The IUE scientific instrument consists of a telescope, acquisition camera, two
ultraviolet spectrographs, and four cameras.
Telescope: The telescope is a 45-cm aperture, f/15
telescope of Ritchey-Chretien design. The primary mirror is made of beryllium.
Thermal heaters mounted on the back of the mirror and on the camera
deck are used for focus control.
Acquisition camera: The Fine Error Sensor (FES) is
an image dissector device sensitive to a broad range of visual
wavelengths. It has a 16 arcmin field of view and effective image
resolution of about 8 arcsec. The device is used in camera mode to
obtain a visual star field image for target identification and acquisition.
It is then used in tracking mode for offset guiding, providing a pointing
stability of about 1/2 arcsec.
Apertures: Each spectrograph may be used with either of
two apertures. The large aperture is a slot (approximately 10 by 20 arcsec)
and the small aperture is a circle about 3 arcsec in diameter. The
apertures are known as LWLA (long-wavelength large aperture), LWSA
(long-wavelength small aperture), SWLA (short-wavelength large aperture),
and SWSA (short-wavelength small aperture). The image quality of
the IUE telescope's optics yields a roughly 3 arcsec image, so observations
using the small aperture result in some light loss. The large apertures
are used most frequently since they give photometric reliability with
little or no loss of resolution.
Spectrographs: The long-wavelength spectrograph operates
in a wavelength range of 1850 to 3300 Å. The short-wavelength
spectrograph operates in a wavelength range of 1150 to 2000 Å.
Dispersion modes: Each spectrograph has two dispersion
modes. High resolution employs an echelle grating and cross-disperser,
giving roughly 0.2 Å resolution). Low resolution employs the
cross-disperser grating alone, and yields approximately 6 Å resolution.
Cameras: There are four cameras, two for each spectrograph.
One is designated prime and the other backup. In the long-wavelength
range, both the prime (LWP) and redundant (LWR) cameras were used during
the mission. With the short-wavelength spectrograph, only the prime
camera (SWP) was fully functional. The redundant camera (SWR) was used
to obtain a handful of images early in the mission. Each camera consists
of an SEC Vidicon with and ultraviolet converter. Each exposure is
obtained by integrating over the requested exposure time, then performing
a destructive readout of the camera target. The signal is digitized and
transmitted to the ground via the spacecraft telemetry stream.
Data format: Each IUE image consists of a 768 by 768
pixel array of 8-bit values (0 to 255). The raw data are archived with
a header recording Telescope Operator comments, automatic activity log,
and spacecraft engineering data.