PSF shaped photometry

The theoretically best PSF shape for good photometric results is a top hat or mesa shape. Point sources, however, come in only one shape, round, and they approximate a Gaussian distribution when imaged on a CCD. This is not the case anymore! OTCCDs have the ability to shift their collected charge in the CCD during integration in order to provide tip-tilt corrections (see Chapter 2). This same feature can be put to use to manipulate the incoming photons into whatever shape the user desires in order to increase the output science. Howell et al. (2003) used this property of OTCCDs to produce square stars (Figure 5.10).

The photometric precision available with square stars is generally greater than with normal stars as the PSFs are better sampled and contain higher S/N for a given source magnitude. The shaped stars have none of the drawbacks of similar techniques (i.e., defocusing of bright stars) but do render faint background sources and 2-D objects (such as galaxies) unusable (Tonry et al., 2005). One of the great benefits of PSF shaped photometry is its ability to greatly increase the high photometric precision dynamic range of a CCD. Figure 5.11 shows the relationship between the recorded magnitude of a source and the photometric precision of its light curve obtained with ensemble differential techniques. The panel on the left is a conventional CCD (with normal star images) and the right hand side shows a similar result for a PSF shaped data set obtained with an OTCCD. The solid line in the left hand panel is the theoretically expected result using the CCD S/N equation (Chapter 4). The difference in the two plots is obvious revealing that the PSF shaped result yields the highest photometric precision over nearly 5 magnitudes of dynamic range (Howell et al., 2005).

Fig. 5.10. PSF shaped star profiles using an OTCCD. The left hand panel shows a sub-section of an OTCCD image in which the stars were moved in a square shaped pattern (of 20 :■: 20 pixels) during each 300-second integration. The right hand panel shows a 3-D plot of one of the mesa-like PSFs.

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Fig. 5.10. PSF shaped star profiles using an OTCCD. The left hand panel shows a sub-section of an OTCCD image in which the stars were moved in a square shaped pattern (of 20 :■: 20 pixels) during each 300-second integration. The right hand panel shows a 3-D plot of one of the mesa-like PSFs.

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Fig. 5.11. Relationship between recorded stellar magnitude and the resulting photometric precision of the light curve for a sample of stars. The left panel is from a conventional CCD while the right panel is from an OTCCD using PSF shaping techniques. The shaped profiles (squares) provide similar high precision for about 5 magnitudes of brightness. Stars on the right, brighter than ~ 11. are saturated.

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