The Neil Gehrels Swift Observatory

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UVOT Coincidence Loss Corrections

The following table combines the coincidence loss relation with the quickmag calibration (for an A0V star) to give the co- I correction (in magnitudes) as a function of magnitude. It assumes the standard UVOT hardware read-out window and zero background.

For example, from the table the white filter saturates at 15th mag, requires a large (0.62 mag) correction at 16th mag, and the co-I correction doesn't become negligible until 20th magnitude.

A non-zero background (usually higher for the optical filters) will increase the size of the co-I correction.

Mag U B V W1 M2 W2 White
13 Sat Sat Sat 0.64 0.34 0.57 Sat
14 0.51 Sat 0.30 0.18 0.11 0.17 Sat
15 0.16 0.36 0.10 0.07 0.04 0.06 Sat
16 0.06 0.12 0.04 0.03 0.02 0.02 0.62
17 0.02 0.04 0.02 0.01 0.01 0.01 0.18
18 0.01 0.02 0.01 0.00 0.00 0.00 0.07
19 0.00 0.01 0.00 0.00 0.00 0.00 0.03
20 0.00 0.01 0.00 0.00 0.00 0.00 0.01

More information on the UVOT coincidence loss expression can be found in The Count Rate to Flux Ratio UVOT CALDB document (PDF).

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