2. Brief History of IMAPS

  The origins of IMAPS can be traced to a confluence of scientific, technical and programmatic incentives that materialized in 1978. On the scientific side, it was clear that Copernicus was in its waning years, and the limitations in its wavelength resolution were a source of frustration in solving certain scientific problems. In the technical arena, devices that could record images in the ultraviolet were coming of age. It was clear that enormous multiplexing advantages offered by these devices could allow an information gathering power that far outstripped the older technology of scanning photomulitpliers, i.e., those primitive devices that had to sample the spectrum one element at a time. On top of this, Princeton University was eager to develop a detector of the type employed by IMAPS and prove that it could function effectively in a space environment. Finally, a programmatic impetus came from an Announcement of Opportunity (AO) issued by NASA to develop astronomical instruments to fly as attached payloads on the Shuttle. We responded to the AO with a proposal showing an instrument design very similar to actual IMAPS one, but, fortunately, we were not selected because the reviewers judged the detector development to be too risky. Soon thereafter, we proposed to develop the instrument for flights on sounding rockets, and this program was accepted by NASA.

The detailed design and fabrication of IMAPS was carried out by the Ball Aerospace Systems Division in Boulder, Colorado. End-to-end optical testing of IMAPS was performed at the laboratory of G. R. Carruthers at the US Naval Research Laboratory. The first flight on a sounding rocket in October 1984 was not successful because an inrush of air from the parachute and instrumentation section subjected the internal volume of IMAPS to an initial pressure of 10^-3 torr, just at the time our high voltage potential was applied. By the time the pressure decreased to a point that the high voltage discharge had ceased, it was too late to gather data.

The second flight of IMAPS in April 1985 was a resounding success: a spectrum of $\pi$ Sco was obtained at R= 150,000 and a peak S/N = 25. On the basis of data from this flight, conclusions were derived about the properties of H₂ in different stages of rotational excitation (Jenkins et al. 1989), the depletions of various elements within interstellar clouds of differing radial velocity (Joseph & Jenkins 1991), and the extremely clumpy nature of the ionized gases near the star (Bertoldi & Jenkins 1992). The third and final flight of IMAPS on a sounding rocket occurred in September 1988, where a spectrum of $\epsilon$ Per was obtained at a slightly higher resolution but with incomplete wavelength coverage.

On the technical side, a major accomplishment of the sounding rocket flights was proving that the IMAPS detector could perform in the harsh environment of a suborbital mission, one that is worse than the usual orbital environment because of the low altitude (near the top of the ionosphere) and the short time for dissipating the residual gases associated with the rocket systems. Skeptics had predicted that, on account of the high voltages that were employed for the detector, it would be plagued by discharges or a flurry of events caused by ions that either entered from the outside or were created within the image section. Their assertions proved to be wrong. In fact, the IMAPS detector can withstand a vacuum as bad as 4 × 10 ^-4 torr, more than 10 times the safe limit for detectors that use microchannel plates.

After demonstrating that IMAPS could work on a sounding rocket and gather scientifically useful data, we were eager to have the instrument working in an orbital environment, a circumstance where substantially more than 5 minutes observing time would be at hand. In 1985, IMAPS was accepted as a candidate for an initial round of Spartan missions, but soon after the Challenger disaster in 1986 NASA abandoned further development of the Spartan program. In early 1989 it appeared both technically and programmatically feasible to integrate IMAPS with an already planned flight of Astro-SPAS carrying another far uv experiment called ORFEUS (Grewing et al. 1991). This program was supported as a collaboration between the US and German space agencies, NASA and DARA.