1. Introduction

This document describes operating requirements and procedures for the WISCONSIN ULTRAVIOLET PHOTO-POLARIMETER EXPERIMENT (WUPPE), an instrument which will be flown on the Space Shuttle as a portion of the Astro missions. Two launches of Astro with a duration of about 10 days each are anticipated. The launch date of the first Astro mission, Astro-1, is presently scheduled for spring of 1990.

WUPPE is one of 3 major instruments which are mounted on the Instrument Pointing System (IPS). The other instruments are the Hopkins Ultraviolet Telescope (HUT) and the Ultraviolet Imaging Telescope (UIT). Because all three experiments operate in the UV and because their observing goals are complementary, they are co-aligned on a common structure, called the cruciform, that is attached to the Instrument Pointing System (IPS).

Figure 1.0-1 : The UV instruments are mounted on the cruciform which is connected to the IPS.

The Broad-Band X-Ray Telescope (BBXRT) has been added as a second payload. The BBXRT is mounted independently of the three UV telescopes on the Two-Axis Pointing System (TAPS) in a support structure placed just behind the UV telescopes in the Shuttle payload bay.

Since the Astro-Observatory instruments and components are reusable, they can be flown again together, separately, or with other astronomical instruments. The observing goals are defined by the Astro Investigators Working Group (AIWG). Although the X-ray and the UV telescopes have independent pointing systems, they share common resources, such as the orientation of the Shuttle. The observing time is therefore nominally divided evenly among the four instruments. Each instrument will have its share of primary pointings and will do secondary pointings that support joint science goals. The first mission will naturally emphasize targets that are of interest in both the X-ray and the UV spectral ranges.

While the first mission will be dedicated to targets specified by the AIWG, the second mission will call for guest-observer programs that will be implemented by the AIWG.

The purpose of WUPPE is to obtain spectropolarimetric and spectrophotometric observations of astronomical objects brighter than magnitude 16 from 1400 to 3300Å WUPPE consists of a 0.5-m telescope, grating spectrometer, polarization analyzer, intensified photodiode-array detectors and associated electronics.

WUPPE is a classical Cassegrain telescope with an aperture of 0.5-meters and an effective focal ratio of f/10. This telescope feeds light to a low-resolution spectrometer which is equipped with polarimetric analyzers followed by a beam splitter. The two senses of polarization are measured simultaneously by an intensified, analog-readout dual-Reticon detector of well proven design which gives it an exceptionally high effective dynamic range.

WUPPE's Lyot polarimetric mode allows it to measure all four Stokes parameters simultaneously in roughly 40Å band widths. Since fields of view up to 300 square arcseconds are possible, the instrument is particularly sensitive for surface-brightness measurements, i.e. extended objects. Polarization can also be measured with a spectral resolution of about 6Å by the insertion of a sequence of waveplates.

All three of the telescopes of Astro are to be co-aligned to a precision of about 2 arc-minutes. WUPPE has the ability to off-set from the other two instruments by as much as 15 arc-minutes by virtue of its articulated secondary mirror. The zero order of the WUPPE spectrometer is diverted to an intensified CCD camera which makes it possible for the experiment team to acquire and guide upon faint targets and targets in crowded fields. In order to operate and coordinate all of these diverse and complex functions, WUPPE is provided with a highly flexible and sophisticated monitoring, command and control system.

WUPPE interfaces to the Shuttle through the Spacelab avionics. A drawing of the WUPPE telescope and major components is shown in Figure 2.1-1. It is mounted on the IPS which is used to acquire and track targets of interest. Target fields are displayed on the Shuttle Closed Circuit Television (CCTV) system and the Payload Specialist (PS) plays an active role in acquiring the targets. Spectral data and video images are sent to the ground and displayed on the Experiment Ground Support Equipment (EGSE) at the Payload Operations Control Center (POCC) in real time for evaluation by the experimenters. Although normally the PS will control WUPPE, instrumental parameters may also be changed by the experimenters via an uplink from the POCC, which is located at the Marshall Space Flight Center (MSFC) in Huntsville, Alabama.

Observations obtained with WUPPE will provide new polarimetric diagnostic tools in a region of the spectrum which is unexplored polarimetrically. It is expected that polarization effects from scattering, absorption, and non-thermal emission are far more easily measured in the UV than at visible wavelengths.

The major classes of observations are:

i) The UV-wavelength dependence of interstellar polarization, to investigate the shape and composition of the UV-absorbing interstellar grains.

ii) Polarization by electron scattering to probe the geometry of Be stars, supergiants, and close binary systems.

iii) Polarization by dust scattering in solar-system objects, stellar envelopes and diffuse reflection nebulae.

iv) The UV circular polarization wavelength dependence of the magnetic white dwarfs.

v) The geometry and composition of the scattering envelopes around Seyfert-galaxy nuclei.

vi) The relationship of non-thermal and scattering polarization processes in normal and variable QSOs.

vii) Spectroscopic observations of objects not in these classes to support cooperative observations with the HUT, UIT and BBXRT experiments.