The Center for Backyard Astrophysics: Theory and Practice

Jerry Foote
Society for Astronomical Sciences 24th Annual Symposium on Telescope Science
May 2005, Page 91


The author will discuss the collaboration between professional and amateur astronomers in the study of Cataclysmic Variable stars. In addition two case studies, V442 Oph and FS Aur, demonstrating this collaboration will be given with emphasis on the photometric process, data format and data submission.

1. Introduction

The Center for Backyard Astrophysics (CBA),, was formed to harness many amateur astronomers and their telescopes to perform long time series photometry of Cataclysmic Variable (CV) stars. In order to continuously cover these stars members of the CBA have been solicited from around the globe at all longitudes and latitudes. This group has been augmented over the years by professional astronomers that have recognized the power of a network of telescopes with motivated owners that are willing to observe selected CV’s.

The end product of this network are light curves that, due to the multi-longitude nature of the observers, are continuous over many weeks, months and occasionally seasons. These long observing runs are easily able to tease out those periods that are near 24 hours that would otherwise be lost for single observers.

2. CBA History

The beginning of the CBA was the Laurel, MD basement of Dave Skillman in the 1970’s. He called himself the Center for Basement Astrophysics as play on words of a better-known institute in Cambridge, Massachusetts. He started with photomultiplier tubes and ultimately progressed to a CCD camera, which he made himself. He produced light curves of Cataclysmic Variable stars and determined some of their basic properties.

In 1980 Dave’s work came to the attention of Joe Patterson of Columbia University. The two of them collaborated successfully with Dave providing the long-term photometry and Joe augmenting Dave’s light curves with stints at larger telescopes and spectrographs. In those early years they published multiple papers on the Superhump phenomena that they observed in these stars.

In 1993 they were able to bring in Dave Harvey in Tucson, AZ, which now broadened their longitude coverage. Rapidly as more scientific papers were published that demonstrated the worth of this broad time coverage approach, more amateurs joined the group. Now a problem developed. Most of the members did not work out of their basements as Dave did. It was decided that a better name would be the Center for Backyard Astrophysics. Some of this early history appeared in Sky & Telescope: Skillman (1981), Skillman (1993), Patterson (1998).

3. The CBA Today

The CBA network today has over 48 contributing members with many more followers that contribute on selected objects. The world coverage ranges in longitude from 175º East to 155º West and in both hemispheres.

The typical member has a telescope of 10” to 14” diameter with a good drive to which is coupled a CCD camera. The majority of the telescopes are computer controlled to some extent with many having scripting capability that allows un-attended operation. The images that are collected throughout the night are analyzed with commercially available software and the results are posted to an archive maintained by one of the members.

4. The Process

4.1 Data Collection

The goal is time-series differential photometry of the target star. What is needed is a series of sequential images of the program star field using 1-2 minute integrations. The integration time should be chosen such that the variable, comparison and check stars are not saturated. Over a nights time this can amount to 100-200 images. This process is repeated over ~50 nights. Usually the images are collected un-filtered because with smaller apertures, every photon that can be collected is precious.

4.2 Data Reduction

Once these set of images are collected they must be reduced to differential photometry magnitudes. In an ideal world each observer would use the same Comparison and Check star. However, due to different fields of view from each telescope this is not always possible. Often during a long campaign, members will e-mail each other and discuss which stars are being used. In any event the Comparison star should be chosen that is nearby in the field and as blue as possible. Again watch that the Comparison and Check stars don’t saturate the CCD chip.

With the hundreds of images that will be collected over the night it is imperative that some form of software be used that can analyze these images in a consistent manner. Fortunately there are several packages out there that will do this for a reasonable price.

4.3 Data Format

The format of the submitted data is very simple: a list of time series differential (Var-Comp) photometry and Julian Date. A text file that looks like this:

   3472.63286   –0.112
   3472.63359   –0.121
   3472.63433   –0.145

Sometimes people add a third column, which could be Check-Comp or air mass. Both are somewhat useful. Note that the Julian Date is truncated with no heliocentric correction. Time should refer to the middle of the integration, not the beginning or the end.

4.4 Data Submission

The data can be either imbedded in an e-mail or attached to an e-mail and sent to the New Data Archive: cba-data .at. The subject line of the email should contain the name of the CV and perhaps the JD range of the observations or the date the observations were made.

5. Contacting the CBA

The CBA is always looking for new members. If you are interested in joining the network or even receiving the news e-mails, send an e-mail to cba-tech .at. Tell us a little about yourself and your desired level of involvement.

6. Case Studies

6.1 V442 Ophiuchi

On June 12, 2002 those of us in the network received the following e-mail from Joe Patterson:

“… It’s high time to take up the cause on V442 Oph. Long runs, densely spaced at all terrestrial longitudes you can dream up (including you naughty North Americans out there) – that’s what we want to properly subdue V442”

Over the course of 42 nights, there were 203 hours of time series photometry provide by six astronomers.

Once the data from all observers was analyzed it was presented in a paper published in the Publications of the Astronomical Society of the Pacific, Patterson (2002). An exceptional aspect of the CBA publications is that observers that contribute data to the understanding of the CV are included as authors of the resulting publication

6.2 FS Aurigae

The call for observations this season of this CV came on December 2, 2004. This dwarf nova, with an orbital period of 85.7 minutes, has been a CBA candidate previously. FS Aur’s strange behavior was the topic of a publication, Tovmassian (2003).

This recent request for observations was coordinated with both spectroscopic and Chandra observations. As yet the relationships between these have not been delineated. Perhaps another season of coordinated observations will clarify this CV.

7. Conclusion

The Center for Backyard Astrophysics has been in existence for over 30 years and has produced over 34 publications of its findings. The concept that small telescopes operated by dedicated amateurs can produce good science is proven again. The CBA network of astronomers have proven invaluable to the understanding of Cataclysmic Variable stars and they have proven to be dedicated and reliable in the quest for time series photometric data.


  • Skillman, D. R., Gleanings for ATMs – Running a Telescope with a Microcomputer (1981). Sky & Telescope, January, 71.
  • Skillman, D., Astronomical Computing – An evening of CCD Photometry (1993). Sky & Telescope, May, 83-85.
  • Patterson, J. O., Amateur Astronomers – Our Cataclysmic-Variable Network (1998). Sky & Telescope, October, 77-81.
  • Patterson, J., Fenton, W. H., Thorstensen, J.R., Harvey, D.A., Skillman, D. R., Fried, R. E., Monard, B., O’Donoghue, D., Beshore, E., Martin, B., Niarchos, P., Vanmunster, T., Foote, J., Bolt, G., Rea, R., Cook, L. M., Butterworth, N., Wood, M., Superhumps in Cataclysmic Binaries. XXIII. V442 Ophiuchi and RX J1643.7+3402 (2002). PASP, 114:1364-1381.
  • Tovmassian, G.,Zharikov, S., Michel, R., Neustroev, V., Greiner, J., Skillman, D. R., Harvey, D. A., Freid, R. E., Patterson, J., FS Aurigae: A new Class of Cataclysmic Variables or the Missing Link between Intermediate Polars and SW Sextantis Objects? (2003). PASP,115:725-738.

Copyright © 2005 Society for Astronomical Sciences.