Re-imaging Optics Research Articles

HiPERCAM: a quintuple-beam, high-speed optical imager on the 10.4-m Gran Telescopio Canarias

ABSTRACT HiPERCAM is a portable, quintuple-beam optical imager that saw first light on the 10.4-m Gran Telescopio Canarias (GTC) in 2018. The instrument uses re-imaging optics and four dichroic beamsplitters to record $u_{\rm s}\, g_{\rm s}\, r_{\rm s}\, i_{\rm s}\, z_{\rm s}$ (320–1060 nm) images simultaneously on its five CCD cameras, each of 3.1-arcmin (diagonal) field of view. The detectors in HiPERCAM are frame-transfer devices cooled thermo-electrically to 183 K, thereby allowing both long-exposure, deep imaging of faint targets, as well as high-speed (over 1000 windowed frames per second) imaging of rapidly varying targets. A comparison-star pick-off system in the telescope focal plane increases the effective field of view to 6.7 arcmin for differential photometry. Combining HiPERCAM with the world’s largest optical telescope enables the detection of astronomical sources to gs ∼ 23 in 1 s and gs ∼ 28 in 1 h. In this paper, we describe the scientific motivation behind HiPERCAM, present its design, report on its measured performance, and outline some planned enhancements.

Installation of solar chromospheric telescope at the Indian Astronomical Observatory, Merak

We report the observations of the solar chromosphere from a newly commissioned solar telescope at the incursion site near Pangong Tso lake in Merak (Leh/Ladakh). This new H$_{\alpha}$ telescope at the Merak site is identical to the Kodaikanal H$_{\alpha}$ telescope. The telescope is installed in the month of August, 2017 at the Merak site. A 20-cm doublet lens with additional re-imaging optics makes the telescope. A Lyot filter with 0.5~\AA~passband isolates the Balmer line of the hydrogen spectra to make the observations of the solar chromosphere. The observations made in H$_{\alpha}$ wavelength delineates the magnetic field directions at the sunspot and the quiet regions. A CCD detector records the images of the chromosphere with a pixel resolution of 0.27$^{\prime\prime}$ and covers 9.2$^{\prime}$ field-of-view. This telescope has a good guiding system that keeps the FOV in the intended position. We report the development of control software for tuning the filter unit, control detector system, observations, and calibration of the data to make it useful for the scientific community. Some preliminary results obtained from the Merak H$_{\alpha}$ telescope are also presented. This high altitude facility is a timely addition to regularly available H$_{\alpha}$ images around the globe.

Coherent Multibeam Arrays Using a Cold Aperture Stop

To increase the mapping speed of a given area-of-sky, multibeam heterodyne arrays may be used. Since typical heterodyne arrays are spatially arranged sparsely at approximately 4· Nyquist sampling (i.e., two full-width-half-maximum beam widths), many pointings are required to sample fully the area of interest. A cold aperture stop may be used to increase the packing density of the detectors, which results in a denser instantaneous spatial sampling on-sky. Combining reimaging optics with the cold stop, good aperture efficiency can be obtained. As expected, however, a significant amount of power is truncated at the stop and the surrounding baffling. We analyze the consequence of this power truncation and explore the possibility of using this layout for coherent detection as a multibeam feed. We show that for a fixed area-of-sky, a “twice-Nyquist” spatial sampling arrangement may improve the normalized point source mapping speed when the system noise temperature is dominated by background or atmospheric contribution.

Improved mid infrared detector for high spectral or spatial resolution and synchrotron radiation use.

When using bright, small effective size sources, such as synchrotron radiation light beam, for broadband spectroscopy at spectral or spatial high resolution for mid-IR FTIR measurements, a marked detectivity improvement can be achieved by setting up a device matching the detector optical étendue to that of the source. Further improvement can be achieved by reducing the background unmodulated flux and other intrinsic noise sources using a lower temperature cryogen, such as liquid helium. By the combined use of cooled apertures, cold reimaging optics, filters and adapted detector polarization, and preamplification electronics, the sensitivity of a HgCdTe photoconductive IR detector can be improved by a significant factor with respect to standard commercial devices (more than one order of magnitude on average over 6-20 μm region) and the usable spectral range extended to longer wavelengths. The performances of such an optimized detector developed on the AILES Beamline at SOLEIL are presented here.

ULTRASPEC: a high-speed imaging photometer on the 2.4-m Thai National Telescope

ULTRASPEC is a high-speed imaging photometer mounted permanently at one of the Nasmyth focii of the 2.4-m Thai National Telescope (TNT) on Doi Inthanon, Thailand's highest mountain. ULTRASPEC employs a 1024 × 1024 pixel frame-transfer, electron-multiplying CCD (EMCCD) in conjunction with re-imaging optics to image a field of 7.7 × 7.7 arcmin2 at (windowed) frame rates of up to ∼200 Hz. The EMCCD has two outputs – a normal output that provides a readout noise of 2.3 e− and an avalanche output that can provide essentially zero readout noise. A six-position filter wheel enables narrow-band and broad-band imaging over the wavelength range 330–1000 nm. The instrument saw first light on the TNT in 2013 November and will be used to study rapid variability in the Universe. In this paper we describe the scientific motivation behind ULTRASPEC, present an outline of its design and report on its measured performance on the TNT.

Anti-reflection Coating for Cryogenic Silicon and Alumina Lenses in Millimeter-Wave Bands

A dielectric lens with high refractive index is suitable for focusing cryogenic devices in millimeter-wave bands when an appropriate anti-reflection (AR) coating is applied. Two types of AR coatings for silicon and alumina were studied at the millimeter-wave (220 GHz) band: one is by direct machining of mixed epoxy for a silicon lens array, while the other is by laser machining of an antireflective subwavelength structure for a large alumina lens used in a re-imaging optics system. The millimeter-wave optical properties of silicon, alumina, aluminum nitride, and Stycast epoxies were measured with a Fourier Transform Spectrometer (FTS) at cryogenic temperatures. The measured refractive index of the AR coating with a mixture of Stycast 1266 (n = 1.68) and Stycast 2850FTJ (n = 2.2) for silicon at 30 K was 1.84. The thickness of the epoxy AR coating was precisely controlled with direct machining. Transmittance of the AR-coated silicon substrate, measured with FTS, was approximately 95 % at the center frequency of the 220 GHz band with a bandwidth of 25 % at 27 K. An antireflective subwavelength structure was designed for an alumina sample with periodic cylindrical holes. The measured 220-GHz-band transmittance was above 90 % with a bandwidth of 25 % at 25 K.

Optical design of the Atacama Cosmology Telescope and the Millimeter Bolometric Array Camera

The Atacama Cosmology Telescope is a 6 m telescope designed to map the cosmic microwave background simultaneously at 145, 215, and 280 GHz with arcminute resolution. Each frequency will have a 32 by 32 element focal plane array of transition edge sensor bolometers. The telescope and the cold reimaging optics are optimized for millimeter-wave observations with these sensitive detectors. The design of each is described.

Predictions and Strategies for Integral-Field Spectroscopy of High-Redshift Galaxies

We investigate the ability of infrared integral-field spectrographs to map the velocity fields of high redshift galaxies, presenting a formalism which may be applied to any telescope and imaging spectrograph system. We discuss the 5-sigma limiting line fluxes which current integral-field spectrographs will reach, and extend this discussion to consider future large aperture telescopes with cryogenically cooled adaptive reimaging optics. In particular, we simulate observations of spectral line emission from star-forming regions at redshifts z = 0.5 to 2.5 using a variety of spatial sampling scales and give predictions for the signal-to-noise ratio expected as a function of redshift. Using values characteristic of the W.M. Keck II telescope and the new OH-Suppressing InfraRed Imaging Spectrograph (OSIRIS) we calculate integral-field signal-to-noise ratio maps for a sample of U_nGR color-selected star-forming galaxies at redshift z ~ 2 - 2.6 and demonstrate that OSIRIS will be able to reconstruct the two-dimensional projected velocity fields of these galaxies on scales of 100 mas (~ 1 kpc at redshift z ~ 2). With signal-to-noise ratios per spatial sample up to ~ 20, OSIRIS will in some cases be able to distinguish between merger activity and ordered disk rotation. Structures on scales smaller than 1 kpc may be detected by OSIRIS for particularly bright sources, and will be easy targets for future 30m class telescopes.

BOOMERANG: A Balloon‐borne Millimeter‐Wave Telescope and Total Power Receiver for Mapping Anisotropy in the Cosmic Microwave Background

We describe BOOMERANG, a balloon-borne microwave telescope designed to map the cosmic microwave background at a resolution of 10' from the Long Duration Balloon (LDB) platform. The millimeter-wave receiver employs new technology in bolometers, readout electronics, cold reimaging optics, millimeter-wave filters, and cryogenics to obtain high sensitivity to cosmic microwave background anisotropy. Sixteen detectors observe in four spectral bands centered at 90, 150, 240, and 410 GHz. The wide frequency coverage, the long-duration flight, the optical design, and the observing strategy provide strong rejection of systematic effects. We report the flight performance of the instrument during a 10.5 day stratospheric balloon flight launched from McMurdo Station, Antarctica, that mapped ~2000 square degrees of the sky

THE FIRST DIFFRACTION-LIMITED IMAGES FROM THE W. M. KECK TELESCOPE

ABSTRACT The first diffraction limited, 0.05" resolution, images on the W. M. Keck Telescope have been obtained at a wavlength of 2.2 microns. These images were part of an experiment to test the suitability of the Keck Telescope for speckle imaging. In order to conduct this test, it was necessary to modify the pixel scale of the Keck facility Near Infrared Camera (NIRC) to optimally sample the spatial frequencies made available by the Keck Telescope. The design and implementation of the external reimaging optics, which convert the standard f/25 beam from the secondary mirror to f/182 are described here. Techniques for reducing speckle data with field rotation on an alt-az telescope are also described. Three binary stars were observed in this experiment with separations as small as 0.05". With only 100 frames of data on each, a dynamic range of at least 3.5 mag was achieved in all cases. These observations imply that a companion as faint as 14.5 mag at 2.2 microns could be detected around an 11th magnitude point source.

Infrared imaging and spectroscopy with arrays at the University of Montreal

The Montreal Infrared Camera (MONICA) consists of a LN2 cryostat enclosing a NICMOS3 array, re-imaging optics and a filter wheel mechanism containing broad-band and circular variable filters. A polarimetry mode with a warm half wave plate and a cold polarizer/filter combination is also supported. At the CFHT 0.5″ (FWHM) images are routinely obtained with limiting magnitudes of 22.8 at J and 21.7 at K (S/N=3 in 1hr).

A Suggested Procedure for Testing Large Cassegrainian Optical Systems

The optical elements of a cassegrainian telescope are commonly tested individually, with their axes in a horizontal position. When these optical elements are inserted in the telescope, the resulting imagery is often disappointing. The quality of the imagery in the telescope may be predicted more accurately if the primary and secondary mirrors, with their axes in the vertical position, are tested against each other with the aid of null compensating reimaging optics. An example is given to illustrate the application of the technique.