Getting Started with CCD Variable Star Observing

Observing variable stars with a CCD camera is an immensely rewarding and valuable experience. However, it can also be quite daunting to someone just starting out with a CCD camera and casual interest in variable stars. This guide is intended to help introduce relatively new CCD owners to the world of variable star photometry.

Background

The first thing you should do is start making visual estimates of variable stars. This will familiarize you with the techniques, hazards, and tips of observing. It will also probably get you addicted, so be warned!

We have a list of Easy To Observe Stars which is a good place to start. The best resource available for learning how to make an observation (besides a human mentor) is the AAVSO Manual for Visual Observing of Variable Stars. Either download it online or order a paper copy.

Equipment

Finding the right CCD for your telescope, drive, and computer system is beyond the scope of this document. You will need to choose between cost, performance, pixel size, as well as many other variables.

However, there are a few items semi-unique to variable star observing that you may want to consider while investigating equipment. First, the larger the field of view, the better. You will need a field of view that not only shows the variable star, but also comparison (a.k.a "comp") and check stars to compare against the variable. Second, resolution is not as important as sensitivity. Third, if you can afford it, get a camera without antiblooming. This will help you down the road as you become more advanced, however it is not a requirement to do good work.

Finally, you must have a proper filter for the CCD. A good place to start is by buying a Johnson V filter. This filter best represents the spectral response of the human eye so stars in the CCD will look similar to how they appear to the naked eye. Without a proper filter, your observation is of very limited value. Johnson V filters should run between US$40-$75. In the future you may want to invest in a complete BVRI filter set, but for now the V will do.

You will also need a software package that performs aperture photometry. Most of the popular CCD cameras come with such a package, but it may be of limited value. The following commercial packages are popular among many of our observers: AIP4Win, AstroArt, Mira, and MaximDL. Investigate them all and choose the one right for your camera and budget.

Let's Start Observing

The clouds have parted, you've made some visual estimates, your CCD has arrived, and now it's time to get started. First of all, let's set our expectations. We are only trying to get to .1 mag accuracy right now. You may have seen others who report observations to .01 mag accuracy, but that involves a lot of work and experience. For this document we are going to stick with .1, which is still very valuable data and better than what visual observers can do with an eyeball. (Note: That is not meant to say that CCD observations are more important than visual. A good variable star observing program incorporates both visual and CCD observations.)

You will need to subtract dark frames from your images so make a set of "darks" for various exposure lengths. (I do 30s, 60s, and 120s.) If your lenses are clean and exposures short, you do not have to worry about flat fields for .1 accuracy photometry.

Now it is time to find the field. The best way to do this is to slew to the coordinates of the star and use an AAVSO D or E scale chart to identify the field. Then zero in on the star and use an E or F scale chart to make the actual observation. This is where experience making visual observations really helps out. Sometimes finding the field can take quite a long time. Be patient, have a cup of coffee, and take your time. After a few weeks you'll start to get it down pat.

CCD image of UU Lyr with 3 comp stars. FOV is about 12'x9'.

Different software packages use different techniques to perform photometry so I will not go into detail here on the specifics. Consult your documentation. In general, though, you want to use 1 comp star and 1 check star. A check star is a star that you compare to the comp star to make sure the comp star is not a variable itself! If it "checks" out, then you can use the comp star to make an estimate of the variable. This usually involves telling your software program the magnitude of the comp star, which the software then uses to calculate the magnitude of the variable star.

If all this sounds complicated, don't worry. With experience it almost becomes second nature. Eventually you'll probably be able to make an estimate every 5 minutes or so. (Way down the road you can even get advanced enough to script all of this.)

What Now?

When you report the observation to the AAVSO be sure to put "CCDV" in the comments code field. That tells us that it is a CCD measurement with a V filter. Read CCD Views or subscribe to our Special Notices to get ideas of good targets for your new equipment. CCD observers are encouraged to go after objects that are too faint for visual observers.

After a few months to a couple of years you may want to start making even more observations with your CCD. The AAVSO has many programs for advanced CCD observers including work on small-amplitude variables, multi-colored photometry of faint LPV and CVs, faint new eclipsing binaries, and chasing gamma ray bursts (GRBs). Many of these programs are on the cutting edge of science and their results get published. But any program is important if you enjoy it and submit your data to the AAVSO International Database. For more information visit the CCD Committee section of the AAVSO web site. You will also find tutorials on how to take the step to the next level and reach the .01 mag level of accuracy.

If you have any questions please feel free to contact us at aavso@aavso.org or post them to the AAVSO Discussion Group. Thanks and good observing!

By aaronp@aavso.org