5. Conversions to Fly on ORFEUS-SPAS

  In the years that followed the decision to discontinue sounding rocket flights for IMAPS and put it on Astro-SPAS, a number of significant modifications and enhancements were needed:

1.

To protect the Shuttle and its crew, payloads must meet some strictly enforced, rigorous requirements for fracture control. IMAPS was originally built to meet the less demanding structural integrity standards for a rocket flight. To satisfy the Shuttle requirements, we mounted IMAPS in a large canister that would be strong enough to contain any pieces of the payload that might break off.

2.

On sounding rocket flights, we evacuated the payload prior to launch. For an orbital mission, there would be plenty of time for the instrument to reach a good vacuum, so we dispensed with the pumping option to simplify ground operations.

3.

A thermal control system had to be added, so that we would not have a loss of focus caused by dimensional changes in the main instrument housing that holds the optical elements. (On sounding rockets, we relied on insulation and thermal inertia, which was adequate for the short flight times.)

4.

On sounding rocket flights, we had the luxury of being able to send the raw detector signals in real time to the ground over a telemetry link with a large bandwidth. The data were captured by a high-speed tape recorder and retained for processing after the flight. For Astro-SPAS, the data storage capacity of the spacecraft recorder was nowhere near enough to hold all of the frames, so they had to be added together in an accumulating memory, as described in §3.5, for subsequent transmission to recorder.

5.

A computer system that could receive and execute commands was built, so that we could have explicit control of the many instrument functions as the mission progressed. (On sounding rocket flights we had no real-time control over the events, all of which were very simple and executed by a series of timed commands arranged before the flight.) Commands sent from the ground would be relayed through the Astro-SPAS data system to our experiment. The computer also assumed the function of data handling. For quick look monitoring, we could have the contents of the accumulating memory (or even individual frames) telemetered to the ground through a data link with a limited bandwidth (taking 39 to 125 seconds to send a picture, depending on how much data compression was implemented).