Astrophysics Division, Space Science Department of ESA, ESTEC, 2200 AG Noordwijk, The Netherlands
The present ESA/NASA memorandum of understanding on HST dates from 1977 and is due to expire 11 years after the original launch, i.e., on 24 April 2001. However, present NASA planning assumes that HST will be kept operational well beyond this date, and an additional four HST servicing missions are foreseen for 1997, 1999, 2002 and 2005.
During preparatory discussions between ESA and NASA on extending the collaboration on HST, the possibility of ESA providing a new scientific instrument for installation during the fourth HST servicing mission in 2002 was broached. To explore this option further, an internal ESA/NASA working group was appointed at the management and project level and charged with the task of drawing up a detailed proposal for a possible second ESA instrument to succeed the Faint Object Camera.
In order to solicit the advice of the astronomical community and assist in identifying the scientifically most worthwhile instrument concepts, a second more scientifically oriented study group consisting of a number of representative HST users was set up. This so-called ` ad hoc Study Group' is composed of F. Paresce (ESO, Chair); J. M. Deharveng (Marseille); M. Franx (Groningen); R. Kudritzki (Munich); P. Stockman (STScI) and M. Ward (Oxford), with P. Benvenuti and R. Fosbury (ESA/ST-ECF, Garching); P. Jakobsen (ESA/ESTEC, Secretary) and D. Leckrone (NASA/GSFC) as `ex officio' members.
Over the summer of 1995, the ad hoc Study Group carried out a broad survey of present and future HST capabilities, and focused on the general theme of two-dimensional panoramic spectroscopy as the area that holds the most scientific promise and augments naturally present and planned future scientific capabilities of HST. Two candidate instruments in this area that promise to bring novel---albeit rather different---capabilities to HST have been outlined. These two instrument concepts are presently undergoing technical feasibility study, with the aim of reaching a final decision concerning ESA's level of participation in the 2002 HST servicing mission in early 1997.
Brief descriptions of the two instrument concepts under consideration are given below. A more detailed interim report on the scientific deliberations of the ad hoc Study Group can be found in the September 1995 issue of the ST-ECF Newsletter.
The more radical of the two instruments under consideration aims at capitalizing on the recent significant advances in detector technology being made within ESA's Astrophysics Division at ESTEC (Peacock et al. 1996). At the heart of the so-called `STJ Camera' concept is a highly advanced detector array consisting of Superconducting Tunneling Junction (STJ) devices. Although still in the early stages of development, these devices promise to ultimately yield the near-ideal astronomical photon-counting detector in which not only the location, but also the energy of each photon is recorded at extremely high efficiency (Perryman, Foden, & Peacock 1993).
The proposed STJ Camera concept envisages a simple COSTAR style two-element corrective re-imaging system feeding an array of STJ devices, perhaps as large as elements. This instrument would constitute the ultimate `three-dimensional' camera capable of yielding the simultaneous UV through near-IR low-resolution () spectrum of each pixel in the image. The STJ Camera would be highly competitive with respect to its ground-based counterpart in that it exploits to the full the classical HST advantages of high spatial resolution, lower sky background and access to the ultraviolet.
A key scientific driver for considering this highly advanced instrument concept is the important goal of measuring the redshifts and spectral energy distributions of the extremely faint remote galaxies that HST is presently uncovering through very deep imaging. In fact, the dramatic gain in sensitivity promised by the STJ Camera's `dispersionless' approach to spectroscopy may well represent the only hope of following up on this important discovery with the present HST.
The second, competing instrument concept being considered is an adaptation an existing instrument, the so-called ``3D'' near-infrared imaging spectrometer developed by the Max Planck for Extraterrestrial Physics in Garching for ground-based use (Genzel et al. 1995, Krabbe et al. 1995, Weitzel et al. 1996). Modified for HST, this instrument would essentially pick up where the NICMOS instrument leaves off in the area of near-IR spectroscopy. The 3D concept employs an opto-mechanical image slicer feeding a conventional grating spectrograph and NICMOS-derived solid state detector to obtain simultaneous spectra of a grid of spatial pixels on the sky. The HST version of this concept would operate in the wavelength range 0.9--2.3 microns. The spectral resolution would be selectable between a low resolution () `spectrophotometric' mode and a high resolution () `kinematic' mode.
The key scientific drivers behind the H3D proposal spring principally from kinematic and emission line studies of AGNs, galaxy cores, young stellar objects, and planetary astronomy. The main advantages of H3D over its ground-based progenitor are sharp and stable PSF and avoidance of atmospheric OH band emission, giving higher spatial resolution and complete and unhindered access to the full near-IR spectrum.
Genzel, R., Weitzel, L., Tacconi-Garman, L. E., Blietz, M., Cameron, M., Krabbe, A., Lutz, D., & Sternberg, A. 1995, ApJ, 444, 129
Krabbe, A. et al. 1995, ApJ, 447, L95
Peacock, A., Verhoeve, P., Rando, N., van Dordrecht, A., Erd, C., Perryman, M.A.C., Venn, R., Howlett, J., Goldie, D.J., & Lumley, J. 1996, Nature, in press
Perryman, M.A.C., Foden, C.L., & Peacock, A. 1993, Nuc. Instr. Meth. in Phys. Res., A325, 319
Weitzel, L., Krabbe, A., Kroker, H., Thatte, N., Tacconi-Garman, L. E., Cameron, M., & R. Genzel 1996, A&A, in press