Description of the early AS&E work to image celestial x-ray sources with a grazing incidence Wolter telescope

By Edwin Kellogg

Smithsonian Astrophysical Observatory

Cambridge, Massachusetts

Last updated 2/3/2005 12:07 PM ET

Before the Einstein Observatory (ref) was built and launched, the x-ray astronomy group at American Science & Engineering, Inc. (AS&E) was developing an imaging grazing incidence x-ray telescope payload that could be flown on a sounding rocket to demonstrate the feasibility of such a system for orbiting observatories. The program was proposed to NASA by Riccardo Giacconi and Herbert Gursky. After NASA gave the go-ahead, several scientists at AS&E worked on this instrument, which included a grazing incidence mirror and a microchannel plate detector.

Scientists, in rough chronological order of arrival on the project, approximate duration of participation:

Giuseppe Vaiana, ~1967 – 1969

Ted Zehnpfennig ~1969

Edwin Kellogg, 1968 - 1972

Scott Andrus, 1969 - 1971

Leon Van Speybroeck, ~1970 – 197?

Stephen Murray ~1970 – 197?

The rockets were launched from White Sands Missile Range. The first flight, NASA’s flight number 4.262CG on 06 Feb 1970, yielded no data, because of a leaky vacuum door on the microchannel plate detector. The second flight, 13.26CG on 28 Sep. 1970, suffered from faulty electrical relays, so the detector high voltage didn’t get switched on. The third flight, 13.030CG on 05 Aug. 1972, gave the first detected counts from a celestial x-ray source seen through a Wolter x-ray telescope. Results are shown below. Three counts were recorded from Sco X-1 in about ten seconds of observing time out of a planned ~150 second observation, before the pointing system became unstable and moved the telescope off the target. However, the background counting rate in the detector observed over the entire flight was quite low, which indicated good potential for use of microchannel plates in future x-ray astronomy missions.

A report on the third flight, presented at the summer 1973 meeting of the AAS

(1973 Kellogg, E.; Murray, S.; van Speybroeck, L.; Vaiana, G.; Giacconi, R.; Gursky, H. A High Resolution Imaging Rocket X-Ray Telescope for Celestial Observations.

Bull. Amer. Astron. Soc. Vol. 5, p.342.)

1. Schematic of X-ray rocket telescope payload:

The mirror had been used very successfully by Vaiana and his group (1968ApJ...151..333R, ) to take pictures of the Sun in x-rays earlier. The major technological challenges in using this mirror to observe celestial sources were threefold. First, we needed a detector with much higher quantum efficiency than direct exposure of film, as used for the solar observations. Second, we needed a way to point the telescope to ~ 0.5° at a target that was optically very faint. Third, we needed to know where the telescope axis was located on the sky after the fact so we could determine the celestial coordinates of the recorded x-ray events. The solution to the QE problem was to use a microchannel plate, that we began developing in 1967. For pointing, we used an offset mirror, so the standard NASA star tracker could use a star a few degrees away. For location and reconstruction of the x-ray axis on the sky, we used a cinema camera to photograph the star field, and a fiducial light system optical relay using a cube corner prism to project reference locations from the x-ray focal plane into the star camera focal plane. The micro channel plate detector and the fiducial light system and its relay into the star camera found their way into Einstein and Chandra.

Here’s a somewhat irreverent photo of me sitting on top of the payload at the Vehicle Assembly Building at White Sands Missile Range. The shiny aluminum front section is the payload provided by AS&E. Near the rear of the conical section the separation seam of the nosecone can be seen. This was ejected at high altitude to allow the telescope to view the sky. The ion pump power supply is also visible just behind the tip of the nose cone. This kept the detector an high vacuum and allowed us to test its operation until shortly before launch. The grey cylindrical section behind the payload is the liquid fuel tank of the Aerobee 170, and one of its tail fins is just visible to the rear. The liquid fuel rocket is the second stage. Behind that at launch was a Nike solid fuel booster that give the initial thrust. It went off with a great bang, like a giant firecracker. Then the liquid fuel motor took over and burned for about a minute to reach an altitude of about 100 miles.

2. Point Response function of Rocket Telescope mirror.

It was repolished after being used for solar observations, using the “Superpolish” process. The figure shows the point response function before and after repolishing. It was a rather primitive mirror compared to the excellent Einstein mirror and the superb Chandra mirror developed under Leon’s supervision.

3. Observed Data

Just three counts! We scanned the 35 mm movie camera film from the x-ray detector and the 16(8?) mm film from the aspect camera to locate the position of x-ray flashes spots and star images respectively, using a high power microscope with a precision stage to measure positions. As I recall, the error bars were the position of the x-ray source calculated using two and three counts respectively, showing consistency with the position of the blue star. This result gave us confidence that not only could the microchannel plate detector detect x-rays and the mirror focus them, but we could build an imaging x-ray telescope system that could do accurate source locations.