The Wide Field Spectrograph (WiFeS)

This paper describes the Wide Field Spectrograph (WiFeS) under construction at the Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (ANU) for the ANU 2.3 m telescope at the Siding Spring Observatory. WiFeS is a powerful integral field, double-beam, concentric, image-slicing spectrograph designed to deliver excellent throughput, wavelength stability, spectrophotometric performance and superb image quality along with wide spectral coverage throughout the 320–950 nm wavelength region. It provides a 25×38 arcsec field with 0.5 arcsec sampling along each of twenty five 38×1 arcsec slitlets. The output format is optimized to match the 4096×4096 pixel CCD detectors in each of two cameras individually optimized for the blue and the red ends of the spectrum, respectively. A process of “interleaved nod-and-shuffle” will be applied to permit quantum noise-limited sky subtraction. Using VPH gratings, spectral resolutions of 3000 and 7000 are provided. The full spectral range is covered in a single exposure at R=3000, and in two exposures in the R=7000 mode. The use of transmissive coated optics, VPH gratings and optimized mirror coatings ensures a throughput (including telescope atmosphere and detector) >30% over a wide spectral range. The concentric image-slicer design ensures an excellent and uniform image quality across the full field. To maximize scientific return, the whole instrument is configured for remote observing, pipeline data reduction, and the accumulation of calibration image libraries.     

Image Stabilization System for Hinode (Solar-B) Solar Optical Telescope

The Hinode Solar Optical Telescope (SOT) is the first space-borne visible-light telescope that enables us to observe magnetic-field dynamics in the solar lower atmosphere with 0.2 – 0.3 arcsec spatial resolution under extremely stable (seeing-free) conditions. To achieve precise measurements of the polarization with diffraction-limited images, stable pointing of the telescope (<0.09 arcsec, 3σ) is required for solar images exposed on the focal plane CCD detectors. SOT has an image stabilization system that uses image displacements calculated from correlation tracking of solar granules to control a piezo-driven tip-tilt mirror. The system minimizes the motions of images for frequencies lower than 14 Hz while the satellite and telescope structural design damps microvibration in higher frequency ranges. It has been confirmed from the data taken on orbit that the remaining jitter is less than 0.03 arcsec (3σ) on the Sun. This excellent performance makes a major contribution to successful precise polarimetric measurements with 0.2 – 0.3 arcsec resolution. K. Kobayashi now at NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA.     

Adaptive Optics on Large Telescopes

Observations withground based telescopes suffer from atmospheric turbulence.Independent of the telescope size the angular resolution inthe visible is equivalent to that of a telescope with adiameter of 10–20 cm. This effect is caused by the turbulentmixing of air with different temperatures in the atmosphere.Thus, the perfect plane wave from a star at infinity isaberrated before it enters the telescope. In the following,we will discuss the physical background of imaging throughturbulence, using Kolmogorov statistics, and the differenttechniques to sense and to correct the wave-front aberrationswith adaptive optics. The requirements for the control loop ofan adaptive optics system are discussed including formulas forthe limiting magnitude of the guide star as a function of thewave-front sensing method, of the quality of the wave-frontsensor camera, and of the degree of correction. Finally, ashort introduction to deformable mirror technology will begiven followed by the presentation of a new method to measureand to distinguish individual turbulent layers in order toincrease the isoplanatic angle.     

Mechanical design of the solar telescope GREGOR

The mechanical structure of the GREGOR telescope was installed at the Observatorio del Teide, Tenerife, in 2004. New concepts for mounting and cooling of the 1.5‐meter primary mirror were introduced. GREGOR is an open telescope, therefore the dome is completely open during observations to allow for air flushing through the open, but stiff telescope structure. Backside cooling system of the primary mirror keeps the mirror surface close to ambient temperature to prevent mirror seeing. The large collecting area of the primary mirror results in high energy density at the field stop at the prime focus of the primary which needs to be removed. The optical elements are supported by precision alignment systems and should provide a stable solar image at the optical lab. The coudé train can be evacuated and serves as a natural barrier between the outer environmental conditions and the air‐conditioned optical laboratory with its sensitive scientific instrumentation. The telescope was successfully commissioned and will start its nominal operation during 2013 (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Las campanas observatory seeing measurements

Instrumentation built to record seeing data automatically via image motion measurements of bright stars in small telescopes is described. The centroid of the star image is found 256 times s^-1 in one dimension and is analyzed on-line. The device works over a range of FWHM values as would be seen through a large telescope between <0.1 and 3.0 arcsec. The first results for two identical instruments set up at two locations near the duPont Telescope at Las Campanas Observatory are reported. For a total of 61 nights of data (450 h at each site), the median seeing is 0.6 arcsec, with quartiles at 0.4 and 0.8 arcsec. These are FWHM values referred to 5000 Å at the zenith. So far, the two sites are indistinguishable on average.     

A wide-field corrector at the prime focus of a Ritchey-Chrétien telescope

We propose a form of a lens corrector at the prime focus of a hyperboloidal mirror that provides a flat field of view up to 3° in diameter at image quality D₈₀<0.8 arcsec in integrated (0.32–1.1 µm) light. The corrector consists of five lenses made of fused silica. All lens surfaces are spherical in shape, so the system is capable of achieving better images, if necessary, by aspherizing the surfaces. The optical system of the corrector is stable in the sense that its principal features are retained when optimized after significant perturbations of its parameters. As an example, we calculated three versions of the corrector for the Blanco 4-m telescope at Cerro Tololo Inter-American Observatory with \(2\mathop .\limits^ \circ 12\), \(2\mathop .\limits^ \circ 4\) and \(3\mathop .\limits^ \circ 0\) fields of view.     

Herschel SPIRE FTS telescope model correction

Emission from the Herschel telescope is the dominant source of radiation for the majority of SPIRE Fourier transform spectrometer (FTS) observations, despite the exceptionally low emissivity of the primary and secondary mirrors. Accurate modelling and removal of the telescope contribution is, therefore, an important and challenging aspect of FTS calibration and data reduction pipeline. A dust-contaminated telescope model with time invariant mirror emissivity was adopted before the Herschel launch. However, measured FTS spectra show a clear evolution of the telescope contribution over the mission and strong need for a correction to the standard telescope model in order to reduce residual background (of up to 7 Jy) in the final data products. Systematic changes in observations of dark sky, taken over the course of the mission, provide a measure of the evolution between observed telescope emission and the telescope model. These dark sky observations have been used to derive a time dependent correction to the telescope emissivity that reduces the systematic error in the continuum of the final FTS spectra to ～0.35 Jy.     

The SWAP EUV Imaging Telescope Part I: Instrument Overview and Pre-Flight Testing

The Sun Watcher with Active Pixels and Image Processing (SWAP) is an EUV solar telescope onboard ESA’s Project for Onboard Autonomy 2 (PROBA2) mission launched on 2 November 2009. SWAP has a spectral bandpass centered on 17.4 nm and provides images of the low solar corona over a 54×54 arcmin field-of-view with 3.2 arcsec pixels and an imaging cadence of about two minutes. SWAP is designed to monitor all space-weather-relevant events and features in the low solar corona. Given the limited resources of the PROBA2 microsatellite, the SWAP telescope is designed with various innovative technologies, including an off-axis optical design and a CMOS–APS detector. This article provides reference documentation for users of the SWAP image data.     

The optical reflector system for the CANGAROO-II imaging atmospheric Cherenkov telescope

A new imaging atmospheric Cherenkov telescope with a light-weight reflector has been constructed. Light, robust, and durable mirror facets of containing carbon fiber reinforced plastic laminates were developed for the telescope. The reflector has a parabolic shape (f/1.1) with a 30 m² surface area, which consists of 60 spherical mirror facets. The image size of each mirror facet is 0°.08 (FWHM) on average. The attitude of each facet can be adjusted by stepping motors. After the first in situ adjustment, a point image of about 0°.14 (FWHM) over 3° field of view was obtained. The effect of gravitational load on the optical system was confirmed to be negligible at the focal plane. The telescope has been in operation with an energy threshold for γ-rays of 300 GeV since May 1999.     

山东大学威海天文台视宁度研究

山东大学威海天文台拥有口径1 m的赤道式反射光学望远镜,于2007年6月建成并投入使用。对天文台2008年、2009年的所有观测数据用编写的IRAF自动处理程序进行处理得到了大气视宁度值,对得到的大气视宁度进行了分析研究,并与天文台气象站获得的气象数据一起进行了分析。经分析得到了山东大学威海天文台的大气视宁度状况,同时得到了一些大气视宁度随气象因素变化的规律。     

Calibration of the ART-XC mirror modules at MSFC

The Astronomical Röntgen Telescope X-ray Concentrator (ART-XC) is a hard X-ray telescope with energy response up to 30 keV, to be launched on board the Spectrum Röntgen Gamma (SRG) spacecraft in 2018. ART-XC consists of seven identical co-aligned mirror modules. Each mirror assembly is coupled with a CdTe double-sided strip (DSS) focal-plane detector. Eight X-ray mirror modules (seven flight and one spare units) for ART-XC were developed and fabricated at the Marshall Space Flight Center (MSFC), NASA, USA. We present results of testing procedures performed with an X-ray beam facility at MSFC to calibrate the point spread function (PSF) of the mirror modules. The shape of the PSF was measured with a high-resolution CCD camera installed in the focal plane with defocusing of 7 mm, as required by the ART-XC design. For each module, we performed a parametrization of the PSF at various angular distances Θ. We used a King function to approximate the radial profile of the near on-axis PSF (Θ < 9 arcmin) and an ellipse fitting procedure to describe the morphology of the far off-axis angular response (9 < Θ < 24 arcmin). We found a good agreement between the seven ART-XC flight mirror modules at the level of 10%. The on-axis angular resolution of the ART-XC optics varies between 27 and 33 arcsec (half-power diameter), except for the spare module.     

Design and development of the Advanced Technology Solar Telescope

Led by the National Solar Observatory, plans have been made to design and to develop the Advanced Technology Solar Telescope (ATST). The ATST will be a 4‐m general‐purpose solar telescope equipped with adaptive optics and versatile post‐focus instrumentation. Its main aim will be to achieve an angular resolution of 0.03 arcsec (20 km on the solar surface). The project and the telescope design are briefly described.     

SARG: The High Resolution Spectrograph of TNG

SARG is a cross dispersed echelle spectrograph in operation since late spring 2000 at the Italian Telescopio Nazionale Galileo (TNG) 3.5 m telescope, La Palma. SARG offers both single object and long slit (up to 26 arcsec) observing modes covering a spectral range from λ = 0.37 up to1 μm, with resolution ranging from R = 29,000 up to R = 164,000. Cross dispersion is provided by means of a selection of four grisms; interference filters may be used for the long slit mode (up to 26 arcsec). A dioptric camera images the cross dispersed spectra onto a mosaic of two 2048 × 4096 EEV CCDs (pixel size: 13.5 μm) allowing complete spectral coverage at all resolving power for λ < 0.8 μm. In order to reach a high wavelength calibration precision an iodine-absorbing cell is provided. A Distributed Active Temperature Control System (DATCS) maintains constant the temperature of all spectrograph components at a preset value. Early results show that SARG works according to original specifications in terms of wavelength coverage, efficiency (measured peak efficiency is about 13%),resolution (maximum resolution R = 164,000 using a 0.3 arcsec slit, R ∼144,000 using an image slicer), and stability (preliminary estimates of radial velocity accuracy is ∼3 m/s using the iodine cell and ±150 m/s without the iodine cell). This revised version was published online in July 2006 with corrections to the Cover Date.     

Development of a low-order Adaptive Optics system at Udaipur Solar Observatory

A low-order Adaptive Optics (AO) system is being developed at the Udaipur Solar Observatory and we present in this paper the status of the project, which includes the image stabilization system and calibration of wavefront sensor and deformable mirror. The image stabilization system comprises of a piezo driven tip-tilt mirror, a high speed camera (955 fps), a frame grabber system for sensing the overall tilt and a Linux based Intel Pentium 4 control computer with Red Hat Linux OS. The system operates under PID control. In the closed loop, an rms image motion of 0.1–0.2 arcsec was observed with the improvement factor varying from 10–20 depending on the external conditions. Error rejection bandwidth of the system at 0 dB is 80–100 Hz. In addition to that, we report the on-going efforts in the calibration of lenslet array and deformable mirror for sensing and correcting the local tilt of the wavefront.     

Frequency spectra of solar image motion

The frequency analysis of image motion (IM) at the solar limb was carried out in the frequency range from 0.5 to 50 Hz using a photoelectric equipment. For a telescopic aperture of 35 cm and a bandwidth of 0.65 Hz a typical frequency spectrum under average observing conditions shows a decrease of amplitude from 2 arcsec at 0.5 Hz to 0.4 arcsec at 5 Hz, 0.03 arsec at 50 Hz (and < 0.01 arcsec at 500 Hz). Visually estimated values of image steadiness seem to be in better agreement with the r.m.s. value of image motion (scattering parameter ) than with the amplitude at a certain frequency (Figures 5a, b). The influence of IM on the quality of photographic pictures or on spectra of solar fine structures is calculated as a function of exposure time. Table II gives the IM scattering parameters (0.01 arcsec to 4 arcsec) calculated for exposure times from 0.001 to 0.5 sec — valid for a time average. The modulation transfer functions (MTF, one-dimensional) derived from the IM scattering parameters are presented in Figure 7 together with the MTF for a diffraction-limited telescope of 35 cm aperture at 6000 Å. Exposure times of less than approximately 0.01 sec (certain within a factor of 2) render the influence of IM negligible compared to the MTF of the objective used for this investigation.Mitteilungen aus dem Fraunhofer Institut Nr. 88.     

The Solar Optical Telescope of Solar-B (Hinode): The Optical Telescope Assembly

The Solar Optical Telescope (SOT) aboard the Solar-B satellite (Hinode) is designed to perform high-precision photometric and polarimetric observations of the Sun in visible light spectra (388 – 668 nm) with a spatial resolution of 0.2 – 0.3 arcsec. The SOT consists of two optically separable components: the Optical Telescope Assembly (OTA), consisting of a 50-cm aperture Gregorian with a collimating lens unit and an active tip-tilt mirror, and an accompanying Focal Plane Package (FPP), housing two filtergraphs and a spectro-polarimeter. The optomechanical and optothermal performance of the OTA is crucial to attain unprecedented high-quality solar observations. We describe in detail the instrument design and expected stable diffraction-limited on-orbit performance of the OTA, the largest state-of-the-art solar telescope yet flown in space.     

Fiber-optic magnetometer for prime focus of 6-m BTA telescope based on suspended echelle spectrograph

We have developed and manufactured a fiber-optic magnetometer for the prime focus of the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences based on a suspended echelle spectrograph. The magnetometer is designed to improve the spectrum stability and eliminate the effect of instrumental polarization of the diagonal mirror on the results of magnetic field measurements. The magnetometer is to be used for measurements of stellar magnetic fields in stars and to study their chemical composition. The instrument operates in the 5000–6800 Å wavelength interval with mean reciprocal dispersion of 0.15 Å/pixel. According to the estimates for 9 ^m .5 stars, the standard error of magnetic field measurements based on 400 spectral lines would be 100 G for a half-hour exposure.