On April 8, 1998, at 23:07z, Helium depletion was announced when the ISO
instrument temperatures rose above 4.2K. A period of testing the satellite
control systems, the Technology Test Phase (TTP), started on April, 20
and will be completed in mid-May. As mentioned in a previous note, any vacant timeslots in the TTP schedule will be
filled with SWS band 1 operations.
In the period prior to the start of this phase, the SWS Instrument Dedicated
Team at VILSPA conducted a series of tests to verify the proper functioning of
the instrument. During these days the temperatures were found to increase at a
very slow rate (about 4 K per day) and with some adjustement to the instrument
settings, both Detector Bands 1 and 2 were seen to work properly. The previously
planned science observations making use of these bands were then executed
normally. On Sunday April 12, Band 2 ceased to function.
The stellar classification programme started on Monday April 13. At that time,
the instrument temperature sensors were out of range (T > 20 K), and the upper
baffle temperature was around 25K. About 20 band 1 observations were scheduled
and the data quality looked very good. As compared to nominal mission conditions
detector dark current and dark noise had not significantly changed, while the
responsivity had increased. On the second day, responsvity had increased by 25%
to 45% across the detector 12-element array. The wavelength calibration had been
slightly drifting, by up to 1 scanner step per day, depending on wavelength.
High quality data was confirmed by spot checks of a few science observations.
As an example, spectra of three carbon stars of the same spectral type were
found to be practically identical. As these observations successfully met all
previously set pass-fail criteria the go-ahead was given to continue with the
SWS observations up to, and throughout, the Technology Test Phase.
As of today, a preliminary analysis has been carried out on calibration data up to April 19. At that time, the telescope temperature sensors were close to the limit of their calibrated range, around 40K. Detector dark current had stabilised to values a few percent different to nominal mission conditions, dark noise had remained unchanged, and detector signals to the internal calibrator had steadily increased to a level 50...100% of their nominal values. The spectrum of the calibrator Alpha Lyra showed a change in the flux calibration, not, however, of the same amount as expected from the internal source measurement. This may be explained by either a drift in the internal source emitted power or by telescope defocussing. The wavelength calibration shift was seen to continue in the same direction and gradient. These trends will be fully characterised by thorough analysis of the various calibrations, coupled to a consistent processing of the complete, well defined set of observations (stellar spectra, all performed with the same observation strategy). BR>
For more information, please contact:
Rens Waters rensw@astro.uva.nl