Dear LWS Observer:
LWS spectra of stars that are bright in the near-IR sometimes contain features
which resemble broad spectral features, but which do not occur at the same
wavelength in detectors which cover the same range. (see section 7.3 of the
LWS ISO Data User's Manual IDUM. The origin of these features is believed
to be a near-infrared leak in the blocking filters located in front of the
detectors. This supposition is supported by a statistical test, which showed a
correlation between these features and the J, H, and K bands. The strength of
these features is best correlated with the strength of the sources in the
H-band (1.6 um). In a small sample of post-main-sequence stars observed in the
Core Programme, all those which had H-band emission brighter than about 2.2
magnitudes (~140 Jy) were seen to exhibit these spurious features.
Follow-up observations of one of the affected sources gave results consistent
with a near-IR leak as the origin of the features. There are no in-flight
changes which can be made to the LWS configuration on order to prevent near-IR
radiation affecting the spectra, so the features must be removed "after the
fact" from the spectra.
The spurious features are now known to be reproducible, in terms of wavelength,
shape and FWHM from one source to another and for repeated observations of the
same source. This means that a template profile can be created which observers
can fit to their data and use to remove the spurious features. A more
sophisticated profile has yet to be derived, but to first order the profile is
Gaussian for each of the three most-affected detectors: SW2, LW1 and LW2. The
short-wavelength wing of a similar feature also affects the longest-wavelengths
of detector SW1's range, while the long-wavelength wing of a spurious feature
affects the shortest wavelengths of detector SW3's range. The best estimates of
central wavelength and FWHM of the gaussians are tabulated below.
Detector Peak WL FWHM
(um) (um)
SW2 53.60 1.86
LW1 105.08 4.30
LW2 109.39 3.91
SW1 >51 ~2?
SW3 <57 ~2?
Using these parameters, it has proved possible to effectively remove the
spurious features and to detect narrow emission lines that were undetectable
before removal of the spurious features.
Based on the small number of affected sources in the Core Programme, we can
offer the following estimates:
1)The spurious features become noticeable when [H] is brighter than 2.2
magnitudes, and the ratio of flux densities F(1.6um)/F(60um) > approx 4.
2)A zero-magnitude source at H (roughly F(1.6)=1050Jy) gives a feature
with peak flux 1.5e-18 W/m2/um in detector SW2.
Studies of the properties and techniques for removal of the spurious features
have now advanced sufficiently that we recommend observers to unblock
their observations of sources that are bright in the near-IR.
Best regards,
ISO Science Operations Team and
LWS Instrument Team
(Roger Sylvester and Mike Barlow)