ISO LWS Calibration Notes

1. For many L01 and L02 data, the error in the flux levels seen for individual detectors is dominated by uncertainties in the dark current removal in the data reduction. These uncertainties yield spectra with continuum values in adjacent portions of the spectrum that may vary significantly. Some dark currents may be over estimated and leave a detector with negative continuum fluxes. In general, dark current subtractions may be in error by as much as 100% (if a large glitch occurred in dark measurement or detector temperature increased during observation). For raster map observations, negative continuum fluxes may appear as the dark current is not calibrated individually for each raster position, but rather at the beginning and end of the observation. The nominal dark levels for detectors 0-9 are approximately these values in the table below as given in the lcdk.fits calibration file found in the LWS data deliveries and ISAP distributions.

Nominal Detector Dark Currents
Detector Dark Current (Amps) Dark Flux (W/cm^2/um) Flux Density (Jy)
0 4.96e-16 5.01e-18 36.6
1 2.08e-16 4.07e-18 44.1
2 2.20e-16 3.34e-18 45.4
3 1.18e-16 2.07e-18 38.7
4 1.56e-16 3.03e-18 72.9
5 2.50e-16 7.92e-19 25.2
6 7.30e-18 7.47e-21 0.34
7 5.31e-17 2.04e-20 1.30
8 1.76e-16 9.30e-20 7.67
9 1.21e-16 3.77e-19 40.5

2. The LWS is calibrated on a point source (Uranus). The optical design of the LWS has a complex mirror M2 inclined to the primary mirror and incoming light. The substrate supporting M2 appears to be reflective, especially at longer wavelengths which may explain the off-axis fringing seen in extended sources. For extended sources larger than the average LWS beam (75"), an extended source correction must be applied to the data. This correction adjusts the point source fluxes for additional off-axis contributions to the source flux. These corrections have been derived under the ideal assumption that the source is smooth with a very extended flux distribution which may not be the case if the observations contain structure or embedded sources. The extended source correction has been tested on the Trumpler 14 and 16 extended source fields and compared to IRAS 100 micron fluxes. The LWS/IRAS ratio was about 0.9 +/- 0.2. For a more detailed explanation of this correction, please see the LWS Handbook. This table gives the approximate effective radius, effective solid angle (omega x 10^6), and correction of the beam for each detector. The corrected source flux for an observed flux F (Jy), is S = F * (correction) / (omega x 10^6) MJy/sr

LWS Beam Characteristics
Detector Effective Beam
Radius (")
omega x 10^6 (sr) Correction
0 39 0.114 0.88
1 42 0.130 0.88
2 43 0.135 0.84
3 41 0.117 0.73
4 40 0.114 0.70
5 38 0.107 0.69
6 39 0.111 0.69
7 35 0.092 0.62
8 34 0.088 0.55
9 33 0.083 0.48