An instrument for measuring the polarization of stellar X-ray emission has been designed, constructed, and tested. The polarization dependence of incoherent Thomson scattering in lithium metal is utilized. Means for suppression of cosmic ray background effects have been provided. The apparatus has been flown in rockets to obtain data in the polarization of X-ray sources in Sco X-1 and Tau X-1.

Following the successful flights of large area survey experiments on three unstabilised Skylark rockets in 1967, 8 and 9, the Leicester Group has now changed the emphasis of its Cosmic X-ray Astronomy programme to the detailed observation of specific sources using attitude-controlled vehicles. Two new Skylark experiments for flights in the current year are described and some details are also given of an X-ray polarimeter/spectrometer being prepared for launch in 1973 on the UK-5 satellite.

We describe a detection unit for X-rays in the energy range 30-200 keV of high directivity (2° FWHM). This directivity is obtained by the use of a honeycomb collimator and completed by an efficient passive shielding. The angular resolution of the detector necessitates the use of a solar sensor so as to track the sources observed during the balloon flight and to localize them with an accuracy of 3.8 arc min. This experiment was launched with balloon at Aire-sur-L'Adour and Gap (France) and the results obtained must permit the flux limit detected to be reduced.

The performances of this detector are analysed as well as their consequences in the study of galactic discrete sources.

At the Institute of Applied Physics TNO-TH at Delft a strongly off-plane grating mounting has been developed for the soft X-ray wavelength region. In this mounting a parallel beam is incident on the grating surface nearly parallel to the grooves. We found herewith a remarkable improvement of the grating efficiency compared with the grazing incidence perpendicular to the grooves. The focal distance is nearly independent on the wavelength, so scanning of the spectrum can be provided by a simple rotation of the cylindrical grating around the central groove. The exit slit and the detector have fixed positions. The lightspot on the detector cathode does not move and has a moderate size; a standard detector can be used.

A rocket payload is being prepared for the purpose of examining the spatial distribution of line emission from two important ions, Ovii and Neix, in the solar corona. The payload will contain the following integrated set of instruments.

(1) A pair of X-ray spectrometers utilizing KAP crystals of approximately 100 cm2 area.

(2) An optical aspect camera with a 1 Å bandpass H-α filter to measure the location of the field of view of the X-ray systems on the sun through out the rocket flight.

(3) A collimated proportional counter spectrometer operating in the 3 to 15 keV range.

The characteristics of an X-ray detection system based on an array of parabolic reflectors that provide a line-shaped focus will be described. The most recent rocket instrument has an overall length of about two meters and utilizes a pair of thin-window gas-flow proportional counters for detecting the X-rays. The variation of effective aperture with X-ray energy, the amount of scattering from the mirror surfaces, and the quality of the optical focus will be discussed. Estimates of the sensitivity of a larger set of X-ray optics used as a survey system, and when combined with a special grating to form a spectrograph, will be presented.

A rocket payload utilizing focusing optics has been designed and constructed for the detection and localization of cosmic X-ray sources. The instrument is designed to be used in a scanning mode. The sensitivity of the instrument covers the wavelength bands 80 Å to 44 Å and 20 Å to 10 Å with a total field of view of 2° × 8°. A telescope operating on the principle of grazing incidence reflection instantaneously focuses the X-rays from a distant source in one dimension upon one member of four proportional counter elements located in the focal plane. Location precision from a single scan is about 0.1° × 1° for the stronger sources. The telescope consists of eight nested sheets of re-enforced glass whose reflecting surfaces are overcoated with an evaporated layer of chromium to improve reflection properties at the shorter wavelengths. The effective collecting area of the mirror assembly for on-axis radiation is about 160 cm2 at 80 Å, and 40 cm2 at 10 Å. The focal plane detector is a multiwire proportional counter with a one micron polypropylene entrance window and contains a supply of propane gas regulated to a constant pressure of 50 cm. In the wavelength range 80 Å–10 Å the focusing technique results in (1) greater sensitivity, (2) better angular resolution, and (3) improved detector reliability compared to non-focusing techniques. (Published, Nuclear Instruments and Methods91 (1971) 451.)

A balloon-borne X-ray instrument with the modulation collimator was designed and constructed to study the structure of solar X-ray flares. The angular resolution was approximately one arc minute. The instrument was used on an occasion of X-ray flare on Sept. 27, 1970.

In this paper we describe an experiment designed to measure solar and celestial X-rays in the energy range between 15 to 250 keV. The experiment was flown on the fifth Orbiting Solar Observatory which was launched on 22 January 1969. Up to the time of this writing the instrument continues to operate satisfactorily.

Glancing incidence telescopes of the kind first described by Wolter have now been physically realized, so that it is now possible to obtain high resolution images of celestial objects at all wavelengths greater than about 3 Å. In this paper we shall describe two such instruments: the GSFC-MSFC X-ray telescope for the Apollo telescope mount uses Wolter type 1 optics and is capable of forming images of the sun in the 8–70 Å region with spatial resolution of the order of one arc second. The GSFC extreme ultraviolet spectroheliometer for OSO H uses type 2 optics and can obtain images of the Sun in spectral lines in the 170–400 Å region with a spatial resolution of about ten arc seconds. Theoretical (ray trace) and laboratory data on these systems will be presented.

As it is well known one can obtain images of the Sun in the soft X-ray region and far UV region by use of zone plates. The main advantage of such imaging systems is the good resolution over a large field. There are however some disadvantages by using zone plates as imaging systems. These disadvantages are the wavelength dependence of the focal length and the zero order halo which diminishes the contrast of the image.

A Fresnel zone plate (sometimes called a Fresnel-Soret zone plate) is an image formation device consisting of a large number of concentric circular rings which are alternatively transparent and opaque for the radiation concerned.

In order to observe celestial X-ray sources an instrument is under development to be launched in the Astronomical Netherlands Satellite (ANS) in 1974. The aim of the experiment is to measure the spectral distribution of the sources. If there is some evidence that a source should be a pulsar the instrument can be switched from the normal mode to the pulsar mode. In this latter mode the detected photons are labeled in time using an on board clock. The ANS will have a pointing mode and a slow scanning mode. The maximum observing time per orbit for one object will be about 2000 s.

Multi-anode gas proportional counters, when used in cosmic X-ray research, offer several advantages over more conventional large area detectors. Some of these are:

1. 1. Low detector background, effected by excluding from the sensitive volume, areas in the proximity of the counter walls, and by requiring that the region of interaction is localized inside the counter.

2. 2. Uniform response which makes possible energy resolution limited primarily by the statistics on the number of electron-ion pairs in the interaction, rather than by partial charge collection due to weak field regions inside the counter.

3. 3. Information relating to the interaction depth within the gas, which may be used in applying consistency checks regarding the nature of the incident radiation.

Methods have been devised for the modular construction of such counters, with particular emphasis on such items as, large areas, ease of assembly, matching of the many counter modules, accurate collimation, window supports, and shape integrity under differing amounts of differential pressure. Detector background at rocket altitudes was found to be flat above 2 keV, and less than 10−3 cm−2 s −1keV−1.

The silicium solid-state detector has been used as control-detector in the X-ray photometer system on board of the satellite Interkosmos 1.

The effectiveness of the use of such detector as control-unit for radiation-belts has been confirmed also as independent indicator of high energy X-ray quants during high levels emission of solar active regions.

In this survey I shall discuss only the results of studies which were obtained with instruments of moderate spectral and spatial resolution. The use of such instruments allows for the study of extremely weak fluxes in the UV region which are unaccessible to diffraction spectrometers of high resolution. In addition a broad field of view dispenses of strict attitude control system. Apparently, investigations of this kind are to be related to a study of sky background, including stellar and non-stellar components, and also to observations of planetary atmospheres from spacecraft in a near fly-by (Earth, Venus, Mars). Results obtained so far are impressive. It would be sufficient to say that due to UV observations the upper atmosphere of Mars has been studied for the moment with the same precision as that of Earth about 15 years ago.

Following a brief historical review, new observations of the sun in the wavelength range 3000 to 20 Å are surveyed for the period since about 1958. Vehicles employed have been sounding rockets, the OSO (Orbiting Solar Observatories), balloons for the window 2300–1900 Å and for λ > 2700 Å, and small orbiting observatories such as Solrad, for XUV solar monitoring. Advances have been made in spectral resolution, using echelle gratings and also Fabry-Pérot interferometers. Much progress has been made towards increased spatial resolution, to obtain spectra of specific solar features and to analyse the chromosphere and corona. Methods employed include spectrographs that are stigmatic, or that have a stabilized solar image projected onto the slit; slitless objective-type spectrographs; and observations during a total eclipse. Spectra have been obtained of a solar flare, showing its form and intensity in the emission lines between 171 and 630 Å. From OSO 4–6 many XUV spectroheliograms and spectra have been obtained over the range 300 to 1350 Å, with spatial resolution 35 × 35 arc sec in OSO 6. Photographic XUV spectroheliograms have provided solar images with spatial resolution as great as 3 arc sec in some cases. Although much effort has been spent to increase the accuracy of XUV intensity measurements, a great deal remains to be done before the requirements of solar physics theory are satisfied. Line identification, however, is proceeding well, although more laboratory spectroscopy is needed. Not included in the survey is the Bragg Spectrometer X-ray range below about 20 Å.

A grating monochromator has been flown from Aire sur l'Adour (Landes) on May 10, 1968, April 19, 1969 and October 3, 1969. The instrumentation and the experimental procedure have been described previously (Ackerman et al., 1968) as well as the results of the first flight. For the two subsequent flights the wavelength marking was improved by increasing the number of the telemetered wavelength marking pulses. For the last flight the FM-FM analog telemetry system was replaced by a PCM telemetry system in such a way that the data analysis was completely made by means of a computer. Float altitudes were respectively 34, 38 and 37 km. From 1850 Å to 3000 Å up to 90 spectra per flight of the solar radiation penetrating into the atmosphere were telemetered to the ground. The solar zenith distance was ranging from 30° to 60°. The equivalent slit width was equal to 19 Å in the case of the first experiment and was equal to 14.6 Å for the two subsequent flights.

The echelle diffraction grating can be used to provide high spectral resolution in compact instrumentation which is very suitable for space astronomy applications. Spectrographs in which the high dispersion of the echelle is crossed with that of a concave grating have been flown in ‘Skylark’ sounding rockets to record the solar spectrum in the wavelength range 1200 Å–2200 Å, and further instrumentation is in preparation for observation of stellar spectra. Efficiency measurements have been made on some echelle replicas used in the instruments, and are correlated with surface conditions on the reflecting facets of the rulings. Results are also presented to demonstrate a spectral resolving power of λ/Δλ ≈ 105 over the wavelength range.

For orbiting astronomical telescopes and for spectroscopic studies from rocket and balloon-borne platforms the great angular dispersion of the Fabry-Pérot interferometer should permit easier guidance tolerance for a given spectral resolving power with the added profit of the physical compactness of an etalon spectrometer or spectrograph. In addition, the superiority in luminosity and illumination of the interferometer permits shorter exposures and greater time resolution.