An Observational Program to measure stellar energy distributions - in the ultraviolet between 3000 Å and 1200 Å has recently been completed. The instrumentation used was similar to that reported in reference 1. A necessary preliminary for discussion of this material is some knowledge of the extinction due to interstellar absorption. The trend of this extinction was first reported in 1964. (See ref. 2.) Additional observational material has since been obtained and is presented herein.

Five pairs of stars with similar Morgan-Keenan (MK) spectral classifications have been observed that are suitable for obtaining extinction: ζ Per and ρ Leo; ξ Per and ζ Pup; o Per and β Can Maj; σ Sco and π SCO; and δ Sco and τ SCO. The telemetry records were hand reduced by the author, each star was multiplied by the absolute calibration curve for the appropriate instrument, and the resulting flux curve was placed on a magnitude scale.

The discovery and measurement of stellar proper motions has always been associated with machines: for proper motion measurements involve four activities: observation, recording, comparison and measurement. Participation by the astronomer in these activities has step by step been replaced partically or wholly by machines. First the observation and recording functions changed from visual to photographic – with the fine guiding done by the astronomer; then the comparison by the blink microscope and the measurement by visually operated measuring machines. On a comparative time scale, the next step – automation of the comparison and measurement function – has been much money, time, and effort away from the previous steps, but as this presentation and other presentations at this conference will show, machines of varying degrees of automation and astronomer participation are now in operation.

The Theory That Graphite Particles could be responsible for interstellar extinction is presented in references 1 and 2, and this paper will present evidence that further supports this contention. The complex dielectric constant (m = n(l — ik) for graphite has been measured as a function of energy by the authors of reference 3, and the optical constants as function of wavelength λ have been obtained from this measurement. A large variation occurs in both n and k at wavelengths below 3000 Å.

The results reported in the preceding paper indicated the desirability of performing Mie scattering calculations on graphite. The measured values of the complex index of refraction corresponding to 17 wavelengths were used in the calculation for spherical particles. The IBM 7094 computer program for Mie scattering follows that of van de Hulst. (See ref. 4.) The Oort-van de Hulst size distribution (ref. 5), which may not be applicable to the type of particle discussed herein, has been used to obtain an integrated cross section for extinction.

The Automated Proper Motion Survey (APMS) has three broad goals-accuracy, completeness, and efficiency in the discovery and measurement of stellar proper motions on pairs of red-sensitive photographic star plates taken with the forty-eight inch Schmidt telescope. The specific range of motions sought is from 0.1 to 2.5 seconds-of-arc per year. The lower limit of 0.1 arc sec/year is consistent with the inherent uncertainties in the photographic emulsion and the typical epoch difference between plate exposures. At the opposite end of the scale, extension of the search radius beyond that corresponding to 2.5 arc sec/year would result in a prohibitively large number of spurious matches and a significant increase in processing time while yielding extremely few (though individually interesting) additional stellar motions. The specific range of stellar magnitudes sought is from 12 to 19 red. Significant motions for stars brighter than the limit mred = 12 are already fairly well documented and the corresponding bright Schmidt images begin to show extensive contamination from diffraction spikes, “blazes” radially away from the plate center, and photographic “bloom”. At the other limit, although images of stars fainter than mred = 19 are visually discernible on the plates (the plate limit is typically mred= 20), inspection of the faintest images reveals that they are amorphous and often quite asymmetric clusters of photographic grain. Thus, both the motion limits and the magnitude limits for the survey have been selected to cover the range of reliable and largely unexplored data on the plate material. The implementation of APMS, then, is tailored to these goals and ranges.

Even Before any Measurement of stellar fluxes in the ultraviolet had been obtained, predictions were being made on the basis of stellar atmosphere models. At the moment, it seems that such com-putations (for example, see ref. 1) are in reasonable agreement with actual observations from rockets (ref. 2) in the spectral region down to about 2000 Å. At shorter wavelengths it is necessary to take into account the appreciable blanketing by spectral lines which unavoidably is present in the transmission bands of any wide-band photometric system, even in the case of early B stars, as has been shown in reference 3.

It must be anticipated that the reduction of extinction measurements in the UV from spaceborne instruments will follow the same procedure that has been used so far: stars with the same intrinsic energy distribution but with different amounts of reddening material in the line of sight are compared, the difference in energy distribution being attributed to the wavelength dependence of interstellar extinction.

The GALAXY (General Automatic Luminosity and X, Y) measuring machine of the Royal Observatory, Edinburgh was completed in 1969 and has been in continuous use since October. It scans a plate, notes the position of all images between preassigned limits and measures some, or all, of these for magnitude and position. The machine was conceived as an attempt to answer the “Schmidt Problem”: how best to handle the immense amount of data that plates taken with Schmidt cameras contain. The general concept of the present machine is due to Professor P. B. Fellgett but the credit for actually bringing the concept to fruition belongs to Dr. V. C. Reddish of the Royal Observatory, Edinburgh and a small design team of what is now Faul Coradi Scotland Ltd.

The Object of the Edinburgh Spectrophotometry Survey is to determine the interstellar extinction law as precisely as possible and to search for intrinsic variations in it. The observations have been extended to early type stars up to a limiting magnitude of 11ṃ0 in regions extending from lII = 50° to lII = 200°. The 16/24/60-inch Schmidt telescope of the Royal Observatory has been used in conjunction with an objective prism and grating. Spectra extend from λ = 3300 Å to λ = 9000 Å with a dispersion of 1000 Å/mm at Hγ. Since few stars fainter than 9m have been measured in earlier investigations, the present survey extends considerably the amount of information available.

In determining the extinction from a comparison of pairs of stars, one reddened and the other unreddened, one is faced by the problem of having to find two intrinsically similar stars in the same galactic region.

During recent years it has often been stated that the introduction of automatic electronic equipment can provide two benefits for the astronomical observer. The first is that he can make observations which he could not make with older methods. The second is that he can do the same job more rapidly, more thoroughly, and more efficiently than he had been able to do previously. The first benefit has obviously been realized with many pieces of equipment and I will not concentrate on this point. The second has been a very real one also, and I would like to report on some of the gains which have been made at the Hale Observatories by the introduction of modern electronic equipment. Because of the power which can be brought to bear on astronomical observing problems by the use of digital electronic techniques, almost all of the equipment which I will describe is in one way or another digital in nature.

When the Observed Spectral Intensity Distribution of an early-type star has been freed from the distortions caused by absorption in the Earth's atmosphere and by instrumental sensitivity factors, it is a measure of the true spectral intensity distribution of the star and of the modifications to that distribution caused by the interstellar material between the Earth and the star. Thus, if one knew the true intensity distribution from the star, one could derive the shape of the interstellar reddening curve. In practice it is customary to compare the intensity distributions of pairs of stars of apparently the same spectral type, within the errors of classification, but which suffer different amounts of reddening, and to attribute the difference to interstellar reddening. In this way one can determine the shape of the interstellar reddening curve.

The Proper Motion Survey on plates taken with the Bruce telescope of the Harvard Observatory was begun in 1928 with a fellowship from the Guggenheim Foundation. The plates used have a scale of 1 mm = 59”, generally go down to 16mpg though a few long exposure plates almost reach 18mpg and the survey is considered reasonably complete for the larger motions down to 14.5mpg.

A proper motion survey of the northern hemisphere, utilizing first epoch plates made some thirty years ago with the 13 – inch Pluto search telescope of the Lowell Observatory, was begun in 1957. The relative proper motions of stars fainter than the eighth magnitude and with motions greater than 0.″26 per year are measured directly on the calibrated grid of the projection blink comparator.

As is Well Known, the surface brightness within the galaxies decreases strongly with increasing distance from the center. Therefore, if the diameters of the galaxies are defined by the isophote of a distinct apparent surface brightness, these diameters become smaller and smaller with increasing galactic absorption. The diameters are defined in reference 1 in this manner (by the isophote 26ṃ15/sec2); therefore, the mean surface brightnesses derived in that reference correspond more and more to the inner, brighter parts of the galaxies as the absorption increases. Consequently, in the derivation of the half optical thickness a of the galactic absorption layer from the variation of the mean surface brightnesses S of 113 spiral galaxies with galactic latitude β, the assumption in reference 1, dS/d(csc β) = a, is not correct but has to be replaced by the more complicated formula

Measurements of the Interstellar Polarization of Starlight at different wavelengths have been reported by several observers in references 1 to 8. With a few exceptions the wavelength range of the measurements is still relatively small, and up to now only a few dozens of stars have been measured with high precision. There is still a remarkable discrepancy between the results of different observers, as can be seen in figure 1. The scattering of the individual values is significantly larger than the probable error given by the respective authors. One of the most serious error sources seems to be the instrumental polarization which itself is highly color dependent. (See ref. 9.)

Nevertheless, even if a much greater individual error is assumed, a remarkable conclusion can be drawn: the wavelength dependence of polarization is not the same for all stars.

At the General Assembly of the I. A. U. in Prague in 1967 I became involved in a discussion on the merit of using stars with positions taken from the Astrographic Catalogue as reference stars in finding positions of faint members of the solar system. The answer was that little choice exists if the positions are being found from photographs of small field taken with large telescopes.

The Polarization of 19 Bright, Highly Polarized Stars was measured from March until July 1965, with a differential polarimeter installed at the Cassegrain focus of the 90-cm telescope of the Toruń Observatory in Poland. The stellar light was split by the calcite Wollaston prism into two beams falling upon the cathodes of the photomultipliers. The observations were made at eight position angles of the polarimeter differing by 45°, alternately with and without the quartz depolarizer. The yellow 2-mm Schott GG 14 and the blue Schott BG 12 (l-mm) + GG 13 (2-mm) filters were used. No optical elements except the depolarizer were placed between the Wollaston prism and the telescope mirrors.

Usually only the Stokes parameters having the larger absolute values were accurately measured. The resulting polarization ratios pv/pB of the Stokes parameters expressed in magnitudes, measured with yellow and blue filters, are given in the ninth column of table I.

The determination of secular parallaxes of stars is usually based on meridian observations of proper motions of bright stars, this introducing known systematic errors. The mean parallaxes of stars can be obtained by means of radial velocities which are known for the bright stars. The more perspective method, the reference of stars to galaxies, is not applicable at low galactic latitudes.