How do astronomers get the DATA from a ccd image?

The American Association of Variable Star Observers publishes a manual for analyzing CCD images to mine their data on star magnitudes; see below. They also have VPHOT, an online tool for photometric analysis of CCD images. The major source of information is spectra, and here it's a matter of scanning the spectrum generated by the diffraction grating longitudinally to measure the luminosity of the spectrum at different wavelengths.

After you've done the reduction to standardize the field, the pixel count above the background can be converted into a magnitude using magnitudes of known stars in the field or a calibration image. Most professional astronomers use IRAF to do the reduction and analysis of the images, but some schools teach undergrads to do it using MIRA.

After you've done the reduction to standardize the field, the pixel count above the background can be converted into a magnitude using magnitudes of known stars in the field or a calibration image. Most professional astronomers use IRAF to do the reduction and analysis of the images, but some schools teach undergrads to do it using MIRA.

The American Association of Variable Star Observers publishes a manual for analyzing CCD images to mine their data on star magnitudes; see below. They also have VPHOT, an online tool for photometric analysis of CCD images. The major source of information is spectra, and here it's a matter of scanning the spectrum generated by the diffraction grating longitudinally to measure the luminosity of the spectrum at different wavelengths.

Each pixel in a CCD camera responds ONLY to the level of luminosity (or more precisely, the number of photons) that hits it. And it does so in a linear fashion. After you correct the image for defects in the camera itslef and in the telescope, you are left with a pixel-by-pixel recording of the level of luminosity. If you compare two stars, you add up the pixel values for each star (the pixel value is recorded as a number, representing the amount of luminosity it received during the exposure). If one star's value is 16 times larger than the other, then we know that the brilliance difference is 3.0 magnitudes. For Cepheids, if you are lucky, there will be a standard star in the same field. This is a star with a known (and fixed) magnitude. Since the exposure may change every time (length of exposure, extinction due to altitude, temperature, air transparency...), you compare, on each frame, the Cepheid's total intensity with the intensity of the standard star. In theory, this should work all the time. In practice, there are some uncertainties. Sometimes one star includes a faulty pixel that was not included in previous shots. Sometimes the amplifier (that counts the number of photons in each pixel) may overheat (or be colder than usual), etc. Therefore, you need quite a few shots to statistically make your observation reliable.