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NASA NTRS1988

A consideration of the use of optical fibers to remotely couple photometers to telescopes

The possible use of optical fibers to remotely couple photometers to telescopes is considered. Such an application offers the apparent prospect of enhancing photometric stability as a consequence of the benefits of remote operation and decreased sensitivity to image details. A properly designed fiber optic coupler will probably show no significant changes in optical transmisssion due to normal variations in the fiber configuration. It may be more difficult to eliminate configuration-dependent effects on the pupil of the transmitted beam, and thus achieve photometric stability to guiding and seeing errors. In addition, there is some evidence for significant changes in the optical throughputs of fibers over the temperature range normally encountered in astronomical observatories.

Heacox, William D.↗

NASA NTRS1989

Interacting binary galaxies. VI - The fast encounter of NGC 2672 and NGC 2673 (Karachentsev 175, Arp 167)

The isolated pair of interacting ellipticals NGC 2672/2673 (Arp 167; Karachentsev 175), whose line-of-sight relative velocity is about 520 km/s, is studied. This very unequal mass system features two low surface brightness plumes emerging from the secondary galaxy NGC 2673. Photometric data on the pair are described, and the distortions in the galaxies and their photometric properties are discussed. Spectroscopic observations of the pair and the velocity structures derived from these are addressed. A physical solution for the pair is presented and explained.

Balcells, Marc↗

NASA NTRS1989

Centroid position measurements and subpixel sensitivity variations with the MAMA detector

Initial measurements of the position sensitivity of a visible-light multianode microchannel array detector show that centroid calculations for image spots are accurate to better than 0.04 pixels even with sources that are essentially delta functions at the photocathode.Subpixel sensitivity variations of 10-15 percent are typically found for pixels in the array. Variations as large as 30 percent are possible in the worst conditions. These variations limit the photometric accuracy of the detector when very small scale features are observed. The photometric accuracy and the position sensitivity of the detector appear to be limited by cross-coupling effects within the anode array. Initial measurements with more recent designs of the detector show that most or all of this cross-coupling has been eliminated.

Morgan, Jeffrey S.↗

NASA NTRS1988

The All Sky Monitor for the X-ray Timing Explorer

The All Sky Monitor (ASM) for the X-ray Timing Explorer (XTE), whose purpose is to record a photometric history of known objects and to detect and locate transient objects, is discussed. This instrument uses the multiple pinhole approach to avoid photon starvation for good photometric capability together with good angular resolution and an excellent capability for locating transients. While the XTE ASM does not have the ideal imaging qualities of the uniformly redundant array-based designs, it is simpler to build and requires a modest telemetry bandwidth; since the ASM is intended for detection, location, and photometry of discrete point sources, imaging is of secondary importance.

Doty, John P.↗

NASA NTRS1989

A CCD portrait of Comet P/Tempel 2

The development of activity in Comet P/Tempel 2 is studied from aphelion (R = 4 AU) to perihelion (R = 1.4 AU) using extensive time-series CCD photometry and CCD spectra. The comet undergoes a profound morphological change at R of about 2-2.5 AU, from a bare nucleus at larger distances to an active comet supporting a coma of gas and dust. Cyclic photometric variations with the period T = 8.95 + or - 0.01 hr. are present at all R, and are attributed to the rotation of the nucleus at this period. The nucleus is prolate (axes a:b:c = 1.9:1:1), a property shared with other nuclei studied using CCD photometry. Novel results include a limit on the bulk density of the nucleus, rho above 300 kg/cu m, and a 20-A-resolution CCD spectrum of the nucleus. Spatially and temporally resolved photometry is used to study the effects of nucleus rotation on the coma. The coma does not share the dramatic photometric variations shown by the nucleus. It possesses a steep surface-brightness distribution, which is attributable to progressive destruction of the coma grains with increasing space exposure.

Jewitt, David↗

NASA NTRS1988

Three-meter balloon-borne telescope

The Three-Meter Balloon-Borne Telescope is planned as a general purpose facility for making far-infrared and submillimeter astronomical observations from the stratosphere. It will operate throughout the spectral range 30 microns to 1 millimeter which is largely obscurred from the ground. The design is an f/13.5 Cassegrain telescope with an f/1.33 3-meter primary mirror supported with a 3-axis gimbal and stabilization system. The overall structure is 8.0 m high by 5.5 m in width by 4.0 m in depth and weighs 2000 kg. This low weight is achieved through the use of an ultra lightweight primary mirror of composite construction. Pointing and stabilization are achieved with television monitoring of the star field, flex-pivot bearing supports, gyroscopes, and magnetically levitated reaction wheels. Two instruments will be carried on each flight; generally a photometric camera and a spectrometer. A 64-element bolometer array photometric camera operating from 30 to 300 microns is planned as part of the facility. Additional instruments will be derived from KAO and other development programs.

Hoffmann, William F.↗

NASA NTRS1989

Theory of wide-angle photometry from standard stars

Wide angle celestial structures, such as bright comet tails and nearby galaxies and clusters of galaxies, rely on photographic methods for quantified morphology and photometry, primarily because electronic devices with comparable resolution and sky coverage are beyond current technological capability. The problem of the photometry of extended structures and of how this problem may be overcome through calibration by photometric standard stars is examined. The perfect properties of the ideal field of view are stated in the guise of a radiometric paraxial approximation, in the hope that fields of view of actual telescopes will conform. Fundamental radiometric concepts are worked through before the issue of atmospheric attenuation is addressed. The independence of observed atmospheric extinction and surface brightness leads off the quest for formal solutions to the problem of surface photometry. Methods and problems of solution are discussed. The spectre is confronted in the spirit of standard stars and shown to be chimerical in that light, provided certain rituals are adopted. After a brief discussion of Baker-Sampson polynomials and the vexing issue of saturation, a pursuit is made of actual numbers to be expected in real cases. While the numbers crunched are gathered ex nihilo, they demonstrate the feasibility of Newton's method in the solution of this overdetermined, nonlinear, least square, multiparametric, photometric problem.

Usher, Peter D.↗

NASA NTRS1990

HR 1362 - A test case for stellar dynamo theories

An analysis of 11 yr of photometry of HR 1362 = EK Eri has shown a well-established photometric period of 335 days. This confirms an earlier period determination of about 310 days from 3 yr of APT data and is consistent with a new v sin i determination of 2 + or - 2 km/s. Recent Ca II H and K observations show moderately strong H and K emission lines at a flux level of log F prime (K) of about 6.5 ergs sq m/s. If the photometric period is interpreted as the rotation period, this surface flux is more than one order of magnitude larger than what can be expected from empirical activity versus rotation relationships. The spectrum and the U-B and B-V colors are consistent with a single G8 III-IV spectral classification, while the V-R and V-I values indicate a small color excess, typical of chromospherically active stars.

Strassmeier, Klaus G.↗

NASA NTRS1990

Optical variability of extragalactic objects used to tie the HIPPARCOS reference frame to an extragalactic system using Hubble space telescope observations

Observations of a set of 89 extragalactic objects (EGOs) will be made with the Hubble Space Telescope Fine Guidance Sensors and Planetary Camera in order to link the HIPPARCOS Instrumental System to an extragalactic coordinate system. Most of the sources chosen for observation contain compact radio sources and stellarlike nuclei; 65 percent are optical variables beyond a 0.2 mag limit. To ensure proper exposure times, accurate mean magnitudes are necessary. In many cases, the average magnitudes listed in the literature were not adequate. The literature was searched for all relevant photometric information for the EGOs, and photometric parameters were derived, including mean magnitude, maximum range, and timescale of variability. This paper presents the results of that search and the parameters derived. The results will allow exposure times to be estimated such that an observed magnitude different from the tabular magnitude by 0.5 mag in either direction will not degrade the astrometric centering ability on a Planetary Camera CCD frame.

Bozyan, Elizabeth P.↗

NASA NTRS1990

Television image of a large upward electrical discharge above a thunderstorm system

A low light-level TV camera is used to obtain an unusual image of luminous electrical discharge over a thunderstorm 250 km from the observation site. The image is presented and the discharge in the image is described. It is suggested that the image is probably due to two localized electric charge concentrations at the cloud tops. The hazard of these discharges for aircraft and rocket launches is examined. Consideration is given to the possibility that these discharges may account for unexplained photometric observations of distant lightning events that show a low rise rate of the luminous pulse and no electromagnetic sferic pulse like that in cloud-to-earth lightning strokes. The photometric events of this type that occurred on September 22-23, 1989 during hurricane Hugo are noted.

Franz, R. C.↗

NASA NTRS1989

Properties of interstellar dust in the region of the CEP OB4 association

Regional variations in the interstellar extinction law was examined. The region of Cyg was noted among the areas with abnormal interstellar extinction law (IEL). An attempt was made to study interstellar extinction in the region of the Cep OB4 association. The Vilnius seven color photometric system was used for this purpose. The effective wavelengths and their reciprocals for the Vilnius photometric system are presented. This system was applied for the study of variations in the IEL in four regions of the sky: Cyg, Cep, Per, and Mon (Sudzius, 1974). The results of multicolor photometry were combined with the data of Whiteoak (1966) and detailed interstellar extinction curves were derived for Cyg and for Cep-Per-Mon (average) regions in the wavelengths interval of 3000 to 8000 A (Sudzius, 1974). The obtained IEL for the Cep OB4 association does not match any of the laws determined for other regions of the sky. However, it is very close to Cyg OB2 law in the ultraviolet. Results suggest that Cep OB4 IEL have no very broadband structure (VBS) in the interval of wavenumbers 1.84 to 2.15/micron.

Sudzius, J.↗

NASA NTRS1990

An atlas of mixed-morphology pairs

Current theories of galaxy formation imply that environment is the most important factor in deciding whether a collapsing region becomes a spiral galaxy or an elliptical galaxy. If this is the case, then isolated pairs comprising a galaxy of each morphology provide an especially interesting test, since, strictly speaking, they should not exist. The first phase of studying E-S pairs is to define a sample which is genuinely isolated and genuinely E-S. The catalog compiled by Karachentsev is the most useful Northern Hemisphere starting point, containing 603 pairs which obeyed certain well-defined angular isolation criteria. All of these pairs now have velocities, enabling further removal of at least some of the non-physical binaries. The conventional cutoff at 500 km s(exp -1) was used for this purpose. However, the morphological types were estimated from Palomar Sky Survey prints, and are consequently not very accurate. It was therefore decided to obtain good charge coupled device (CCD) images in the B-band of all pairs from the Karachentsev sample which were classified as either E-S (73 pairs) or E-SO (91 pairs), for a total of 164 pairs. This includes the 24 pairs with known redshift differences greater than 500 km s(exp -1) (9 E-S and 15 E-SO), which were also imaged in this survey, not only for completeness but also in case the criteria were later changed, or the redshifts were found to be in error. A total of 198 CCD frames were obtained between February 1986 and the present, with most of the observations being completed by the summer of 1987. A number of missing stragglers and replacements for poor quality images were taken more recently. Most observations used the number 1-0.9m telescope at Kitt Peak, with a few frames from the 2.1m. Some pairs required more than one image due to their separation. The morphological study did not require photometric conditions, but calibrations were made for almost all images anyway, and the weather was probably photometric at least half of the time, at least to an accuracy of around 0.1 magnitudes. The results of this first phase will be published in the form of an Atlas, from which any interested readers will be able to select their own sub-sample using their preferred morphological criteria.

Sharp, N. A.↗

NASA NTRS1990

Ultraviolet, visual, and infrared studies of galactic extreme emission line objects with very large IR-excess

Some preliminary results of a study of extreme emission line objects (EELOs) are presented. These stars are selected from H alpha emission line stars having a strong IR excess. The study is based on multiwavelength photometric and spectroscopic observations. Special attention is paid to the detailed study of the correlation between spectroscopic and photometric variations, which indicate the activity of the star. The ultraviolet and infrared extinction characteristics of the circumstellar material are given special attention.

Dewinter, D.↗

NASA NTRS1991

Determination of lunar ilmenite abundances from remotely sensed data

The mineral ilmenite (FeTiO3) was found in abundance in lunar mare soils returned during the Apollo project. Lunar ilmenite often contains greater than 50 weight-percent titanium dioxide (TiO2), and is a primary potential resource for oxygen and other raw materials to supply future lunar bases. Chemical and spectroscopic analysis of the returned lunar soils produced an empirical function that relates the spectral reflectance ratio at 400 and 560 nm to the weight percent abundance of TiO2. This allowed mapping of the lunar TiO2 distribution using telescopic vidicon multispectral imaging from the ground; however, the time variant photometric response of the vidicon detectors produced abundance uncertainties of at least 2 to 5 percent. Since that time, solid-state charge-coupled device (CCD) detector technology capable of much improved photometric response has become available. An investigation of the lunar TiO2 distribution was carried out utilizing groundbased telescopic CCD multispectral imagery and spectroscopy. The work was approached in phases to develop optimum technique based upon initial results. The goal is to achieve the best possible TiO2 abundance maps from the ground as a precursor to lunar orbiter and robotic sample return missions, and to produce a better idea of the peak abundances of TiO2 for benefaction studies. These phases and the results are summarized.

Larson, Stephen M.↗

NASA NTRS1991

Terrestrial polarization imagery obtained from the Space Shuttle - Characterization and interpretation

An experiment to measure the polarization of land, sea, haze, and cloud areas from space was carried aboard the Space Shuttle in September 1985. Digitized polarimetric and photometric imagery in mutually perpendicular planes was derived in the red, green, and blue spectral regions from photographs taken with two synchronized Hasselblad cameras using type 5036 Ektachrome film. Digitization at the NASA Houston Video Digital Analysis Systems Laboratory permitted reduction of the imagery into equipolarimetric contours with a relative accuracy of + or - 20 percent for comparison to ground truth. The Island of Hawaii and adjacent sea and cloud areas were the objects of the specific imagery analyzed. Results show that cloud development is uniquely characterized using percent polarization without requiring precision photometric calibration. Furthermore, sea state and wind direction over the sea could be inferred as well as terrestrial soil texture.

Egan, Walter G.↗

NASA NTRS1991

Measuring the direct sky brightness on CCD images

A new method of determining the background sky brightness for stellar photometric measurements with CCDs is described. By fitting and subtracting stellar profiles from a CCD image, one can measure the sky brightness directly at the position of each star after subtraction. These sky values can then be used for the next round of profile fitting and subtraction. The 'direct sky' brightness for a star is the convergent value of the sky after a few such iterations and is the most direct measurement of the line-of-sight contribution of the background to the observed magnitude of a star. Using this method, a decrease of the random photometric errors by more than a factor of two over standard methods is possible, with the most significant improvements for stars in very crowded regions and areas of strong and highly variable nebulosity.

Parker, Joel WM.↗

NASA NTRS1991

Gas and dust production by Comet P/Halley (1910 II)

Selected long-slit spectrograms of Comet P/Halley, photographed during the postperihelion period between April 16 and June 3, 1910 have been quantitatively analyzed for comparison with the 1985/86 apparition. The Fe-V-Na spark spectrum was used to determine characteristic curves for these plates, and flux calibrations based on contemporaneous broad-band photometric measurements were applied. Haser model production rates for C2 and values Afp were computed for each of the nights selected and compared with the photometric observations obtained by Schleicher et al. (1987) during the comparable postperihelion period of the 1985/86 apparition. As found for the 1985/86 apparation, the gas and dust production in 1910 varied in phase, but the rates of production were higher than observed in 1986 by a small but statistically significant amount.

Howell, Ellen S.↗

NASA NTRS1991

UBVRI photometry of stars in the fields of X-ray selected BL Lacertae objects

UBVRI photometry is presented of stars in the fields of 38 X-ray selected BL Lacertae objects, two X-ray selected AGNs, and one X-ray binary system (possibly a cataclysmic variable). These stars are suitable for use as photometric comparison stars for optical monitoring programs involving these X-ray selected objects. A total of 166 observations of 87 stars were made on seven photometric nights. At least two stars were observed in every field. UBVRI measurements of three stars were made in the fields of MS 0950.9 + 4929, MS 1552.1 + 2020, and H 2154 - 304. Four stars were measured in the field of H 1426 + 428. Finding charts for all of these objects are included.

Smith, Paul S.↗