Astronomical Photoelectric Photometry
Manual
Foreword
I often ask my beginning astronomy students to list the characteristics of remote objects that can be measured by going outside and looking at the nighttime sky (either with or without a telescope). Their responses are almost limitless -- distances, sizes, colors, temperatures, etc. Certainly, these are examples of the ultimate goals of a scientific investigation, but they don't really answer the question that was posed. I then try to convince these students that there are only two basic quantities that one can define about the radiation coming from distant "points of light" -- the objects we call planets, stars, and galaxies -- namely, "I see an object of a certain brightness in a particular direction." From these two very rudimentary pieces of information, all of the other characteristics must be derived. The roots of this generality extend back to the ancient star catalogs of Hipparchus and Ptolemy. Without doubt, our modern catalogs contain much more information. We collect and analyze light at slightly different frequencies (i.e., forming a spectrum) or from slightly different directions in space (i.e., forming an image), but the underlying information is the same: how bright is the object seen in some carefully specified direction.
The Hopkins Phoenix Observatory Astronomical Photoelectric Photometry Manual is the latest of Jeff Hopkins' contributions to this quest. For 25 years, he has built his own equipment, collected/reduced data, and contributed to the study of variable stars. It is said that to teach is to learn twice. This is certainly true of Jeff's endeavors. In these pages he describes some of the techniques that work. Readers should take heart that this is not necessarily a cookbook. The process is as much a form of art as it is science. In fact, the title of one of Jeff's earlier texts was Zen and the Art of Photoelectric Photometry. There is no simple way to throw together some equipment, point a telescope at a star, and miraculously have the answer come out. Thus, I encourage readers to look at this manual as a starting point. What are some of problems that might or probably will be encountered?
In an era of relatively inexpensive CCD detectors, it is reasonable to question the value of single-channel photoelectric photometry (PEP). The manual addresses some of the justifications. One reason is that PEP is better suited to observing bright stars -- and there certainly are many entries in the Bright Star Catalog that would benefit from extensive observing! A second reason is that major photometry systems were developed using the PEP approach. Hence, the best way of matching new observations to those in a well established data base is to use equipment and techniques as similar as possible to the original. This idea is often forgotten in today's sea of panoramic detectors. Even the faint (12th-14th magnitude) secondary standard stars of the UBV system often used to calibrate CCD images were defined using the photoelectric methods described here. Finally, I find the most compelling reason to use modern PEP techniques is that one becomes much more intimate with the data. Careless image processing can easily produce pretty, but otherwise worthless, pictures. Push a few buttons and see a result pop up on the screen. However, if one critically examines the PEP process and questions the validity of each individual step, then one produces not only an excellent product but also pride of ownership. Nature will reveal her secrets to those who look carefully and ask the right questions. So, I encourage readers to use this new manual as a fresh resource. Then, adopt a star or two as your favorites and try to determine the stories they are telling.
Paul Schmidtke
Arizona State University
May 31, 2004
Present Page Version as of 31 May 2004
