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To effectively eliminate optical end loss for small-scale linear solar concentrators, a fixed linear-focus Fresnel lens solar system (FLFS) with triangle cavity receiver is described and the principle is provided. The effects of the receiver parameters including receiver position f, solar declination angle δ , solar hour angle ω , internal surface absorptivity α a b and end plane reflectivity ρ r on the end loss ratio ( μ o e l ) and relative optical efficiency ( η r e − o p t ) of the proposed system have been studied by the control variable method and Monte Carlo ray tracing (MCRT) method which is implemented by software TracePro®. Significance tests of critical factors are conducted to assess the effect of receiver parameters. These results revealed that the FLFS could effectively solve the problem of optical end loss of linear solar concentrators. When f = 650 mm and δ = 0°, 8°, 16°, and 23°27 ′ , the corresponding average η r e − o p t at ω from 0° to 60° are 0.9459, 0.9422, 0.9145, and 0.8144, respectively. As δ increases, reducing f can ensure the system operates at a high-efficiency state. The η r e − o p t shows a positive relationship with α a b and no relationship with ρ r . δ occupies the greatest significant influence on η r e − o p t , which is followed by ω , α a b and f.

The method for precisely determining the astronomical azimuth using the local hour angle (LHA) of Polaris is only applicable in northern middle latitudes. In addition, the meridian star pair method is limited by the conditions of star selection and matching. A new hour angle method for multiple meridian stars is proposed, which is suitable for any part of the global region. When multiple south and north stars have been observed, the influence of longitude error and timing error on the azimuth is modified by linear regression. A precise astronomical position is not required, and there is no limit to the specific position of the meridian stars. Therefore, both the number of observable stars and work efficiency are greatly increased. Our experiment indicates that the results of the new method can reach the first-order accuracy level; i.e., the root mean square error (RMSE) is less than 0.5 ″ , even when the amount of observed data is reduced to half that of the Polaris method.

International audience; The Dark Energy Spectroscopic Instrument (DESI) is a Stage IV ground-based dark energy experiment that will measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 square degrees will be measured during the life of the experiment. We describe the installation of the major elements of the instrument at the Mayall 4m telescope, completed in late 2019. The previous prime focus corrector, spider vanes, and upper rings were removed from the Mayall’s Serrurier truss and replaced with the newlyconstructed DESI ring, vanes, cage, hexapod, and optical corrector. The new corrector was optically aligned with the primary mirror using a laser tracker system. The DESI focal plane system was integrated to the corrector, with each of its ten 500-fiber-positioner petal segments installed using custom installation hardware and the laser tracker. Ten DESI spectrographs with 30 cryostats were installed in a newly assembled clean room in the Large Coude Room. The ten cables carrying 5000 optical fibers from the positioners in the focal plane were routed down the telescope through cable wraps at the declination and hour angle axes, and their integral slitheads were integrated with the ten spectrographs. The fiber view camera assembly was installed to the Mayall’s primary mirror cell. Servers for the instrument control system replaced existing computer equipment. The fully integrated instrument has been commissioned and is ready to start its operations phase.

Extraterrestrial solar irradiance (KEX) is the solar irradiance at the “top” of the Earth’s atmosphere, on a plane perpendicular to the Sun’s rays, and at mean Earth–Sun distance. To calculate the amount of irradiance available in the photic zone, the maximum possible extraterrestrial irradiance or the solar constant, which is 1366 + 3 Watts per square meter (or W m−2), is adjusted downward to account for atmospheric attenuation and finally attenuation rates in the water body, as a function of the reciprocal of the inverse square law—the radius vector—the Earth–Sun distance d divided by the mean of d throughout the year, along with the zenith angle (z) formed by the Sun, the surface point of interest, and the vertical. The zenith angle can be computed as a function of the latitude of interest, the latitude experiencing the direct rays of the Sun on that day, i.e., solar declination (δ), and the angle that the Earth has yet to rotate through to reach solar noon or hour angle (h). On a clear day, perhaps 80% of KEX will be transmitted to the surface. However, under heavy clouds, as little as 10% of KEX reaches the surface. When solar radiation strikes a water surface at any angle other than the vertical, refraction occurs because of the density difference between air and water. In oceanography and coastal sciences, modeling the radiation emitted by a point or area source of light (usually the Sun) can often be important. Three-dimensional radiant intensity can be used to derive the amount of energy radiated by and onto points or bodies in the ocean.

The Ultraviolet Forward Spectrometer is a new type of spectrometer for monitoring the vertical distribution of atmospheric trace gases in the global middle atmosphere. It is on the TianGong-2 space laboratory, which was launched on 15 September 2016. The spectrometer uses a solar calibration mode to modify its irradiance. Accurately calculating the solar orientation is a prerequisite of spectral calibration for the Ultraviolet Forward Spectrometer. In this paper, a method of calculating the solar orientation is proposed according to the imaging geometric characteristics of the spectrometer. Firstly, the solar orientation in the horizontal rectangular coordinate system is calculated based on the solar declination angle algorithm proposed by Bourges and the solar hour angle algorithm proposed by Lamm. Then, the solar orientation in the sensor coordinate system is achieved through several coordinate system transforms. Finally, we calculate the solar orientation in the sensor coordinate system and evaluate its calculation accuracy using actual orbital data of TianGong-2. The results show that the accuracy is close to the simulation method with STK (Satellite Tool Kit), and the error is not more than 2 %. The algorithm we present does not need a lot of astronomical knowledge, but only needs some observation parameters provided by TianGong-2.

The brightness temperature (TB) data of the Moon acquired by the microwave radiometer (MRM) on-board the Chinese Chang’e-2 (CE-2) lunar probe are valuable and comprehensive data, which can be helpful in studying the physical properties of the lunar regolith, such as thickness, physical temperature, and dielectric constant. To construct the accurate and high-resolution lunar TB model with the TB data obtained by the MRM on-board CE-2, 2401 tracks of the original TB data are quantized by using the hour angle processing, and the hierarchical MK splines function (HMKSF) method is presented, which uses a hierarchy of coarse-to-fine control lattices to generate a sequence of TB model functions. The TB model constructor is the sum of the TB model functions derived at each level of the hierarchy. In addition, the lunar TB models with a resolution of $0.5 {^{\circ }} \times 0.5 {^{\circ }}$ in all four frequency channels are constructed for both the daytime and the nighttime. The obtained models show rich information, e.g., the global distribution of TB over the lunar surface, the effect of frequency on the TB model.

The purpose of this work is to develop a simple control law to implement stable orbits about a small rotating celestial body to achieve global coverage as well as fixed-body hovering on the equatorial plane. The celestial body is assumed to be rotating about a principal axis, with constant rotational velocity along the largest moment of inertia. A simple three dimensional closed-loop guidance law function of position and velocity is defined and analyzed, enabling the determination of the guidance constants to assure convergence to any desired circular orbit about the irregular celestial body, controlling independently five orbital parameters: inclination, right ascension of the ascending node, orbital radius, orbital rate and equatorial longitude. Representative numerical results are presented for an Eros type asteroid.

Firstly I plan to describe briefly growth of the Radio Astronomy group that was established at TIFR, Mumbai, in early 1963. The Ooty Radio Telescope (ORT) was designed and built indigenously and became operational in early 1970. It consists of a 530m long and 30 m wide parabolic cylindrical antenna that is located on a suitable hill with its axis of rotation parallel to that of the Earth, which allows it to be steered mechanically in hour angle for about ten hours. A phased array allows coverage from - 45 to +45 degrees in declination. Using the lunar occultation observations, angular size of over 1000 radio galaxies with arc-second resolution were made by 1976 for the first time in the world. The observations supported the Big Bang model. An Ooty Synthesis Radio Telescope (OSRT) was operational during 1982-1986. Currently, ORT is being used for Inter-planetary Scintillation observations of compact radio galaxies and Quasars for daily measurements of the velocity of the Solar wind. An active programme for observations of Pulsars is also carried out.

We present the results of 15184 astrometric observations of 322 visual binaries carried out in 2007–2014 at Pulkovo observatory. In 2007, the 26-inch refractor (F = 10413 mm, D = 65 cm) was equipped with the CCD camera FLI ProLine 09000 (FOV 12′ × 12′, 3056 × 3056 pixels, 0.238 arcsec pixel−1). Telescope automation and weather monitoring system installation allowed us to increase the number of observations significantly. Visual binary and multiple systems with an angular distance in the interval 1.ʺ1–78.ʺ6 with 7.ʺ3 on average were included in the observing program. The results were studied in detail for systematic errors using calibration star pairs. There was no detected dependence of errors on temperature, pressure, and hour angle. The dependence of the 26-inch refractor’s scale on temperature was taken into account in calculations. The accuracy of measurement of a single CCD image is in the range of 0.ʺ0005 to 0.ʺ289, 0.ʺ021 on average along both coordinates. Mean errors in annual average values of angular distance and position angle are equal to 0.ʺ005 and 0.°04 respectively. The results are available here http://izmccd.puldb.ru/vds.htmand in the Strasbourg Astronomical Data Center (CDS). In the catalog, the separations and position angles per night of observation and annual average as well as errors for all the values and standard deviations of a single observation are presented. We present the results of comparison of 50 pairs of stars with known orbital solutions with ephemerides.

In this section we shall develop some approximations for the most basic aspect of an ephemeris for the Sun and the stars which will enable the navigator to calculate approximations to the required Greenwich Hour Angle (GHA) and Declination δ of the Sun and stars. Those results can be classified as low precision ephemerides for the Sun and stars.

In this article, we report all results obtained with a fiber scrambler on the Hamilton spectrograph at Lick Observatory. We demonstrate an improvement in the stability of the instrumental profile using this fiber scrambler. Additionally, we present data obtained with a double scrambler that further improves the stability of the instrument by a factor 2. These results show that errors related to the coupling between the telescope and the spectrograph are the dominant source of instrumental profile variability at Lick Observatory. In particular, we show a strong correlation between instrumental profile variations and hour angle, most likely due to pointing-dependent illumination of the spectrograph optics.

We describe the motivation, design and implementation of the CORNISH survey, an arcsecond resolution radio continuum survey of the inner Galactic plane at 5 GHz using the Karl G. Jansky Very Large Array (VLA). It is a blind survey co-ordinated with the northern Spitzer GLIMPSE I region covering 10 deg < l < 65 deg and |b| < 1 deg at similar resolution. We discuss in detail the strategy that we employed to control the shape of the synthesised beam across this survey that covers a wide range of fairly low declinations. Two snapshots separated by 4 hours in hour angle kept the beam elongation to less that 1.5 over 75% of the survey area and less than 2 over 98% of the survey. The prime scientific motivation is to provide an unbiased survey for ultra-compact HII regions to study this key phase in massive star formation. A sensitivity around 2 mJy will allow the automatic distinction between radio loud and quiet mid-IR sources found in the Spitzer surveys. This survey has many legacy applications beyond star formation including evolved stars, active stars and binaries, and extragalactic sources. The CORNISH survey for compact ionized sources complements other Galactic plane surveys that target diffuse and non-thermal sources as well as atomic and molecular phases to build up a complete picture of the ISM in the Galaxy., 17 pages, 14 figures. Accepted for publication in PASP (16th-August-2012)

A generalized analytical model was developed to determine the optimum tilt angle of flat-plate solar collectors for any location worldwide. The developed analytical model was found to be directly related to the latitude φ of the location under consideration, the declination angle δ of the earth axis, and the day hour angle at sunset at any day throughout the year. The optimum tilt angle β opt was mathematically expressed by β opt = φ – tan−1[{tan δ(πω s /180)}/sin ω s ]. This model was applied to Riyadh, Kingdom of Saudi Arabia. The results were compared with those calculated by NASA, and showed very good agreement.

A new design concept, called one axis three positions sun-tracking polar-axis aligned CPCs (3P-CPCs, in short), was proposed and theoretically studied in this work for photovoltaic applications. The proposed trough-like CPC is oriented in the polar-axis direction, and the aperture is daily adjusted eastward, southward, and westward in the morning, noon and afternoon, respectively, by rotating the CPC trough, to ensure efficient collection of beam radiation nearly all day. To investigate the optical performance of such CPCs, an analytical mathematical procedure is developed to estimate daily and annual solar gain captured by such CPCs based on extraterrestrial radiation and monthly horizontal radiation. Results show that the acceptance half-angle of 3P-CPCs is a unique parameter to determine their optical performance according to extraterrestrial radiation, and the annual solar gain stays constant if the acceptance half-angle, {theta}{sub a}, is less than one third of {omega}{sub 0,min}, the sunset hour angle in the winter solstice, otherwise decreases with the increase of {theta}{sub a}. For 3P-CPCs used in China, the annual solar gain, depending on the climatic conditions in site, decreased with the acceptance half-angle, but such decrease was slow for the case of {theta}{sub a}{

International audience; Since the Saturn orbit insertion (SOI) of the Cassini spacecraft, in July 2004, the Cassini Composite Infrared Spectrometer (CIRS) has obtained a large number of thermal infrared spectra of Saturn's rings. Over the two and a half years of observations to date, ring temperatures were retrieved for a large range of unique geometries, inaccessible from Earth. Understanding their dependencies with phase angle and local time is a clue to understanding the thermal properties and dynamics of Saturn's ring particles.Azimuthal scans of rings, which have been obtained by CIRS at constant radial distance from the planet, have been planned to measure ring temperature variations with local hour angle. Over 47 azimuthal scans for Saturn's main rings (A, B, C and Cassini Division) have been retrieved to date, on both lit and unlit sides, at different phase angles and spacecraft elevations. The first measurements of the transient thermal episode of eclipse cooling in the planetary shadow have also been obtained for all three rings.In this paper, we present an overview of all azimuthal scans obtained by the Cassini/CIRS instrument so far and the dependencies of the temperature and the filling factor with the phase angle and the local hour angle. The ring temperature varies with longitude as the input heating flux coming from Saturn and the Sun changes. The decrease in temperature with the increasing phase angle on both the lit and the unlit sides and for most of the local time also suggests the presence of slowly rotating particles. The crossing of the planet's shadow generates drastic azimuthal variations in temperature, up to 20 K in the C ring. The strong anisotropy of emission observed outside the shadow between low and high phase angles decreases when ring particles cross the shadow, suggesting that particles are almost isothermal in the shadow. This suggests a thermal inertia associated with a rotating rate of particles low enough to have a thermal contrast on their surface.The temperature in the B ring is less sensitive to the phase angle effect on the lit side, suggesting that particles are close enough to form a flat layer at a scale larger than the particle's radius. On the unlit side, particles in the B ring are less sensitive to the lack of solar input than in the C ring or in the A ring. Azimuthal variations of the filling factor in the A ring are also detected with changing ring local time. This effect might be created by the presence of gravitational instabilities (wakes).

In the first part of this series, we discussed the celestial sphere and Aryabhata’s principle of axial rotation; in this part we shall discuss in detail the concept of sidereal day and then mention Aryabhata’s computations on the duration of sidereal day.

To investigate the variability of the flux density, observations of four power radio sources (3C144, 3C274, 3C405 and 3C461) were carried out on the radio telescope URAN-4 at two frequencies, 20 and 25 MHz, during 1998–2005. The automatic procedure for obtaining the observations is considered briefly in this paper. The results of previous procedures are presented in the tables. They contained information that allows the vast amount of material that has been gathered to be analysed in more detail as follows: estimation of the source flux densities, study of ionospheric scintillations, investigation of the dependence of the direction pattern on the hour angle and the solving of other tasks. The dependence of the URAN-4 direction pattern on the hour angle is cited as an example of the mean procedure using many observation data. Long-time observation series allowed the cycle year dependence of the ionospheric scintillation index to be obtained.

An investigation of the stability of the transfer function of the European Southern Observatory's Very Large Telescope Interferometer has been carried out through observations of Fomalhaut over a wide range in hour angle. No significant variation in the transfer function was found for the zenith angle range 5-70 degrees. The projected baseline varied between 139.7 m and 49.8 m during the observations and, as an integral part of the determination of the transfer function, a new accurate limb-darkened angular diameter for Fomalhaut has been established. This has led to improved values for the emergent flux, effective temperature, radius and luminosity., Comment: 6 pages, 4 figures, uses an.cls, accepted for publication in Astron. Nacht

I describe a method for quickly and accurately determining the plate parallelism in a tunable Fabry-Perot interferometer. The method takes advantage of the fact that our Fabry-Perot is installed in a spectropolarimeter: we measure profiles of solar or telluric absorption lines in three differently polarized telescope subapertures and use the residual polarization profiles to determine the plate parallelism error. An example of the error as a function of time during the observing day is also shown; it changes in a consistent way with telescope hour angle but the drift is enough to require frequent adjustment.

We present observations of a sample of Herbig AeBe stars, as well as the FU Orionis object V1057 Cygni. Our K-band (2.2 microns) observations from the Palomar Testbed Interferometer (PTI) used baselines of 110m and 85m, resulting in fringe spacings of 4 mas and 5 mas, respectively. Fringes were obtained for the first time on V1057 Cygni as well as V594 Cas. Additional measurements were made of MWC147, while upper limits to visibility-squared are obtained for MWC297, HD190073, and MWC614. These measurements are sensitive to the distribution of warm, circumstellar dust in these sources. If the circumstellar infrared emission comes from warm dust in a disk, the inclination of the disk to the line of sight implies that the observed interferometric visibilities should depend upon hour angle. Surprisingly, the observations of Millan-Gabet, Schloerb, and Traub 2001 (hereafter MST) did not show significant variation with hour angle. However, limited sampling of angular frequencies on the sky was possible with the IOTA interferometer, motivating us to study a subset of their objects to further constrain these systems.

(Abridged) Atmospheric dispersion and field differential refraction impose severe constraints on widefield MOS observations. Flux reduction and spectral distortions must be minimised by a careful planning of the observations -- which is especially true for instruments that use slits instead of fibres. This is the case of VIMOS at the VLT, where MOS observations have been restricted, since the start of operations, to a narrow two-hour range from the meridian to minimise slit losses. We revisit in detail the impact of atmospheric effects on the quality of VIMOS-MOS spectra. We model slit losses across the entire VIMOS FOV as a function of target declination. We explore two different slit orientations at the meridian: along the parallactic angle (North-South), and perpendicular to it (East-West). We show that, for fields culminating at zenith distances larger than 20 deg, slit losses are minimised with slits oriented along the parallactic angle at the meridian. The two-hour angle rule holds for these observations using N-S orientations. Conversely, for fields with zenith angles smaller than 20 deg at culmination, losses are minimised with slits oriented perpendicular to the parallactic angle at the meridian. MOS observations can be effectively extended to plus/minus three hours from the meridian in these cases. In general, night-long observations of a single field will benefit from using the E-W orientation. All-sky or service mode observations, however, require a more elaborate planning that depends on the target declination, and the hour angle of the observations. We establish general rules for the alignment of slits in MOS observations that will increase target observability, enhance the efficiency of operations, and speed up the completion of programmes -- a particularly relevant aspect for the forthcoming spectroscopic public surveys with VIMOS., Accepted to A&A. 11 pages, 15 figures. This paper presents the new recommendations for optimal slit alignment in VLT/VIMOS observations

We demonstrate the global secular rotation of the Celestial Ephemeris Origin (CEO), the nonrotating origin (NRO) for Earth's equator adopted recently by the IAU. The current speed of this global rotation is as high as -415 yr-1, where the minus sign indicates that the CEO rotates clockwise with respect to the inertial frame when viewed from the north celestial pole. Such secular rotation does not exist for some geometrically defined longitude origins. We think that the existence of a global secular rotation means that the CEO, and the NRO in general, is not appropriate to be specified as the x-axis of a celestial coordinate system.

Space impacts retrieved to Earth, such as those on LDEF and EuReCa, have led to an unprecedentedly high definition of the impact environment in Earth orbit. This leads to (1) a penetration flux distribution (2) angular dependence of the flux (3) physical signatures of the penetration through thin films and (4) chemical evidence of impact residues. At micron dimensions, space retrieved surfaces show an “excess” flux, mainly when exposed to the leading (ram) direction, attributable to space debris or to micrometeoroids captured in Earth orbit. We present results of studies to determine the capture efficiency of the Earth to interplanetary dust and the dwell time of microparticles in eccentric orbits. Effects of the solar hour angle of the initial argument of perigee at capture are critical and, especially, the increase or decrease of atmospheric density at subsequent perigee passages due to solar heating cycles. Results will be presented of interest to the relative importance of space debris and of astrophysical sources.

The chapter starts by describing briefly the basic features of sun and then proceeds to deriving the relevant equations that allow the calculation of several parameters pertaining to the sun namely the eccentric anomaly, hour angle, and the position of the sun (azimuth and altitude) amongst others. The relevant formulae are valid at any spacial and temporal location on the earth. The rule which allows us to compute the sun’s position at any time is cross-checked with the US National Renewable Energy Laboratory’s Solar Position Algorithm and is shown to be in good agreement (\(

Space-geodetic measurement systems are capable of determining: (1) a terrestrial, body-fixed reference frame defined in practice by the stated positions and secular motions of a set of observing stations, (2) a celestial, space-fixed reference frame defined in practice by the stated locations of celestial objects, and (3) the rotation parameters linking these two frames together. Five parameters are conventionally used to specify the orientation of the terrestrial frame with respect to the celestial frame: two nutation parameters, two polar motion parameters, and one spin parameter. The celestial ephemeris pole (CEP) is defined as the north pole of that axis about which the spin parameter (UT1) is measured. The two nutation parameters locate the CEP in the celestial frame, and the two polar motion parameters locate the CEP in the terrestrial frame. By examining the frame transformation matrices, an expression relating the location of the rotation pole to that of the CEP can be derived. In order to compare theoretical predictions with observations, results of models for the effect on the nutations of geophysical excitation processes such as diurnal oceanic current and sea level height variations should not only be given in terms of the location of the CEP (rather than of the rotation pole), but must also account for the resonance effects of the free core nutation.

A Microsoft Excel application has been developed to utilize eclipsing binary elements to calculate binary ephemerides that are useful in a program of observing binary systems. Binary star systems consist of two stars orbiting a common center of mass. Consistent monitoring of eclipsing binary stars is important for establishing fundamental astronomical parameters, such as mass and composition of the stellar system. The application that was developed supports effective use of resources in such monitoring applications. The main quantities computed by the application at the time of the eclipse include: local time, Julian date, starting Epoch, altitude of the system, phase of the moon, Local Mean Sidereal Time, and Local Hour Angle. Other quantities computed by the application include current Universal Time (UT), UT/Local Time correction, Julian Date, and current moon phase/percent illuminated. The observing application has been demonstrated to be effective in practical application to obtain differential photometry data for SV Camelopardalis, a system of interest because of variability in its period caused by a third body. The application can be easily modified for use with other highly periodic astronomical systems, for instance transits of extrasolar planets.

The paper deals with the automatic grid azimuth determination of an object on the earth using the electronic theodolite Kern E2 by observing the sun, a star, or a planet. The observation time and the readings of horizontal and vertical circles of the electronic theodolite enter automatically the electronic calculator Hewlett-Packard HP41CX. The calculator computes the grid azimuth of the terrestrial object and directs the observer what to do. Therefore, even a personnel without specific high education can make observations. For the solar observations it is not necessary to have an astronomical almanac, because the program computes the solar coordinates. The general input data, together with the measured ones, can be stored on a magnetic cassette and later on, if necessary, one can correct the general data and recalculate the azimuth. The method proved to be very practical in the field work.

MaNGA (Mapping Nearby Galaxies at Apache Point Observatory) is an integral-field spectroscopic survey that is one of three core programs in the fourth-generation Sloan Digital Sky Survey (SDSS-IV). MaNGA’s 17 pluggable optical fiber-bundle integral field units (IFUs) will observe a sample of 10,000 nearby galaxies distributed throughout the SDSS imaging footprint (focusing particularly on the North Galactic Cap). In each pointing these IFUs are deployed across a 3◦ field; they yield spectral coverage 3600-10,300 Å at a typical resolution R 2000, and sample the sky with 2” diameter fiber apertures with a total bundle fill factor of 56%. Observing over such a large field and range of wavelengths is particularly challenging for obtaining uniform and integral spatial coverage and resolution at all wavelengths and across each entire fiber array. Data quality is affected by the IFU construction technique, chromatic and field differential refraction, the adopted dithering strategy, and many other effects. We use numerical simulations to constrain the hardware design and observing strategy for the survey with the aim of ensuring consistent data quality that meets the survey science requirements while permitting maximum observational flexibility. We find that MaNGA science goals are best achieved with IFUs composed of a regular hexagonal grid of optical fibers with rms displacement of 5 μm or less from their nominal packing position; this goal is met by the MaNGA hardware, which achieves 3 μm rms fiber placement. We further show that MaNGA observations are best obtained in sets of three 15-minute exposures dithered along the vertices of a 1.44 arcsec equilateral triangle; these sets form the minimum observational unit, and are repeated as needed to achieve a combined signal-to-noise ratio of 5 °A−1 per fiber in the r-band continuum at a surface brightness of 23 AB arcsec−2. In order to ensure uniform coverage and delivered image quality, we require that the exposures in a given set be obtained within a 60 minute interval of each other in hour angle, and that all exposures be obtained at airmass less than or similar to 1.2 (i.e., within 1-3 hours of transit depending on the declination of a given field). Publisher PDF

The ESO VLT Interferometer (VLTI) is a general-user facility and is operated in service mode (SM) for a large part of the available time. An important aspect of this SM observing mode is the definition of a set of critical observing conditions that must be met at the time of executing the requested observation. There are a number of observing constraints that are specific to interferometric observations, such as the choice of the array configuration and the hour angle at time of observation, which is processed during the scheduling. On the other hand, classical constraints such as the regular seeing or the lunar illumination are less critical for observations using VLTI instruments than for those using classical VLT instruments. In particular, the use of the adaptive optics system MACAO for VLTI observations employing the Unit Telescopes (UTs) ensures a very good image quality even for moderate environmental conditions. However, the exact dependence between environmental conditions, the performance of the MACAO systems, the wavefront quality at the interferometric instruments, and the accuracy of the final visibility, are not yet known in much detail. In order to investigate this dependence we have started to monitor routinely the environmental conditions, the quality of the MACAO systems, the quality of the acquisition images, and the final data product for all VLTI observations since June 2005. Here, we present the details of this study, as well as first statistics and results.

International audience; Aims.We present the first lab and sky validation of spectro-polarimetric equipment put at the combined focus of an optical long-baseline interferometer. We tested the polarimetric mode designed for the visible GI2T Interferometer to offer spectropolarimetric diagnosis at the milliarcsecond scale. Methods.We first checked the whole instrumental polarization in the lab with a fringe simulator, and then we observed α Cep and α Lyr as stellar calibrators of different declinations to tabulate the polarization effects throughout the GI2T declination range. Results.The difference between both linear polarizations is within the error bars and the visibilities recorded in natural light (i.e. without the polarimeter) for calibration purposes are the same order of magnitude as the polarized ones. We followed the α Cep visibility for 2 h after the transit and α Lyr for 1.5 h and detected no decrease with hour angle due to the fringe pattern smearing by instrumental polarization. Conclusions.Differential celestial rotation due to the dissymetric Coudé trains of the GI2T is well-compensated by the field rotators, so the instrumental polarization is controlled over a relatively wide hour angle range (±2 h around the transit at least). Such a polarimetric mode opens new opportunities especially for studies of circumstellar environments and significantly enhances both the potential of an optical array and its ability for accurate calibration.

We have previously described a system that derives the pointing coordinates of an equatorial telescope by measuring the angular position of a dual-axis tilt-table whose frame is rigidly attached to the telescope's primary mirror cell. In that system, two precision tilt-sensors aligned orthogonally and mounted in the plane of the table are used as nulling devices to close an active servo loop which holds the table level as the telescope moves. Rotary encoders measure the angle by which each tilt-table axis rotates, and a mathematical transform converts those encoder readings into telescope hour angle and declination. Recent work has indicated the feasibility of several simplifications to that system. First, by use of suitable low friction bearings on the tilt-table axes, along with non-contacting encoders, the active servo loop is no longer needed to level the tilt- table. Rather, a simple suspended weight keeps the platform almost level, with the residual small tilt error measured by the precision tilt sensors.Second, by suitable orientation of the weight and the tilt sensors relative to the telescope polar axis, the system can measure telescope hour angel and declination directly, eliminating the need for the complex mathematical transform. Experimental result using these ideas are presented.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

where /is is the hour angle of the Sun (angular distance from the meridian of a site in radians) at sunset. Sunrise and sunset are not exactly symmetrical about the time that the Sun reaches a local meridian, but for biological simulation purposes the symmetry implicit in equation (1) is acceptable. The geometric equation for sin a (where a is solar elevation) can be rearranged to find hs as follows

This report describes a method for using precision tilt-sensors to measure the position of an equatorial telescope relative to the local horizontal plane. Unlike conventional systems which measure the telescope position using position encoders coupled to the telescope axes, this method avoids many sources of non-repeatable error, such as hysteresis in the telescope structure due to inelastic flexure of the fork or yoke, or random slippage in the couplings between the position encoders and the telescope axes. In this respect, it shares many of the advantages of optical gyros, but achieves these at much lower cost. We present a design for a compact and relatively inexpensive dual-axis tilt-table whose frame is rigidly attached to the telescope's primary mirror cell. The table contains two precision tilt-sensors, aligned orthogonally with the tilt axes of the table. The sensors are used as nulling devices to close a servo loop which keeps the table level at all times. This provides a precise and stable reference against which the telescope position is measured. A high resolution incremental encoder is directly coupled to each tilt-table axis and measures the angle by which that axis rotates to keep the table level. A mathematical transform converts these two encoder readings into local hour angle and declination. Preliminary tests of the tilt sensors and of a single-axis prototype tilt-table are reported, and future plans described. The use of tilt-tables for measuring the positions of non-equatorial telescopes is also briefly examined.

Guinot suggested in 1979 to replace the vernal equinox by the so-called “non-rotating origin” as the first axis of the celestial equatorial system. This paper derives the analytical relation between the traditional and the alternative equatorial systems by means of their rotation vectors. Under the assumption of a regular precession of the mean celestial pole, the motions of the rotation vector and the first axis of the alternative mean equatorial system with respect to a space-fixed ecliptical system are investigated and illustrated.

Penn State University and the University of Texas are collaborating on an Arecibo-type opticaltelescope to be employed in a semi-transit mode for spectroscopic survey work. This telescope, clampedfirmly to its foundation during observation, will employ a 5-axis servo-mechanism in the telescope'supper section to track an object's image locus on the focal sphere. Once the telescope is clamped in anazimuth position, a transit time for each object relative to the telescope's structure may be determined,and therefore a strategy may be derived for driving the fiber-carrying optical head into conjunction with the image locus. The transit time is defined as the time when the image crosses the "transit" plane containing the primary mirror's center of curvature and normal to the central tangent of the image locus.The trigonometric mappingbetween the hour angle and declination functions of each object and the five coordinates x, y, 0, p, and PZ , (representing

In 1978, Guinot proposed that, for studies of Earth rotation, the zero point of the apparent “right ascension” coordinate on the true equator should be so chosen that the rate of change of its hour angle is exactly proportional to the inertial rate of rotation of the Earth. It has been subsequently suggested that this concept of the “non-rotating origin” supersede the equinox quite generally as the origin of celestial coordinates. Since this proposal was first put forward, there has been much discussion, and some criticism, from Aoki and his colleagues, both published and in private correspondence. Some of the arguments for and against Guinot’s proposal are discussed, as a contribution to the wider debate on reference systems now being carried out under the auspices of the IAU.

The aim of this study is a uniform system of equatorial spectrophotometric standards to be used with large and intermediatesize telescopes. We plan to obtain absolute energy distributions for 43 equatorial (lcl < 3O) intermediate-brightness (6.5m < V < 8.5m) stars in the spectral region of 3150-7550 A with a resolution of 50 A. The criterion for the selection of the standard stars is the following: at any time, a pair of stars should be observable near the sky meridian (hour angle < 1 h) - one early-type (B9-A2) star and one intermediate-type (GO-G5) star. It is important that both stars are equally observable from both hemispheres of the Earth. The homogeneity of the standard-star system is achieved with the attachment of all the stars to a single primary standard - the circumpolar star HD 221525 (V = 5.58; FOIII); we obtained the energy distribution for this star earlier (Tereshchenko and Glushkova, 1992). Note that we used this star as a primary standard in our work on a spectrophotometric version of the North Polar Sequence (Tereshchenko, 1994). Each star should be observed not less than four times in different nights; the expected standard deviations would be 2-4%. We plan also to observe the spectrophotometric standards in the UBVR system, too. We plan to carry out the spectrophotometric and photometric observations in the high-altitude conditions, on the 1-m and 0.5-m telescopes, with the concave-grating spectrograph and a: UBVB photometer. Partly, this work has already been done. To reveal random and systematic errors in the observations, as well as possible variability of some of the selected standard stars, check-up observations with different instruments in other conditions are highly desirable. In this connection, the author would like to attract the attention of other observers to this program. Table 1 lists the equatorial standard stars.

Solar irradiance received on a horizontal surface depends on the solar output, the semimajor axis of the elliptical orbit of the Earth around the sun (a), the distance from the Earth to the sun (r), and the zenith distance (z). The spectrum of the distance, r, for a given value of the true longitude, lambda, displays mainly the precessional periods and, with much less power, half precession periods, eccentricity periods, and some combination tones. The zenith distance or its equivalent, the elevation angle (E), is only a function of obliquity (epsilon) for a given latitude, phi, true longitude, and hour angle, H. Therefore the insolation at a given constant value of z is only a function of precession and eccentricity. On the other hand, the value of the hour angle, H, corresponding to this fixed value of z varies with epsilon, except for the equinoxes, where H corresponding to a constant z also remains constant through time. Three kinds of insolation have been computed both analytically and numerically: the instantaneous insolation (irradiance) at noon, the daily irradiation, and the irradiations received during particular time intervals of the day defined by two constant values of the zenith distance (diurnal irradiations). Mean irradiances (irradiations divided by the length of the time interval over which they are calculated) are also computed for different time intervals, like the interval between sunrise and sunset, in particular. Examples of these insolations are given in this paper for the equinoxes and the solstices. At the equinoxes, for each latitude, all insolations are only a function of precession (this invalidates the results obtained by Cerveny [1991)). At the solstices, both precession and obliquity are present, although precession dominates for most of the latitudes. Because the lengths of the astronomical seasons are secularly variable (in tenus of precession only), a particular calendar day does not always correspond to the same position relative to the sun through geological time. Similarly, a given longitude of the Sun on its orbit does not correspond to the same calendar day. For example, 103 kyr ago, assuming arbitrarily that the spring equinox is always on March 21, autumn began on September 13, and 114 kyr ago, it began on September 27, the length of the summer season being 85 and 98 calendar days, respectively, at these remote times in the past.

We describe a method for reducing displacements in radio source positions caused by ionospheric refraction. Our method is an improvement on that used by Wild et al. (1959), who assumed all frequencies in the 40-70 MHz band came from the same position above the Sun and applied an f-2 correction for ionospheric refraction. Our method retains the dispersion of source position with frequency, which is inherent in the radio source, but allows for the f-2 ionospheric effect. We also discuss ways of estimating the absolute source positions from a knowledge of ionospheric density gradients. A typical example of ionospheric variations on solar records is shown in Figure 1(a). Here we have plotted the observed source positions on an f-2 scale. The measurements refer to a solar storm continuum burst which occurred on 1981 May 9 following an importance 2B flare at 08°N., 38°E., heliographic coordinates. From previous observations of such events we think it highly likely that the true source positions are stationary and displaced with decreasing frequency outwards along a line close to the radial direction above the flare. Figure 1(a) shows that: (a) the 160, 80 and 43 MHz source displacements vary as f-2; (b) there is a systematic slow drift of the source positions towards the south and east which increases with increasing hour angle; (c) superimposed on this steady drift is a quasi-periodic variation in source position with a period ~20 min.

A microcomputer controlled all sky scanning radiometer covering the short wave (0.3–3.0 μm) and long wave (3.0–80.0 μm) regions and using a pyroelectric detector has been constructed for continuous automatic measurement of sky radiance. Angular distributions of short, long, and total wave sky radiance are recorded and plotted at each solar hour angle from which irradiances of tilted surfaces are computer calculated. Measurement of the local earth surface radiance facilitates estimation of short wave earth surface albedo and net income of radiation to variously oriented black surfaces. Total scanning, computation, and plotting time for each hour angle observation is 8 min.

The parallactic refraction correction should be applied to the photographic observations of the light signals carried either by artificial satellites or by meteorological balloons. The correction to the right ascension and declination are made with the equations where ϕ is the latitude of the observation station, z the zenith distance, α the right ascension, δ the declination and h the hour angle of the light signal. Parallactic refraction is denoted by Δζ.

The two more widely used formulae for the time distribution of the shortwave absorbed radiation are discussed and compared with field data in order to evaluate the net radiative flux on a horizontal surface. A very good agreement with the observed values is obtained by introducing into the formula for the shortwave input, a time-dependent non-dimensional parameter modifying the hour angle towards greater effective values, depending on atmospheric conditions, albedo variations and longwave balance.

A specific class of paraboloidal reflectors namely the offsetfeed paraboloidal reflectors which find application in space communications is utilized for concentration of solar energy at a fixed point (focus) maintained throughout the day in shade. The energy distributions on spherical receiver centered around the focus and on a flat sectorial receiver held in a plane perpendicular to the focal line have been studied for the case of perfect tracking in hour angle and declination axes. The spill-over of energy due to non-tracking in hour-angle or declination axis for specific periods in the case of a flat sectorial receiver has been studied and plotted. A summary of the results and suggested applications of this configuration are presented. A comparison of performance is made between wedge-type and circular aperture type offsetfeed reflectors.

The exact description of the Earth's rotation raises the problem of the choice of a reference point on the instantaneous equator both in space and in the Earth. We propose to use, as the reference point in space, a ‘non-rotating origin’ (Guinot 1979) such that its hour angle, reckoned from the origin of the longitudes (or ‘non-rotating origin’ in the Earth), represents strictly the sidereal rotation of the Earth. Such an origin on the instantaneous equator depends only on the motion of the pole of rotation; it is practically realizable from a chosen fixed reference and we give the formulae to obtain it in space and in the Earth. We show that the estimation of the sidereal rotation is not critically affected by the precision with which the trajectory of the pole is known. We therefore propose a definition of the Universal Time which will remain valid even if the adopted model for the precession and the nutation is revised. We show that the use of the non-rotating origin also simplifies the transformation of coordinates between the terrestrial and celestial reference systems. An additional simplification of this transformation would be obtained when using, in the precession and nutation matrixes the development of the celestial coordinates of the pole as function of time in place of the various usual equatorial and ecliptic parameters. The use of the non-rotating origin instead of the equinox would thus have advantages for both conceptual and practical reasons.

The orbit of Asteroid 1976 AA is described, with attention given to calculations of its period and its distance from earth, both of which could be accurately and quickly determined by measuring the minor planet's position over wide ranges of hour angle on one to three nights. The geometry of the asteroid's orbit is compared to that of earth's orbit, and the periodicity of the minor planet's approaches to earth is projected. The motion of 1976 AA over an interval of seven centuries into both past and future is also studied; the possibility of its libration with respect to earth or to Venus is examined. Some data on closest approaches of the asteroid to Mars and Venus, as well as to earth, are given.