CCD Observing Manual

5.0 Observing Programs

5.1 Cataclysmic Variables

Cataclysmic variables (CVs) are unpredictable stars. Their changes in brightness can be tiny (.01 mag) or steep (>6 mags!) and can occur every few minutes or every few decades.

Most CVs are monitored until they experience a sudden change in brightness, usually either an outburst or a fade. During this period of activity they are monitored closely for small variations in their light curve.

UV Per's 2003 Outburst

It is this second phase where CCD observers are especially needed. High resolution imaging of a star in outburst, for example, can discover superhumps. These superhumps can help determine the orbital period for the binary star and setup models for the system. Discoveries of previously unknown or changing superhump periods can be worthy of publication. However, you need very accurate photometry to detect superhumps in most systems, usually .01 mag accuracy.

Example of superhumps detected by Bruce Gary (GBL). This data, combined with data from 18 other observers, refined UV Per's superhump period and measured it's evolution. It was published in IBVS #5488.

CVs are a great opportunity for visual and CCD collaboration. Visual observers can monitor the CVs every night and then notify the CCD observers when they go into outburst. At that point the CCD observers can try to detect superhumps or other behavior. CCD observers can also help by observing very faint CVs at quiescense. CCD Views has the following two articles about observing faint CVs: Observing Faint CVs at Quiescence (includes a list of recommended CVs) More on Observing Faint CVs Also read Cataclysmic Variables: A background on CVs from the Manual for Visual Observing of Variable Stars.

Cataclysmic Variable Stars: How and Why They Vary is a recommended book for people interested in CVs. Read our review from Eyepiece Views.

5.2 Long Period Variables

Long Period Variables (LPVs) are red giants that have long periods. They are among the easiest targets for CCD observers since they don't need to be monitored every night and tend to be fairly bright. Because of this same reason, they also tend to be neglected by CCD observers who think we don't need any more data on them. On the contrary, LPV research is just as important as other fields of variable stars and the AAVSO receives more data requests for LPV data than any other type of variable star.

LPVs need to be observed only once a week. Since they tend to be red, using a photometric filter is a must. See the Variable Star of the Month section on Miras for more information.

For a list of LPVs that are too faint for visual observers and are showing strange behavior check out CCD Views.

Also read Pulsating Variables: A background on LPVs, SRs and other pulsating variables from the Manual for Visual Observing of Variable Stars.

5.3a Gamma Ray Bursts

The AAVSO International High Energy Network consists of a worldwide network of observers dedicated to searching for optical counterparts of gamma-ray bursts (GRBs) through a coordinated rapid response network.

What does this mean? They await word that a GRB has been detected (usually by satellite). When that word comes, they head to their telescopes and attempt to image the GRB using CCD cameras. Time is of the essence here and that is where the AAVSO GRB Network comes in.

Through a connection with the GCN at NASA's Goddard Space Flight Center, notifications of recent GRB detections by orbiting satellites is distributed to the AAVSO. Our network takes these notices and distributes them, in turn, to our members based on a set of filters each observer specifies. The distribution method is primarily via e-mail, cell phone, and pager.

Satellite detects GRB -> Relay Station -> GCN@Goddard -> AAVSO HQ -> You!
then at AAVSO HQ:
Pagers Sounded -> Email Alerts Sent -> Charts Made & Placed Online -> Photometry Online

Depending upon the satellite that detected the GRB, the entire process can take from a few hours to a few seconds.

The speed of notification is key because most optical counterparts to GRBs discovered so far have been faint and very quickly became even fainter. The faster we react, the better chance we have of finding the counterpart and/or creating a light curve. GRBs may hold one of the keys to better understanding our Universe. They are the most powerful explosions of energy yet discovered. They may be used as distance markers, glimpses into the early days of our universe, and much more. But none of this can happen unless we obtain more information on these strange events.

A sample GRB finding chart

Everything you need to know is available at the web site for the AAVSO International High Energy Network.

5.3b Blazars

The AAVSO International High Energy Network is also observing Blazars in supports of the upcoming GLAST mission. Blazars are optically variable quasars with their jets pointed right at us. They vary on all timescales and at all wavelengths. They are observed in between GRB alerts. For more information visit our blazar web pages.

Different CCD observing strategies are used with blazars. Since they vary so much campaigns of different lengths are devised. All objects are monitored monthly to set a baseline of behavior. Specific campaigns looking for intraday (timescales of 4, 8, or 12 weeks) and microvariability (resolution of several minutes for many days) are planned. In addition, when GLAST is launched blazar alerts will be issued similar to GRB alerts when the GLAST telescope is observing a blazar and detects a gamma ray burst from it.

5.3c Polars

The AAVSO International High Energy Network is also observing magnetic catalcysmic variables, a.k.a "polars" or "AM Her stars". These are CVs with very strong magnetic fields that control the flow of material from one star to another. our observations of polars are in support of the XMM Newton mission. For more information visit our polars web pages.

5.4 Eclipsing Binaries

The Eclipsing Binary Committee is a superb example of CCD and visual observers working together to do good science. The visual observers will follow a recently discovered or suspected eclipsing binary star (along with doing archival research) to try and determine a time of minimum. Then the CCD observers will refine that time and together they usually publish an IBVS paper.

The Eclipsing Binary Committee can tell you what you need to do to observe and report data. They can also give you interesting targets and help coordinate observing programs if you would like to work with others.

5.5 Low Amplitude Stars

It is difficult for visual observers to record variations below a magnitude. CCD observers, on the other hand, can record variations down to the error level of their system. This makes CCD observations ideal for stars that may be bright but vary by small amplitudes. Such stars may be very red also, which increases visual scatter even more. If one wants to detect variations below 0.1 magnitude them precision photometry is essential.

The AAVSO PEP Committee usually monitor the brighter of these stars (>10). CCD observers should follow the fainter stars (<10). Flare and flicker stars (such as FU ORI) also have small fluctuations in brightness which cannot be detected visually.

5.6 Supernovae/Novae Patrols

Most supernovae and novae patrols these days are done with automated systems. However, non-automated systems still have their place and discover tens of supernovae/novae per year. For novae, the key is to have a wide field of view. Many people will simply connect their CCD to a standard 35mm camera lens and then point at the sky along the galactic plane. For supernovae, the best method seems to be to select a group of galaxies to monitor and then image them as often as possible. In both cases, blinking the image will reveal new sources. Blinking is a term referring to flipping back and forth between two images of the same field. There are many programs that can help automate this process.

5.7 BVRI Program

The AAVSO has a program to monitor selected stars in the B, V, R, and I filters using CCD observations. We began the program with a limited number of stars for two reasons. First, we had developed finder charts with comparison stars in the four filters for only 8 stars. Second, we wished to see how well data can be combined from different observers with different telescopes, cameras, and software. Please send BVRI data only for the stars for which we have BVRI charts: S Per, U Ori, VX Gem, DH Dra, VX UMa, W Leo, RU Vir, and RR Boo.

Each night you observe, you should take bias images, dark images, and flat fields with each of the filters you intend to use. All images should be processed before you do the photometry (bias and dark subtracted and divided by the flat). We suggest that you take a minimum of three images in each filter for each star you observe. In order to transform the images to a standard system, you need to observe with at least two different filters in each session.

Choose three comparisons for each variable you observe and find the magnitude of the variable by comparing it to each of the three comparison stars. The data you enter on the report form should be corrected using your transformation instructions.

First you will get differential magnitudes between the variable and each of the three comparisons. Then, obtain the differential colors (B-V, V-R, R-I). Then, using the transformation equations, transform the colors. Calculate the color- corrected values of the differential magnitudes. Finally, add the calibrated value (from the AAVSO CCD chart) for each of your three comparisons to obtain the correct magnitude of your variable for each filter you have observed. An example calculation for observations with the V, R, and I filters is given on the next page.

If you have observed only with the R and I filters, you will need only the R-I and R transformation equations. If you observe only with V and R, use the transformation equation for T_v and T_vr. If you observe with the B filter, then you must also transform B-V.

Sample Calculation of Transformed Magnitudes

Assume your transformation coefficients are as follows:

to transform the instrumental differential colors to standard differential colors:

Using V = (V-R) + R and I = R - (R-I) to obtain the transformed differential magnitudes:

Use the standard magnitudes of the comparison stars to obtain the magnitudes of the variable star:

5.8 Variable Star Chart Calibration

This is a very challenging program for those who have mastered the art of photometry. In order to make new variable star charts we need high quality photometry for the field. Believe it or not, no high quality (errors of .05 or less) photometric database exists for objects fainter than magnitude 10.5.

The AAVSO needs good photometry for newly discovered objects (novae, supernovae, GRBs, etc.) and to update existing charts that have poor photometric data. In order to help us you will need to be able to perform all-sky photometry at the .01-.03 mag level. Our requirements are very strict because making a star chart is a serious business. If there is anything wrong with your data then every observation made with the new chart will be tainted.

If you feel you qualify or have futher questions e-mail charts@aavso.org.

5.8 Designing Your Own

Everyone has a favorite object, type of star, and/or method of observing. When designing a personal observing program think of what you can do with the CCD that visual or PEP observers cannot. Focus on those items to get the most out of your system. Also, be realistic with what you can do. For example, if you don't have the patience to take numerous careful flat and dark fields throughout the night then don't try to detect and report .01 mag variations in a star. Instead, go for an LPV program where .1 mag accuracy is enough.

You will probably find that as you become more experienced you will take a little from here and there and end up with a custom program. You can always send feedback to the AAVSO for help in planning a personal observing plan.