Identification of [Fe III] in the solar ultraviolet spectrum
Solar ultraviolet spectra have been obtained with high spectral and spatial resolution using the Naval Research Laboratory's high-resolution telescope and spectrograph1 (HRTS) flown on rockets since 1975 and most recently on the Spacelab 2 Shuttle flight. Because the solar spectrum between ∼1,170 and 1,719 Å has been well observed for some years, few lines of substantial intensity remain unidentified. (See for example the recent compilation by Sandlin et al.2). The longest exposures during the first rocket flight of the HRTS, which obtained spectra with a spatial resolution of ∼ 1 arc s along the slit, of length ∼ 1 solar radius, and a spectral resolution of ∼0.05 Å, showed a number of weak emission lines at the solar limb, in particular between 1,570 and 1,600 Å (see Plates 16 and 17 in ref. 3), that could not readily be identified. Improved observations of these lines were obtained during the flight of HRTS on the Spacelab 2 Shuttle flight in July–August 1985, by making longer exposures (60,100 and 250 s). The scope of the data obtained has been described4. We have now identified the emission lines concerned as forbidden (electric quadrupole and magnetic dipole) transitions in Fe III, the first detection of these particular transitions in any source. They must now be considered potential candidates for previously unidentified lines in other low-density (Ne ≲ 1010 cm−3) astrophysical sources.
Extreme ultraviolet spectrograph ATM experiment S082B.
The extreme ultraviolet, double-dispersion, photographic spectrograph, Apollo Telescope Mount (ATM) Experiment S082B on Skylab is described. Novel features were the use of a predisperser grating with a ruling whose spacing varied approximately linearly with distance for the purpose of increasing the instrument speed by reducing the astigmatism and a photoelectric servo-system to stabilize to 1 sec of arc the solar image at various near-limb positions. The 970-3940-A range was covered in two sections with effective lambda/Deltalambda congruent with 30,000 from 1100 A to 1970 A. The spatial resolution was 2 x 60 solar sec of arc. During the Skylab mission 6400 exposures were made with the instrument pointed by an astronaut at selected and recorded'solar positions.
The Profile of Solar Hydrogen-Lyman-α
The profile of the Lyman-α line of hydrogen in the radiation from the sun was determined using a photographic spectrograph, flown in an Aerobee-High rocket on July 21, 1959, as a part of the United States effort in the International Geophysical Cooperation, 1959. The instrument employed a 50-cm radius, 1200 I/mm diffraction grating in the 13th order, following a design originated by Dr. Robert J. Meltzer of the Bausch and Lomb Optical Company. Overlapping orders and stray light were effectively eliminated by means of a similar grating as a predisperser; this grating was used at an angle of incidence such that the optical system as a whole was stigmatic. Therefore, points along the spectrum line corresponded to points in a narrow slice diametrically across the sun.