Chromospheric Helium Imaging Photometer (an Instrument for High Time Cadence 1083-nm Wavelength Solar Observations).

The scientific motivation, design criteria, and specifications for a new ground-based instrument to observe the Sun in the He i 1083-nm spectral line is described. The instrument employs a liquid-crystal tunable Lyot-type spectral filter and an array detector that allows the full solar disk to be observed with a time cadence of minutes. We describe the telescope's optical and mechanical features and discuss computer interface and data-reduction procedures employed. Instrument performance during the initial year of operation of the telescope at its high-altitude site is summarized.

New Mauna Loa coronagraph systems.

A new set of instruments, consisting of two coronagraph systems, has been installed and is operating at the Mauna Loa Observing Station, Hawaii, operated by the High Altitude Observatory of Boulder, Colorado. The instruments are the 23-cm objective Mark III K-coronameter (K-III) system, a photoelectric instrument used to observe the inner solar corona from 1.2 R(0) to 2.2 R(0) and the 12.5-cm objective Prominence Monitor system used for the detection of H(alpha) limb activity. New features of the K-coronameter system include the use of achromatic wave plates for wide bandpass operation and linear diode array detectors. Raster scans of the coronal image are obtained in 1.5 min for a critical sampling scheme of 20-sec of arc resolution (10 x 10-sec of arc pixels) in the coronal p(B) image. This represents a 350 information gain factor for each detection channel when compared with the previous Mauna Loa K-coronameters.

Radiance calibration of the High Altitude Observatory white-light coronagraph on Skylab.

The High Altitude Observatory coronagraph produced over 35,000 photographs of the solar corona over 9 mo. Images were obtained on Kodak special film type 026-02, a 3400 (Pan-X) emulsion modified by Kodak for minimal reciprocity losses at the exposure times (3 sec, 9 sec, 27 sec) used on the coronagraph. The film was processed in a specially developed chemistry to obtain the best compromise between speed, low fog, and modulation transfer function. The calibration of a nineteen-step wedge within the coronagraph is based on previously calibrated glass opal filters. The step wedge, illuminated by attenuated sunlight, is imaged on each photograph made by the instrument. Data reduction procedures employ average characteristic curves for each data set (approximately 4000 frames). It is found that the effects of radiation fog and latent image loss are negligible within these sets. The relative coronal radiance error, determined by measuring the coronal plus stray light radiance over the solar pole, is found to be le s than 8%. Based on an estimated error of 15% in the absolute calibration of the step wedge, the net absolute accuracy of a given radiance measurement is estimated to be 20%.

Measurement of stray radiance in the High Altitude Observatory's Skylab coronagraph.

The stray radiance present in the field of view of the High Altitude Observatory's Skylab coronagraph has been measured from results obtained in orbit with two techniques. First, employing images of the lunar disk made near the time of solar eclipse, a comparison of the apparent contrasts recorded by the coronagraph of maria and highlands with those of previous studies, allows the determination of the magnitude of the stray radiance and its variation along a radius. Next, the azimuthal (position angle) variation was found through measurement of the change in the net radiance of a point in the coronal image, as the image (coronagraph) was rotated. From these measurements the stray radiance was found to be a maximum near the inner limit of the instrument field of view, variable in azimuth from 1.4 to 5.6 10(-9)B(O), where B(O) is the mean solar disk radiance. Further, the stray radiance was found to be a steep, decreasing function of distance along a radius, falling to a nearly uniform, azimuthally independent va ue of ~3.5 x 10(-10)B(O) in the outer field. In this latter region, the coronagraph stray radiance is well below a typical midtotality eclipse sky radiance of ~1.8 x 10(-9)B(O). Finally, the radiance of lunar maria and highlands (in units of 10(-10)B(O)) was determined to be 2.2 and 4.5, respectively, for the 30 June 1973 eclipse and 1.4 and 3.0, respectively, for the 24 December 1973 eclipse, in agreement with other observers.

Far infrared airborne spectroscopy.

Equipment and techniques employed in an airborne far ir (80-400 micro) spectral measurement of the solar brightness temperature are described, with particular attention paid to the scanning Michelson interferometer and the radiometric calibration. The airborne performance of several telescopic guiding systems, a liquid helium cooled bolometer, and golay cells are discussed, summarizing experience gained on twenty-five jet aircraft flights.

A computer-controlled infrared eclipse telescope.

An f/8, chopped, dual beam, gyroscopically pointed telescope has been constructed for use in obtaining infrared (7-13 micro) observations of the spectrum of the thermal emission of the solar corona. The instrument employs a scanning Michelson interferometer with germanium beam splitter and a germanium bolometer operated at 2 K. A small general purpose digital computer is used for experiment control and data processing. The computer, using reference signals from the chopper, controls the position of the moving interferometer mirror (through a stepper motor), and the operation of an A/D converter enabling rapid measurements of the bolometer output to be made. Synchronous detection, necessary to minimize sky background radiation, is accomplished within the computer. The resulting interferograms are stored on magnetic tape for future processing and displayed on an x-y recorder. A fourier transformed spectrum may be generated on operator request in nearly real time.

A balloon-borne infrared coronagraph.

A stratospheric balloon-borne coronagraph operating at wavelength 2.2 micro has been successfully flown in order to observe the outer solar corona. The system employed external occultation with three straight edges, and the Lyot portion was axial. Scanning of the corona was accomplished through rotation of the rear Lyot section about a diameter of the primary objective lens. A description of the origins of stray light in the instrument is given, and measurements at wavelengths 0.5 micro, 0.8 micro, and 2.2 micro presented. A wavelength dependence of the scattered light from the objective lens is deduced. Some details of the detection system and solar tracking are discussed.