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WFIPhotometric Properties |
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There are two effects that should be separately dealt with when reducing WFI data. One is the well known colour transformation equation to the standard system. The second is the variation of zero points as a function of position in the mosaic. In a well designed reduction process we must first correct the instrumental magnitudes using a zero point correction map, and then apply the color transformation equation. The problem is complicated in the case of the WFI due to the large color term for the B filter: the zero point for an O3 star, intrinsic color -0.3, reddened by 1 magnitude to +0.7, differs with respect to the zero point of an unreddened star with intrinsic color of +0.7 by more than 0.1 magnitudes! Most of the Landolt standards have small reddenings, so... caveat emptor.
color
equations. |
zero point
correction map. |
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The following Zeropoints (ZP), Colour Terms (CT) and Extinction coefficients should be used in the following format:
B = b + ZP(b) + (B-V)* col(b) - Ext(b)*Airmass V = v + ZP(v) + (V-R)* col(v) - Ext(v)*Airmass R = r + ZP(r) + (V-R)* col(r) - Ext(r)*Airmass I = i + ZP(i) + (R-I)* col(i) - Ext(i)*Airmass |
Below are listed key solutions that have been calculated from a reasonable amount of data. You can also view this table for other solutions that do not contain as much data.
Filters | Zero point | Extinction | Color term |
---|---|---|---|
U | 22.06 +/- 0.11 | 0.48 +/- 0.03 | 0.05 +/- 0.02 |
B | 24.81 +/- 0.05 | 0.22 +/- 0.015 | 0.25 +/- 0.01 |
V | 24.15 +/- 0.04 | 0.11 +/- 0.01 | -0.13 +/- 0.01 |
R | 24.47 +/- 0.04 | 0.07 +/- 0.01 | 0.00 +/- 0.00 |
I | 23.37 +/- 0.05 | 0.00 +/- 0.00 | 0.03 +/- 0.01 |
Filters | Zero point | Extinction | Color term |
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U | 22.09 +/- 0.08 | 0.5 | 0.11 +/- 0.03 |
B | 24.81 +/- 0.04 | 0.21 | 0.26 +/- 0.01 |
V | 24.21 +/- 0.03 | 0.1 +/- 0.02 | -0.12 +/- 0.01 |
R | 24.53 +/- 0.04 | 0.11 +/- 0.02 | 0.00 +/- 0.00 |
I | 23.52 +/- 0.05 | 0.10 | 0.04 +/- 0.02 |
Filters | Zero point | Extinction | Color term |
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U | 21.91 +/- 0.09 | 0.33 | 0.14 +/- 0.05 |
B | 24.86 +/- 0.09 | 0.24 +/- 0.09 | 0.21 +/- 0.02 |
V | 24.25 +/- 0.09 | 0.19 | -0.13 +/- 0.06 |
R | 24.49 +/- 0.09 | 0.09 | -0.05 +/- 0.06 |
I | 23.55 +/- 0.06 | 0.10 | -0.10 +/- 0.08 |
Filters | Zero point | Extinction | Color term |
---|---|---|---|
B | 24.75 +/- 0.04 | 0.22 +/- 0.03 | 0.03 +/- 0.02 |
V | 24.21 +/- 0.04 | 0.19 +/- 0.02 | -0.15 +/- 0.02 |
R | 24.49 +/- 0.09 | 0.14 +/- 0.04 | -0.04 |
I | 23.46 +/- 0.09 | 0.11 +/- 0.03 | -0.12 +/- 0.04 |
Filters | Zero point | Extinction | Color term |
---|---|---|---|
B | 24.70 +/- 0.07 | 0.2 | 0.2 |
V | 24.10 +/- 0.05 | 0.15 | -0.11 |
R | 24.42 +/- 0.05 | 0.12 | -0.04 |
I | 23.41 +/- 0.05 | 0.1 | 0.0 |
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Last modified: Thu Jul 31 2003 |
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