The article summarizes some of the recent advances in the techniques of solar space research, particularly over the last 4 years, in an attempt to review the current state of instrument technology. The present state of development in rocket and satellite vehicles for solar observations, far ultraviolet detectors, optical materials, ultraviolet reflection coatings, filters, and photographic film fogging, are among the topics described.

Ultraviolet astronomy is in a phase of very rapid development and any review of the new techniques involved is necessarily conditioned by the fact that most of the work is in a preparatory stage. This paper, therefore, will not be restricted to a discussion of those techniques which have undergone the ultimate test of their effectiveness, i.e., an actual space mission, but will include a consideration of those techniques which are still under development. Some omissions are therefore inevitable.

The presentation is divided into three categories: (a) the vehicle and its stabilisation (if any), (b) the optics, and (c) detectors. Category (a) presents the major new technological problems and contributes the greater part of the total cost. Experience in the other categories of optics and detectors had, of course, reached an advanced and sophisticated level at the time of the advent of the rocket, but new and difficult problems have been posed mainly by their operation in a space environment, but also by their use in the ultraviolet region of the spectrum.

A review is given of some modern developments in X-ray astronomy. Improved designs of proportional counters and other detectors are described. We further enumerate developments in solar X-ray spectrographs and heliographs, like the ‘gutter’ spectrograph, and the paraboloidal-hyperboloidal frustrum mirror system, the zone plate technique. We also give a review of instruments and collimating techniques for the detection of non-solar celestial X-ray sources.

A few recent observations of interplanetary fields and plasmas are discussed, including the evolution over several years of the longitudinal sector pattern, the corotating filamentary structure that exists within the sectors, guiding of solar cosmic rays along such magnetic filaments, a field-aligned thermal anisotropy of the plasma, and a small component of corotating plasma velocity observed at 1 AU. The solar wind plasma streaming past spacecraft near 1 AU contains a large amount of information about the detailed structure of the Sun.

The Earth’s atmosphere transmits infrared radiation through a number of windows. Table 1 lists the seven photometric systems in use at the University of Arizona which are chosen to fit the windows between 1·0 and 25 microns. An absolute calibration (Johnson, 1965; Low, 1966) has been worked out for each wavelength band and, for reference, we include an estimate of our current limiting magnitudes using a 60-inch telescope. At about 1000 microns, observations from the ground are again possible and both our group and workers in Russia (Fedoseev 1963) and England (Baldock et al., 1965) have succeeded in making observations of celestial sources. Between 25 and 1000 microns a few data have now been obtained from stratospheric altitudes by observers using jet aircraft (Low and Gillespie, 1968) and helium-filled balloons (Hoffman et al., 1967). We can anticipate that activity of this sort will increase greatly in the near future. At present, however, most of what we know concerning the nature of celestial objects at infrared wavelengths was obtained with ground-based instruments.

Modern astronomy includes optical, ultraviolet, infrared and, in recent years, radio, γ-ray, and X-ray astronomy. Such a classification is justified to a certain degree. In fact, the difference in wavelength ranges causes a distinction in the methods and techniques of receiving radiation. Also, the solution of specific problems requires observation in different wavelength regions. In this respect it is possible to describe a new astronomical branch, the submillimeter one.

The submillimeter range is intermediate between the infrared and microwave regions, as shown in Table 1. The boundaries of this region are not very definite. Some authors include in the submillimeter range the wavelengths longer than 50 microns, others, those longer than 100 microns. The long wavelength edge of the range is also diffuse. Formally it is a wavelength of 1 mm, but in some cases the 2-mm or 4-mm wavelengths are also included in the submillimeter range. The measurements of submillimeter receiver performances are sometimes carried out at wavelengths up to 8 mm. The uncertainty of the boundaries is very understandable: their shifting depends on the methods of generation, transmission, and detection of radiation. In this review paper, following Martin’s (1962, 1963) terminology, wavelengths between 50 microns and 2 mm will be attributed to the submillimeter range.

In spite of the title of this paper, it is not easy to point out many significant developments in the techniques of space radio-astronomy in the last few years. Radio-astronomers are limited in their endeavours by several difficulties which they have not yet been able to overcome completely. I still remember F.G. Smith, in his 1965 Liège Symposium talk, saying that “space radioastronomy is in great danger of turning to geophysics”. This statement seems still valid three years later.

However, we might consider that this tendency to deal with geophysical phenomena more than with observations of real astrophysical significance, is after all, not so bad. First of all, it is becoming more and more difficult to define the boundary between geophysics and astrophysics. And second, the Earth environment can be used as a laboratory where a number of processes which are assumed to take place in celestial bodies can be studied in situ. Some types of radio emissions observed by space telescopes are still to be explained and could very well lead to the understanding of mechanisms overlooked by the theoreticians. Radiophysical methods make a very powerful tool to study these phenomena, which occur in a medium that is now being extensively probed by rockets and satellites which send a continuous flow of data to the Earth, about 100000 bits per second, according to Parker.

Two experiments carried out recently at M.I.T. gave results which bear on the problems of extragalactic X-ray sources. One of these is the work of a group under the direction of Hale Bradt (Bradt et al., 1967), who used an attitude-controlled Aerobee rocket to scan a portion of the sky which included the radio galaxy M 87 (Virgo A). Among their detectors were two banks of argon-filled, 2 mil beryllium window proportional counters, each with an effective area of 350 cm2 and a mechanical collimator giving a 2° × 20° FWHM field of view. The fields of view were crossed so that their long directions made an angle of 60° with one another. Aspect was determined to within 5 min of arc by star photography.

An unstabilised Skylark rocket (SL 118) was launched from Woomera, South Australia at 20:20 local time on April 10, 1967. The rocket carried a large-area proportional counter to perform a high-sensitivity survey for cosmic X-ray sources in the Southern sky. The flight was successful and during the period of observation with the rocket above the absorbing atmosphere, from 80 to 350 sec after launch, a total of 50 scans across the sky were obtained. In addition to the predominant roll motion of the rocket, a slow precession about a flat (70° half angle) cone provided at least two separate looks at every part of the celestial hemisphere. At the time of the flight, the Milky Way was almost perpendicular to the horizon and a region around the Galactic equator from Scorpius to Taurus via Centaurus, Carina, Puppis and Orion was visible.

Since the results of early surveys revealed the existence of cosmic X-ray sources (Giacconi and Gursky, 1965), the major effort of X-ray astronomy has been devoted to the investigation of their nature. With the exception of the X-ray source in the Crab Nebula (Bowyer et al., 1964), an object well known prior to the discovery of its X-ray emission, the galactic X-ray sources which have been observed do not appear to coincide with conspicuous visible or radio objects.

The attempt to study their nature has proceeded along two main directions:

(a) Accurate determinations of celestial coordinates for the X-ray emitters permit the identification of their visible and radio counterparts. When a likely candidate is discovered, the techniques of optical astronomy yield information regarding the spectrum, size, distance, temporal variations and polarization of the radiation emitted by the object.

(b) The properties of the X-ray emitter are investigated by careful measurement of the angular size of the X-ray emitting region, its detailed spectrum and its polarization and variability.

In this paper we wish to present briefly the latest results which have been obtained on the hard X-ray spectra of two strong sources in the Northern skies. These observations, which have been discussed in detail previously (Peterson et al., 1967), were made from balloons launched at Palestine, Texas, to 3 gm/cm2 atmospheric depth during September 1966. The Crab Nebula and the Cygnus XR-1 were observed to have a differential number power law spectra with an index of about –2 over the 20–200 keV range. Both sources have the same intensity within about 10%. The Crab Nebula has been observed on two occasions, one year apart, and showed no change in intensity over this range at about a 5% significance level.

The six or eight optically identified X-ray sources comprise starlike objects and extended supernova remnants in the Galaxy, well as as a radio galaxy and a quasar. Both X-ray and radiofrequency radiation penetrate the entire galactic plane, but only two or three galactic radio sources have been identified with X-ray sources. This has led Hayakawa et al. to postulate that detectable X-ray sources are not farther than 1 kpc. However, other studies suggest that there is a cluster of a few intrinsically bright sources actually near the galactic nucleus and a scattering of weaker sources near the sun.

The distances of X-ray sources can be estimated from extinction by interstellar gas or intergalactic gas on spectra above 10 Å, but the method ultimately depends on the radio and optical data of the gas. Conversely, interstellar densities of certain elements with large photo-ionization cross-sections may be determined from the absorption of X-rays, after calibration of source distances by the methods of optical astronomy.