How can I correct for time sensitivity degradation of the OM
detector?
The OM detector (intensified CCD) is affected by aging. This produces a
loss of efficiency, or time dependent sensitivity degradation, slightly
different depending on the filter used. In next figure we can see this
degradation as measured in three white dwarf stars that are observed
regularly as part of the OM calibration.
This degradation is linear and it can be corrected as follows:
- Correction factor:
Y = A + B * MJD
- Corrected countrate:
Co = C * Y
where MJD is the Modified Julian Date of the observation and:
| filter |
A |
B |
| UVW2 |
- 2.72723 |
7.14543e-05 |
| UVM2 |
- 2.91093 |
7.49769e-05 |
| UVW1 |
- 1.16448 |
4.14899e-05 |
| U |
- 0.313603 |
2.51781e-05 |
| B |
- 0.484320 |
2.84507e-05 |
| V |
- 1.25429 |
4.32118e-05 |
This correction is applied in SAS OM image processing (
ommag task
within
omichain) from Version 6.5.
We recommend that data processed with earlier versions be reprocessed with SAS 6.5.
For a quick comparison however, old data can be corrected with the
formulas above.
