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Which magnitudes should I use?
There are several different kinds of magnitudes in the PS1 catalog (aperture, PSF-fitting, Kron, etc.), and there are also several different sources for those magnitudes (means from multi-epoch measurements, measurements from deep stack images, "forced mean" measurements where a stack image is used to identify objects but the photometry is determined by fits to the single-epoch warp images). Which magnitude should you used for your science?
A comparison between different photometric measures provides some guidance. The answer is complex, but here is a high-level summary:
- For point sources use PSF magnitudes.
- Mean PSF magnitudes have the lowest noise (because the PSF model is most accurate in single-epoch images). They are good for brighter objects, but for objects near the single-epoch detection limit they will be biased (due to the absence of sub-threshold detections), and objects too faint to detect in a single epoch are missing.
- Stack PSF magnitudes are noisier because the PSF model is less accurate. But the stack detections are more than a magnitude deeper and so have many more faint objects than mean detections.
- Forced mean PSF magnitudes use PSF-fitting photometry on the single-epoch images at positions of stack detections. They are a reasonable compromise: they have slightly lower noise than the stack PSF magnitudes, and they are deep and unbiased (because they use data from all warps). Their noise is higher than the mean PSF magnitudes, however.
- For extended objects use Kron magnitudes.
- Stack Kron magnitudes are usually the first choice as a general-purpose, deep magnitude.
- de Vaucouleurs and exponential model fits could be better in some cases, and the mean measurements can be useful for objects that are barely resolved (where the PSF is important).
- Extended object photometry using the PS1 catalog will require research and analysis by the user to determine the best approach.
What are the brightest and faintest stars for which the data are reliable?
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