Photoelectric Photometry Newsletter

Message from the Editor

Once again, photoelectric photometry was well represented at the AAVSO Fall Meeting, held this year in Chicopee and South Hadley MA, with Tom Dennis and Ron Zissell as hosts.

On Friday October 24, there was a workshop.on "CCD(V), PEP(V) and Visual Magnitudes: How Do They Relate?". The speakers included visual, photoelectric, and CCD observers, chartmaker Chuck Scovil, vision physiologist Peter Hallett, and AAVSO Director Janet Mattei. The main motivation for the workshop was the AAVSO's plan to revise its visual charts by the year 2000, to reflect more realistic magnitudes.

The topic of the workshop is a complex one. CCD(V) and PEP(V) magnitudes should, in principle, agree, if appropriate filters are used, and the magnitudes are carefully transformed through the use of standard stars. Visual magnitudes, however, use a different and complex detector. Most visual observers are not aware of how they make their measurements. Do they glance, or fixate? Do t ey use the central part of the retina, or an off-axis part? And what are the effects of dark adaptation, or background sky brightness?

There is a second problem for some visual observations: some comparison-sequence magnitudes have been derived from photography, from the BD catalog, or from some other source which may have large systematic errors. Workshop participants agreed that a good PEP(V) magnitude was a better approximation to a visual magnitude than these were.

As a first approximation, it appears that the difference between PEP(V) and visual magnitude may be a simple linear function of the (B-V) color, as Richard Stanton had suggested many years ago. it might be possible to derive a more complex transformation equation involving non-linear terms, or terms in the (U-B) color, if enough data were available. It might even be possible to use photoelectric transformation software with these features built in, such as the program HEC-22 developed by my colleague Petr Harmanec in the Czech Republic.

This workshop, with its diverse but well-focussed presentations, was an interesting and useful first step in the AAVSO's "millenium project-".

As many of you have found, there is a page on the AAVSO web site about the photoelectric photometry program - look under "committees". I have been planning/hoping for a year or two to develop and expand this page. Now, with the help of a keen student assistant, it appears that I can do so.

If any readers have suggestions about content for the web page, please let me know.

Gary Frey asks whether there is a more comprehensive list of red-blue pairs (for determining extinction and transformation coefficients) than the four which are recommended by the AAV@0. The most extensive list, as far as I know, is Appendix B in the book "Astronomical Photometry" by Hended and Kaitchuck, published by Willmann-Bell, 1990.

John B. Chouinavas informs us that he is the new observer at the Larissa Observatory, Greece, Nick Stoikidis having retired. We welcome John Chouinavas, and thank Nick Stoikidis for his contributions to the AAVSO over the years. According to my records, he began contributing visual observations in 1974.

"Calling All Amateurs" was a one-page article in the popular US newsmagazine TIME, August 11, 1997, page 48. It began by highlighting the work of amateur astronomers in monitoring an occultation of Aldebaran with camcorders and small telescopes. It then described the contributions of amateurf to encryption, epidemiology, ornithology and archaeology. Undoubtedly there are many other fields of science and other scholarship in which skilled amateurs could play a role. But amateur astronomers take the lead!

The New York Times also published a long article on the work of amateur astronomers in its March 18, 1997 edition. "Backyard Astronomers Enlist as Foot Soldiers of Astrophysics" dealt specifically with the Center for Backyard Astrophysics, which is directed by Columbia University astronomer Joe Patterson. There are CBA sites in several countries; some use photoelectric photometers, and others use CCD cameras. Reflecting Joe Patterson's interests, the CBA network observes mainly cataclysmic variables. one goal is to establish sites at many longitudes, so variables can be monitored continuously.

Very few amateur astronomers are equipped to monitor variable stars spectroscopically. Yet there is a need for such observations, because very few professional astronomers make them. Be stars, with their bright variable emission lines of hydrogen, are a prime target.

Ernst Pollman, of Leverkusen, Germany, uses a 10cm Maksutov telescope with an objective prism made of flint glass. His detector is a CCD camera with a Philips 386 x 290 chip. In the IAU Newsletter on Active B Stars, #32 (1997), he presents three years of H-alpha measurements of several Be stars. One is Algol, which is not a classical Be star, but has phase-dependent variations in H-alpha due to the accretion disc around the hot star. Another target is P Cygni, which is on the AAVSO photoelectric photometry program. This is also not a classical Be star; its H-alpha emission comes from its dense wind. The H-alpha strength varies on a time scale of about half a year similar to the time scale of the light variations.

Figure 1. Variations in the equivalent width of the H-alpha line in the blue hypergiant P Cygni. Observations by Ernst Pollman.

The IAU Newsletter on Active B Stars is available on-line at: http://www.chara.gsu.edu/BeNews/intro.html