Desiderata for Photometry

The requirements for photometric images are really not much different than for any high-quality CCD image: expose correctly, calibrate properly, shoot high in the sky, and do everything you can to insure a high signal-to-noise ratio. The major differences are that you need to make your images in a well-defined part of the spectrum using colored glass filters, and that star images need to be large enough to insure proper sampling.

Correct Exposure. The integration time must be long enough to obtain a good signal-to-noise ratio for objects of interest, but cannot be outside the linear portion of the CCD's response curve. For most of the sensors used by amateur astronomers, this means that the peak value in star images of interest should not exceed one-half the full-well capacity of the CCD. Blooming is absolutely not allowed in any star of interest. In addition, any camera options that lead to non-linearity (such as an anti-blooming gate) should be shut off or disabled. Taking multiple images and combining them to obtain a better signal-to-noise ratio is acceptable.

Proper Calibration. The images must be calibrated properly, preferably using the advanced calibration protocol (bias removal, dark current subtraction, and flat-fielding; see Section 6.3.3), with a separate set of flat-fields for each filter in the photometric set. The flat-fields should be exposed to one-half of the full-well capacity; and the observer should combine multiple flats, so the signal-to-noise ratio of the master flat for each color is considerably higher than the signal-to-noise ratio expected in the image. For photometry to 0.01 magnitude (1%), the signal-to-noise ratio of the flat-fields should be 500:1 or better.

Minimize Air Mass. Shoot photometric images as high in the sky as possible to reduce the effects of extinction. For all-sky photometry, this means working higher than two air masses (30° elevation) on nights when the sky is free of any trace of clouds. For differential photometry, thin passing clouds are usually acceptable high in the sky; but near the horizon, where cloud motion is slower and extinction is greater, any cloud is unacceptable.

Insure Proper Sampling. Front-surface CCDs (i.e., the inexpensive kind used by amateur astronomers) have a polysilicon gate structure that starlight must pass through to reach the light-sensitive bulk silicon of the chip. The gate structure is not present everywhere on the face of the CCD, but is laid down in strips. If a star image is only one or two pixels across, and it happens to fall on a strip, a significant fraction of the incidental starlight can be lost. If the star images are four or more pixels in diameter, the loss is averaged over multiple pixels and becomes unimportant. With back-illuminated CCDs (i.e., no gate structure), star images can be as small as two pixels in diameter.

As a rule of thumb, the optimum image scale is about two seconds of arc per pixel, but amateurs have done excellent photometry with scales between one and six seconds of arc per pixel. The primary concern is that the star image must cover at least two pixels at full-width half-maximum. If the focal length of your telescope is so short that your star images are only two or three pixels across, set the CCD very slightly out of focus. Enlarged star images don't look nice, but they produce reliable photometric results. An additional benefit from defocusing is that the peak pixel values in "mushy" star images are lower than they are in sharply focused ones; therefore, the camera is more likely to be working in the linear portion of the CCD's response curve.

Shoot Multiple Images. For accurate photometry, why rely on a single image when you can shoot two or even three exposures? Offset the images slightly from one another so the star images fall on different groups of pixels, reduce the images individually, and then check that the magnitudes in the different frames agree with one another. If your darks and flats are good, they will. In sequences of images taken for differential photometry of eclipsing binary stars or rotating asteroids, defective images stand out when the difference between the comparison and check stars falls well outside the normal range of statistical variation.

Shoot through the Right Filters. Unfiltered CCD images have limited value in photometry because too many factors influence the response of the CCD. However, a full UBV(RI) filter set costs several hundred dollars. To keep costs down, order a V filter so that your instrumental measures correspond to visual magnitudes. You can always add other filters later. Few amateurs bother with a U filter because front-illuminated CCDs have so little sensitivity in the U passband that photometry is all but impossible; and with many CCDs, B filters are hardly practical. For many CCD observing programs, a V filter and an I filter may be all you'll ever need. In any case, consult with the organization that you observe with, and use the filters that they recommend.

Some observing programs do not require filters. An example is monitoring cataclysmic variable stars for the Center for Backyard Astrophysics, where the goal is to get good time coverage of these rapidly changing variables.

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