In order to obtain the solar spectrum of the Mgii lines at 2795.5 Å and 2802.7 Å, we built a balloon instrument which consisted of a Cassegrain telescope, with an aperture ratio of F/20 and a focal length of 300 cm to give a solar image on the slit of a spectrograph. The theoretical spectral resolution is better than 10 mÅ. In the laboratory, we measured a spatial resolution of 10 μ in the image plane of the spectrograph, which corresponds to 1 arc sec in this mounting. During a balloon flight of 1969, April 30, which used a biaxial pointing system, we obtained a spatial resolution better than 10 arc sec, limited by the accuracy of the pointing system. Lastly, on 1970 June 24, the same instrument with a servosystem inside the telescope, was launched and spectral and spatial resolutions of 25 mÅ and about 3 arc sec, respectively, were obtained.

This paper gives a general review of the best ways to adapt the UV optical designs to the peculiar conditions of guidance of space vehicles.

1. (1) Conditions to obtain useful photometric direct images of the sky (stars and nebulae) with several different receptors (photographs, image tube, TV camera etc., …). New solutions of imagery are considered.

2. (2) Selection of the different bandwidths of wavelengths (filters; gratings).

An example of a new optical design, the BPM filter especially adapted to the poor conditions of guidance, is described. (Wave length selection is independent of the guidance.)

The last part of this article is devoted to a review of the dispersive optical designs adapted to space research.

Conclusions are in favour of the average size space vehicles on rockets for the time being and for large telescopes in the space platforms even if their guidance is several times worse than their optical quality.

The spectral resolution of the prism or grating spectrograph depends on its angular dispersion which, in a space environment, implies that resolution also depends on the accuracy of pointing of the space platform.

A description is given of a rocket payload designed to record high resolution ultraviolet stellar spectra using the star-stabilised ‘Skylark’ rocket vehicle. The system consists of a 35 cm aperture f/12 Cassegrain telescope and a spectrometer in which the high spectral dispersion of an echelle grating is crossed with that of a plane grating to separate the echelle orders. The spectrum is focussed onto an electrostatic image intensifier tube, the fibre optic faceplate of which is coated with sodium salicylate to give ultraviolet sensitivity. Photographic film is used to record the two dimensional spectrum array with a resolving power λ/dλ ≃ 104 between 1200 Å and 3000 Å. Efficiency measurements of components indicate that the photometric efficiency of the entire system will allow spectra to be recorded for early-type stars which are brighter than mr ≃ 5.

A brief survey is presented and ground based observations are reported aiming at the design of simple spectroheliographs for space research, particularly in the range 1.500–3.000 Å.

The reflectance of aluminium films has been determined with films free of contaminating layers (Feuerbacher and Steinmann, 1969.) It has been shown that the reflectance of smooth aluminium films is 91% at the Lyα line. The influence of the surface plasmon excitation has also been investigated. This shows up as a dip in the reflectance if the film is not perfectly smooth. It imposes stringent requirements on the polishing of mirror substrates if optimum reflectance is required in the 1200–1700 Å region, since the influence of the surface plasmon is caused by surface roughness of lateral dimensions less than 1000 Å.

In the near ultraviolet part of the spectrum filters with narrow bandpasses (smaller than a few Ångströms) are difficult to manufacture. Ordinary birefringent filters suffer from low transmission because of the great number of polarizers usually needed. From this point of view the Šolc-type birefringent filter offers a solution since it only employs two polarizers. With a suitable arrangement of the angles of the optic axes of the birefringent plates adjacent secondary maxima can be suppressed to any desired value.

It is shown that by modulating the distribution of the angles by a cosine function a normal transmission peak may be split into two peaks. The distance of the two peaks depends on the frequency of the modulation function. In this manner the filter can easily be tuned over half the free spectral range without appreciable change of the transmission profile. The theoretical results have been experimentally verified.

We have constructed solid Fabry-Pérot narrow-band filters that can be used in systems having one arc second or better resolution. Our filters operate at Hα, have a three-inch aperture, and typical transmission of 70%. However, the same technology can be applied to construction of filters as narrow as 0.05 Å at any wavelength from 4200 to 11000 Å.

The third Orbiting Astronomical Observatory will be launched in the Fall of 1970 carrying a 92 cm telescope and a scanning spectrometer. The instrument can produce stellar spectra in the range from 1000 to 4000 Å at resolutions of 2, 8 or 64 Å. Several entrance apertures can be used, including a 5′ × 6′ slot, a 1′ circle and two narrow slits for special conditions. The spectrometer employs six pulse counting detectors spaced at 500 Å intervals, and can also be used as a fixed wavelength six channel photometer for faint objects. The detection and data handling systems are designed to yield photometric data at a constant statistical precision of 1.6% with a maximum integration time of 42 s at each wavelength.

Princeton's 80 cm diameter telescope with an ultraviolet spectrometer will be the prime experiment on board NASA's third Orbiting Astronomical Observatory (OAO-C) which is scheduled for launch in late 1971. Two scanning carriage arms within the spectrometer will each have one photomultiplier sensitive to a 1600–3200 Å first order spectrum and another photomultiplier which will register from 800 to 1600 Å in the second order. Since the primary purpose of the instrument will be to study interstellar absorption lines, the design philosophy has emphasized the role of carefully examining the detail of specific lines, rather than surveying a star's entire spectrum. Thus the carriage with the narrowest exit slits, providing 0.1 Å resolution in first order and 0.05 Å in second order, has been programmed to repetitively scan and retrace over any selected 0.7 Å interval in first order (or 0.35 Å in second order). Stepping motion in one direction every 16 s will occur for the second carriage, which will offer 0.4 and 0.2 Å resolutions for first and second orders, respectively. Approximately half of the starlight which is focussed on the entrance slit jaws is reflected into a fine guidance system which will enable the spacecraft to stabilize to within 0.1 arc sec on a star as faint as 7th magnitude.

This paper departs somewhat from the usual interpretation of the subject of this symposium, in which instrumental techniques normally deal with diverse aspects of experiment design or construction. Rather, we will concern ourselves here with some innovations in the ground control optimization of the modes of data acquisition once an instrument is in orbit. Although the fullest possible utilization of the data gathering capability of a device is of vital interest to the success of the experiment and the mission, the subject has not been discussed at length in the open forum of scientific meetings. The planning of the data gathering and processing aspects of a mission must receive as careful attention as instrument design and test if the characteristics of the instrument are to be fully exploited.

Five different Wadsworth mounting spectroheliographs have been flown into the total solar eclipse on 7 March 1970, on top of an Aerobee 170 rocket from Wallops Island, Virginia. Details of the instruments will be discussed. The new pointing system, consisting of a combination of inertial and target guidance, will be described.

The paper describes the high-resolution stellar UV-spectrometer, experiment S 59 for ESRO TD-1 A satellite. The experiment utilizes the orbit and attitude control characteristics of the TD-1 A to obtain spectra of about 1 Å resolution from a few hundred bright early type stars with an instrument of moderate size. Attitude control of this large three-axis stabilized spacecraft is such that observing-instruments can search the sky along ecliptic meridians which precess by 360°/annum so that complete sky coverage is obtained in half a year.

Corona-problems severely can effect absolute calibrated photomultiplier-experiments in sounding rockets. Solutions in case of experiment R-130 (absolute spectral photometry of B-stars in the vacuum UV-region) are presented.

(To be published in Raumfahrtforschung, Berlin/Stuttgart/München.)

The housing of photoelectric equipment in rocket experiments results among others in the following demands: Stability of the structure and nevertheless little weight, exact geometrical alignment and at the same time soft, vibration- and shock-protecting supportings (in case of photomultipliers for instance).

This paper describes a small, low power SEC vidicon camera developed for recording images of the outer solar corona from an OSO satellite. The SEC vidicon has been selected for this application because of its capability to integrate and store low light level and low contrast images. The achieved signal to noise ratio will be discussed and compared with theoretical considerations. A new operational method to readout the tube very slowly will be described. The influence of a zero order data compression scheme on the quality of the coronal images will be discussed.

1. 1. Ultraviolet spectra (1400–1800 Å) of Ap, Am and normal A stars are needed by F. Praderie, R. Bonnet and R. Cayrel.

   The spectral resolution has to be nearly 1 Å. Accurate relative photometry (5%) and absolute calibration (30–50%) are required.

   A rocket experiment, proposed to ESRO by M. Combes and P. Felenbok is planned for launch in 1972.

2. 2. As neutral silicon and magnesium are very efficient ultra-violet absorbents, A stars ultraviolet fluxes are very faint (Praderie, 1968).

   Then a very luminous optical set-up and a high efficiency receiver have to be used. A 30 cm in diameter concave objective grating is associated with a Lallemand electronic camera. The grating (2000 //mm; //l) is holographically made (Labeyrie, 1969). The electronic camera is electrostatically focussed. A semi-transparent solar-blind CsL photocathode is used (Carruthers, 1966).

3. 3. A little mirror, placed against the grating and forming a direct view of the sky, permits to establish an absolute wavelength scale.

   During the fly, before and after stellar observations, a little concave mirror mounted into the opening side-door is used to form on the photocathode a spectrum of a Deuterium calibrated lamp. Two photomultipliers, one on each side of the electronic camera, control the lamp stability.

   The complete mounting is calibrated in the laboratory using a thermopile as reference, before the launch and after the recovery of the waterproof payload.

4. 4. The chosen stars are the brightest Ap and Am stars: α Dra (Ap; mv = 3.64; equivalent type A 0) and α2 Lib (Am; mv = 2.75; equivalent type A3-A7).

   It seems to be possible to obtain spectra (1400-1800 Å) of the Ap star with a spectral resolution of 1 Å and a signal to noise ratio better than 40. But at a pinch one may accept a resolution of 2 Å and a signal to noise ratio of 15 for the shortest range of the Ap star spectrum.

An important feature of observations made outside the atmosphere from manned or unmanned spacecrafts is the reliability and ease of operation, speed and accuracy in the collection of data. A good compromise including the above features is the development of image tube techniques in skilfully matching optics and electronics.

In this paper the bases of the various methods of intensity calibration in the vacuum ultraviolet will be examined. The remarks will be directed primarily at the problem of the measurement of spectral intensities from the sun from above the Earth's atmosphere. It is here that the requirements for intensity calibration are most demanding because of the greater sophistication and because the available range of the spectrum is more extensive. For the stars and other distant astronomical objects, hydrogen absorption limits the spectrum to wavelengths longer than about 900 Å although it transmits again in the soft X-ray region. Some reference will also be made to the problem of intensity calibration as it applies to laboratory spectroscopy, particularly where the object of the laboratory work is to measure astrophysically interesting rate coefficients or cross sections.

Accurate rocket observations are needed to estimate the post-launch sensitivity of the Wisconsin OAO instruments and to set the absolute scale of the OAO broad-band photometry and spectrophotometry. Seven broad-band photometers, equipping a pointable Aerobee rocket, are calibrated against a source of synchrotron radiation in an attempt to determine, as accurately as possible, the absolute energy distribution of OAO reference stars. The first flight (July 1st, 1970) failed for technical reasons but the experiment will be repeated in 1971.