Advanced Concepts Studies

The 4 m Aperture "Hi Z" Telescope

The High-Z mission was proposed as an Advanced Mission Concept in 1994 and is motivated by the scientific goal of observing the initial epoch of star and galaxy formation in the early universe, possibly as early as z=10-15. Although such early formation is not favored by the original cold-dark-matter/inflation theories for cosmology, the most recent HST deep fields and the Keck "K" band images suggest an older, more complex universe, one which is at variance with these theories. A secondary, but important, science goal was the detection and observation of approximately 20 Type 1a supernovae per redshift interval (z=1-5) or approximately 3-10 per month of the nominal 3-year mission. These would provide an independent measure of the cosmological geometry and deceleration parameters. Finally, High-Z would be a powerful general-purpose observatory for a variety of scientific problems best studied at near- and mid-infrared wavelengths.

To observe distant galaxies and SNe at these redshifts requires sensitivities greater than SIRTF and qualitatively better angular resolution, since the sky is mottled with foreground galaxies at these faintness levels. For these reasons, a passively-cooled 4m facility Fig. 15 is proposed which would be placed in an elliptical `trans-asteroid' orbit taking it to 3+ AU to reduce IR background due to the scattering of sunlight by dust in the dust of the Solar System. For the passive cooling concepts, the work of Hawarden and Thronson for the Edison mission was utilized to a maximum extent. Because the mission goals concentrated on distant galaxies and SNe whose light would lie between 0.8-7 microns, the thermal requirements for High-Z were modest (T<70 K) compared to those for Edison and seem easily achievable at a distance of 3 AU, if parasitic heating of the optics are minimized.

A thin meniscus, 4-m fused silica mirror (f/1.67) similar to the ALOT design is the choice for the primary optic. An on-axis, three-mirror system would provide a real pupil on a fast beam-steering mirror to achieve the pointing accuracy, Figure 16. ALOT-style primary actuators would provide slow, active control of the wavefront. The two major instruments are a wide-field, multiband imager (4-simultaneous bands) and a moderate-resolution grism spectrograph. The large array detectors, InSb and Si:Ga/Si:As are cooled by mechanical closed-cycle systems. Sufficient science 1bandwidth (1 MHz after x4 compression @ a 10% duty cycle) would require a 8m deployed antenna, 50 watts, and at least 2 hrs/day of DSN. Two key enabling technologies are identified: ultra-low noise IR detectors to provide zodiacal background-limited performance; and low temperature, low-power actuators for the primary mirror.

Key Features: 4-m meniscus active mirror, passive cooling (<70K), 1-3 AU solar orbit, wide-field simultaneous imaging from 0.8-7.0 microns, Titan IV or Ariane 5 launch.

The nature of such an observatory and the zodiacal background suggests that a 30-40K optical temperature should be the goal and provision should be made to reach 30-40 microns. There is a powerful amount of science in this regime (brown dwarfs, Kuiper belt objects, and cool dust and gas) and it will not be encroached upon by foreseeable sub-millimeter arrays.