Conducting the observations

If you haven't previously observed the night's target, then your preparation for the observing session begins a few hours before sunset. You need to acquire the standard star chart for the target and acquaint yourself with the chart, the object, and the comparison stars. The most convenient method for getting the star charts is to download them from the AAVSO website at www.aavso.org/charts There, you can search for charts by star identity, by coordinates (RA, Dec), and by constellation. For most stars, the charts come in different scales (ranging from the relatively wide field of view C- or D-scale charts, to the smaller field of view F-scale CCD charts). Select the CCD chart if it is available and confirm that its scale is appropriate for your imaging setup. If you have an unusually short focal length telescope and unusually wide field of view imager, then the CCD chart might conceivably be too small, in which case you can use one of the larger-scale charts.

Figure 4.8 illustrates how to find the target field of view, and your target in it, by finding the pattern of stars in your CCD image. On the image, I've added an outline of the area covered by the AAVSO chart (this is a CCD F-scale chart) to help you identify the variable star and the comp stars. For the record, this image was taken with my 11-inch Celestron NexStar, operating at F/6.3, and an SBIG ST-8XE imager, through a V-filter. It is a 3-minute guided exposure. It has been reduced by dark-subtraction and flat-fielding using Software Bisque's CCDSoft. Note that all of this, aside from the V-filter, is typical entry- or medium-level CCD imaging equipment.

Once night falls and your target star is reasonably high in the sky, you're ready to locate the target in your imager. As a general rule of thumb, it is a good idea to have the target at least 30 degrees above the horizon. That will minimize most of the atmospheric effects that can corrupt your photometry. There will, of course, be situations where you have no choice but to observe at lower elevation angles (such

Figure 4.8. Matching an AAVSO chart to a CCD image. (Used with the kind permission of the AAVSO)

as a star just emerging from behind the Sun, visible only low in the sky just before twilight, or a star that is at a low southern declination that never gets high in the sky for northern-hemisphere observers).

Take your photometric images as you would any other CCD images. Save them in FITS format (preferably), or native uncompressed imager format (if FITS is not an option). Do not use a compressed image format such as JPEG; image compression severely garbles the photometric data. It is also good practice to save in "image-only" mode, not with "auto-dark" or "auto-flat". Set the imaging software to record the image time into the FITS header. This makes your record-keeping simpler. If for some reason it isn't possible to have your CCD control software write the image time directly to the FITS header, you'll need to record the time and exposure duration for each image in your notebook. Exposure duration should be long enough that both the target and the comp stars have good signal-to-noise ratio, but not so long that any are saturated. For most targets, and most set-ups, exposures in the range 1-5 minutes will be appropriate.

Even though a single image contains all of the information needed to determine the target star's brightness, it is usually good practice to take at least three images, sequentially, using identical imaging parameters. That gives you two meritorious types of redundancy. First, if by chance either the target star or comp star are corrupted by a cosmic ray hit on one image, odds are that they will be fine on the other two. Second, with two or three images, you will be able to assess their consistency, so that you can toss out an image that gives discordant data, and you can improve your accuracy by averaging the results from "good" images.

At this stage, it is also a good idea to examine each star's image to determine the peak-pixel ADUs, and (if your software provides it) the signal-to-noise ratio. The peak pixel ADU of each star allows you to confirm that the image is not saturated. If your brightest comp star is saturated, just delete it from your analysis. If your target star is saturated, then you'll need to re-take the images, using a shorter exposure. If your target signal-to-noise ratio is low (e.g., less than 30), then your photometric accuracy will not be as good as it could be, and you may want to consider re-taking the images using a longer exposure. If you are not familiar with the concepts of sensor linearity and saturation, refer to Sections 4.5.2 and 4.5.3.

In addition to your science images, you need "dark frames'' and "flat frames'' that match the conditions of your science images. The dark frames should be taken at the same exposure and same chip temperature as your science frames. The flat frames can be twilight flats, "T-shirt" flats, or "lightbox" flats (whichever is most convenient), and they can be taken either immediately before or after your photometry session. One purpose of flat-fielding is to compensate for the effects of "dust donuts'' in the image. Therefore, it is essential that the flat frames be taken through the same V-filter as the science images, and that they be taken before the CCD orientation in the telescope is changed from what it was during the science frames. Otherwise, the "dust donuts'' in your science frames won't line up with those on the flat frames. If your image-processing software provides for use of "dark flats''—dark frames of the same exposure as your flat frames—take those also. If you are an experienced astro-imager, then you probably already understand the purpose and use of darks and flats, and have developed methods that you are comfortable with. If this subject is relatively new to you, then I offer a few suggestions about darks and flats in Section 4.5.

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