CCD Photometry

Aperture photometry

This is the equivalent of photoelectric photometry using one or more apertures to define the area of sky, where the measurement is derived. In CCD photometry one has the option to tune parameters according to the problem as the reduction takes place on a computer.

The technique must be able to deal with different conditions: high or low sky background, readout noise, crowding problems etc.

The technique works well, in fact better than other methods, for bright isolated stars under stable atmospheric conditions.

PSF photometry

Here one fits a function, the Point Spread Function, to single, bright, isolated stars. It is important to choose a properly defined function. It can be an analytical function or a table. Or it can be a mix of both.

At some telescopes you will find, that this function varies over the field of view, and it is necessary to have a PSF, which varies with position. This is the case for the Danish 1.5m telescope at La Silla.

In order to improve noise statistics one can apply weights to the fit of the PSF to the individual stars. For each star one determines the amplitude and the position required to make the best fit of the PSF to each stellar image.

In crowded areas one can apply multistellar fits. The field can be dived in small areas, which contain a small number of stars, which are then fitted with a set of PSF's in an optimal way (LSM?).

Precautions to ensure high accuracy

• Good guiding of the telescope. This means that the same part of the detector is used for the same object for each exposure. For differential photometry this minimizes the effect of flatfield errors on the magnitude determined.
• Defocusing. For bright stars, the exposure becomes very short to avoid overexposure of the CCD. The readout then dominates and the duty cycle becomes very bad. If the crowding of the filed of view is not bad, one can compensate for this by defocusing the telescope. The PSF then sometimes becomes very strange. For a good telescope you will get a nice ring.
• Nonlinearity might be a problem for some telescopes/cameras.
• PSF variations make PSF photometry difficult. In such cases experiments are needed to find the best reduction technique.
• Defects can affect some stars in a very bad way. They can be eliminated by interpolation of the good part of the image, and this may be needed for special reduction techniques.
• Atmospheric effects. Sometimes scattered light from clouds can be really damaging to the images. It is recommended always to look out for effects, when observing close to full Moon.