CBA Center for Backyard Astrophysics

A History of the CBA

Center for Backyard (Basement) Astrophysics

The Center for Basement Astrophysics (CBA) was founded by David Skillman in the 1970s as a sort of counter-culture version of the perhaps better-known institute in Cambridge, Massachusetts. The single product of the CBA was, and still is, light curves of variable stars. The workhorse instrument was a 32 cm reflector with a 1P21 photomultiplier tube. The unique feature was that it took data automatically all night long -- shifting between variable and comparison stars, recentering with an automatic algorithm, integrating again, etc. The telescope roof had to be slid open, and the telescope had to be told the stars' coordinates; other than that, no human attention was needed, and most data were acquired with Dave soundly (well sometimes not so soundly, but you get the idea) asleep.

The CBA accreted me in 1980 and Dave Harvey (Tucson, AZ) in 1993. More importantly, Skillman bought parts for a CCD and followed a 1987 Jim Gunn article on how the Palomar CCD was driven. He duplicated those circuits in his basement and also added a refrigerator and vacuum pump to get the CCD down to -80° C. The first version of the camera head was built in a large tunafish can. This was the first CBA CCD. By 1991 the CCD had replaced the photomultiplier tube at the business end of the telescope. With the p-m tube our magnitude limit for doing photometry was about 13, but we could not reliably and automatically find such stars beyond magnitude 11.5 (the x-y scan with 1 s integrations would fail). Since the CCD is an area detector, the pointing uncertainties were far less critical, and suddenly our practical magnitude limit went to about 16. Suddenly we had a powerful tool for studying the light curves of faint variable stars.

Lebensraum and Bucks

Short histories of the CBA have also appeared in Sky & Telescope (January 1981, May 1993, & October 1998). Starting in 1995, we accreted members from other countries - or, more significantly, other longitudes. Since it's always clear and dark somewhere, this gave us the ability to track variable stars continuously around the clock. Thus we were able to detect and measure periods on timescales normally very difficult to handle -- especially in the range 0.3-2.0 days. These multi-longitude campaigns also enabled us to reach very sensitive limits for the detection of periodic signals; with a global network of 8-14 inch telescopes, I found that I could reach a sensitivity limit about 5 times better than I could earlier with a single 80-inch telescope proudly performing its hijinks on one isolated mountaintop. Over the next three years, we found Tonny Vanmunster in Belgium, Lasse Jensen in Denmark, Bob Fried in Arizona, Jerry Gunn in Illinois, Gordon Garradd in Australia, Stan Walker in New Zealand - the people that made the CBA the world's most powerful machine for finding periods in cataclysmic variables. With the realization that most members do not have basements, or do not commit subversive acts in their basements, we decided that "Backyard Astrophysics" was a better name.

We also began to find financial support. The Dudley Observatory funded the first computers, and the NSF has funded several hardware purchases for our far-flung observing stations. In 1997 we received a major grant from the Research Corporation which put us temporarily in the unfamiliar position of being able to buy all the toys we needed, and pay the bills relentlessly arriving from research journals. Funding is definitely a problem; since nearly all professional astronomers espouse the rhetoric that telescopes must get bigger and more expensive, it's pretty difficult to get an audience for a contrary view! But patience can pay off, and a proven record in research counts for something. So far we've published 27 papers in refereed journals; these plus the usual information/propaganda blast can be found at


We have concentrated on accumulating long, dense segments of light curves on cataclysmic variables. The CCD detector allows us to make use of marginal nights (dividing the variable by the comparison largely removes the effect of thin clouds), and the robotic telescope control allows the astronomer to go to sleep. It has taken me years to become fully persuaded of the importance of robotic operation, but I am now a full convert. Not least among its many advantages is the fact that data are acquired at no human cost of sweat, tears, insomnia, or psychic stress. So when it comes time to evaluate the data, the designated prime mover of the project can say, "hmmm, this particular segment appears to be of lower quality, so I'll reject it." In normal scientific collaboration that's an awkward thing -- you have to tread gently because people put much time and effort into gathering their data. Computers have a tougher hide.1

Of course the main reason we are so fond of robotic operation is that it allows routine acquisition of very long light curves -- say 8 hours on a given star. With observing stations spaced at strategic longitudes around the world, we could in principle obtain 24 hr coverage for days on end. This is a wonderful goal to strive for, because it would allow us to study a large chunk of frequency space (say 0.4-2.5 c/d) which is at present effectively hidden by the insidious effect of Earth rotation.

A good CBA station needs the following:

  • A good telescope (8 inches or larger) with a good quality drive, and probably with some wind protection. Computer control is desirable.

  • A CCD camera, and convenient software for controlling the camera by computer.

  • Software for rapid differential photometry (variable - comparison) of the many images. To be useful a light curve should have at least 40 points in a night, and 100 is usually better.

Most of the (low-end but still very valuable) stations look something like this: Meade 10" LX-200 telescope, SBIG ST-7 or ST-6 CCD camera, 486 or better laptop computer, fast 486 or better desktop computer for processing images and obtaining delta magnitudes. In case you're wondering and don't know about such things, the cost of each of these components runs around US$3000: telescope, laptop, desktop, CCD camera. Each about US$3000 with a few bells and whistles thrown in.


In practice the way our observing goes is like this:

About twice a year we send out a CBA newsletter with news of past, present, and future observing programs, and the status of scientific papers motoring along in the publication pipeline. About monthly we send out more explicit suggestions about targets for observing, and declare campaigns of some stated duration on particular stars. I try to keep the communication frequent but not frenzied -- so nobody gets calls or e-mails saying "hey forget your stupid plans and observe my star instead". A few of our campaigns will perhaps suffer a bit, but by respecting each other's privacy and judgment we will have a much sounder collaboration in the long run. Typically the observer posts the reduced data (just the delta magnitudes, not the full image files) a few days later by e-mail or on floppy disks. At the end of the observing season or perhaps two, the data crop is usually ready for harvest in a scientific paper.

The Program

In principle we could study all variable stars. But to achieve greater scientific power we have (mostly) elected to specialize in photometry of cataclysmic variables (CVs). These stars have great advantages:

  1. They flash their periodic waves on timescales of 10 minutes to 2 days -- short timescales that offer substantial sensory reward per hour invested.

  2. They erupt frequently, keeping e-mail wires and conference programs aglow with reports of their latest hijinks. Good for observer morale.

  3. Their underlying physics mostly involves the study of their accretion disks, a tremendous growth area today in both observation and theory. Being bright and radiating primarily at accessible wavelengths, the accretion disks of cataclysmic variables provide some of the most important checks on rampaging theorists. And, happily enough, the theorists even know this!
More than half of our time is currently devoted to the study of "superhumps" -- light variations at periods near but not exactly at the binary orbital period (typically a few hours). Superhumps are a much-discussed and still mysterious subject today. They're very difficult for professional astronomers to study, because they're often transient, and because they're just not very well constrained by obervations over a short time baseline. Backyard telescopes solve both problems. We find that by amassing large amounts of data over an observing season, especially with robotic telescopes and a range of terrestrial longitudes, we can build an observational record of far better quality. And we do.

The Future

To me, the most wondrous thing about this enterprise is the people I've managed to locate. I was an experienced amateur astronomer before going into research, so I was well acquainted with the great technical skill for which this community has long been famous. But I found that our members were not only tremendously skilled with their telescopes, but endowed with the same curiosity about things unknown that the best of professional scientists show. They had a deep natural instinct for research. In retrospect, this should not have surprised me; it's just what happens naturally when you couple telescopes, starlight, and humans with long attention spans.

Barring some totally unforeseen development (illegality of the internet, world famine, arrival of the killer asteroid, etc.), it seems probable to me that networks of this type will grow substantially and perhaps even dominate the study of variable stars. The sum total of ingenuity and energy among the world's amateur astronomers vastly exceeds that of professionals, and that will likely dominate no matter how greedily the latter guard the world's largest telescopes against invasion by outsiders.

We're always looking for new observers. If you are interested in joining our network or at least getting included in the communications web, please send us some mail. And tell us about yourself!


1 Despite the story in the National Enquirer about the computer who electrocuted the boss after overhearing her discuss plans for buying a better one.