhere in between a cookbook and a reference book. I will try to provide a wide, but not too deep overview of the equipment and practice of photometry with single-channel photometers. I will fudge on the details, occasionally, in the service of clarity.

Table of Contents

A Day in the Life of a “PEPPER”

Chapter 1 — Background

1.1 The Magnitude System
1.2 Time and Date
1.3 Star Identifiers
1.4 Photometric Bands
1.5 Response Curves
1.6 Single Channel Photometry
1.7 The Sinkhole
1.8 Our Mascot: Count von Count of Sesame Street

Chapter 2 — Observing

2.1 Scopes and Mounts
2.2 Photometers and Filters
2.3 Basic Operation
2.4 Setting the Gain and Integration Time
2.5 The Standard Sequence
2.6 Skies: the Good, the Bad, and the Ugly
2.7 Tricks of the Trade
2.8 Extra notes on the SSP5

Chapter 3 — Data Reduction

3.1 Software
3.2 Data Management
3.3 Observational Honesty
3.4 Avoiding Embarrassment
3.5 Gory Details of Reduction
3.5.1 Instrumental Magnitudes
3.5.2 First-order Extinction
3.5.3 Color Contrast
3.5.4 Second-order Extinction in B Band
3.5.6 Transformation
3.5.7 Complete Magnitude Reduction Formulae
3.5.8 Airmass Reduction
3.5.9 Time of Observation
3.5.10 Metadata (data about the data)
3.5.11 Reference Magnitudes
3.5.12 AAVSO Extended Data File Format

Chapter 4 — A Quick Digression on Statistics

4.1 Precision
4.2 Fitting
4.3 Weighted Averaging

Chapter 5 — Calibration

5.1 First-Order Extinction
5.2 Transformation (the easy way)
5.3 Transformation (the hard way)
5.4 Second-Order Extinction

Chapter 6 – War Stories


Appendix A: PEPObs

Appendix B: References

Appendix C: Metadata

Appendix D: Extinction Examples

Appendix E: SSP Electronics

Appendix F: Additional Notes

Appendix G: Airmass Computation



I would like to thank Arne Henden, Richard Berry, Mike Beeler, Terry Moon, Erwin van Ballegoij, and Jim Kay for helpful reviews of this document. Significant content errors are all mine, and I am also responsible for the simplistic diagrams and text formatting.


English - Version 2.0  (1.4 MB, issued 28 May 2020)

English - Version 1.0  (4.5 MB, issued 17 January 2017)


French - Version 1.0 - translated by Bernard Candela (January 2018).