5.13 Critical Analysis

There is a discrepancy between the optimal and aperture photometry produced by the PL in each of the three wavelength reductions. To quantify this the mean rms value of the difference between the optimal and aperture photometries, derived on a point-by-point basis, were calculated and compared to the mean rms photometric errors for each band. These data can be seen in Table 5.11

Filter	Optimal	Aperture	Difference


R	0.13	0.10	0.25
I	0.19	0.11	0.36
Z	0.26	0.11	0.34

Table 5.11:

Comparison data for the prototype and production pipe-lines. The “optimal” column is the mean rms value of the optimal photometry’s errors, the “aperture” column is the mean rms value of the aperture photometry’s errors and the “difference” column is the mean rms value of the difference between the two photometries.

The reason for this difference is unclear. However the profile correction is performed using data extracted from the brightest stars on the frame, whilst the object stars will be amongst the lowest luminosity stars on the frame. It may be possible that this profile correction is not a linear function of brightness and may need to be corrected for in a more luminosity dependent manner. This argument is re-enforced by the fact that the optimal and aperture reduction methods always show two distinct distributions on a point-by-point basis (e.g., the optimal reduction is always above or below the aperture reduction but not both, indicating a systematic difference).

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