Spurious Features in LWS Spectra

1. Introduction

Observers are warned of spurious features which may appear in LWS spectra. In contrast to the broad features caused by the near-infrared leak (see note from 31/01/97), these features reside in the calibration files and are transferred to the observed spectrum during the calibration process.

2. The Calibration of the LWS

The primary calibration source for the LWS is Uranus. For each detector, the measured photocurrent is plotted as a function of wavelength. A model of Uranus is then used to predict the flux-density falling on the entrance pupil of the telescope, also as a function of wavelength. From these two plots, the overall responsivity of the instrument, in terms of current per unit flux-density, is recorded, detector by detector. For historical reasons, these responsivities are known as the detector "filter profiles".

The calibration of any other source is then achieved by dividing the observed photocurrents for that source by the filter profiles. Clearly, any spurious features in the profiles will be transferred to the calibrated spectrum.

3. Spurious Features in the Filter Profiles

3.1 Noise

The signal-to-noise ratio obtained in the calibration observations of Uranus - and therefore in deriving the filter profiles - is comparatively modest. It limits the signal-to-noise ratio on the calibrated spectrum of any other source, no matter how strong. The situation will be improved when we re-derive the profiles using Mars. However, observers should be very circumspect in picking out unknown "features" in their spectra: these should be checked against the published filter profiles (in the LCGR file on the CD ROMs).

3.2 Features in the Spectrum of Uranus

We have identified lines and features in the spectrum of NGC 7027 arising from Uranus. The 112 micron HD line emission line in 7027 has exactly the same equivalent width as the same line in absorption in Uranus. Similarly, the 56 micron HD line has an equivalent width of 3.4 millimicrons in NGC 7027 compared with 2.8 millimicrons in eta Car, which is 100 times brighter. We suggest that only HD lines with equivalent widths very much greater than 3 millimicrons should be accepted as real.

Other features may also be present. Again, the situation should improve as soon as Mars is available.

Peter Clegg/LWS
and
ISO Science Operations

09/04/97