The Cryogenic Near Infrared Spectropolarimeter for the Daniel K Inouye Solar Telescope is designed to measure polarized light from 0.5 to 5 μm. It uses an almost all reflective design for high throughput and an R2 echelle grating to achieve the required resolution of up to R = 100,000. The optics cooled to cryogenic temperatures reduce the thermal background allowing for IR observations of the faint solar corona. Both the spectrograph and its context imager use H2RG detector arrays with a newly designed controller to allow synchronized exposures at frame rates up to 10 Hz. All hardware has been built and tested and the key components met their design goals. 1) The cryogenic system uses mechanical closed cycle coolers which introduce vibrations. Our design uses a two stage approach with a floating mounting disk and flexible cold links to reduce these. The vibration amplitudes on all critical stages were measured and are smaller than 1μm. 2) The grating stage of the spectrograph uses a double stack of harmonic drives and an optical encoder to provide sub-arcsecond resolution and a measured repeatability of better than 0.5 arcsec.
This paper addresses the issue of calibrating the Advanced Technology Solar Telescope for high-precision polarimetry, in particular of the optical train above the Gregorian station (where suitable calibration optics will be placed). Conventional techniques would not be adequate for this telescope given its large aperture. Here we explore two different methods that are currently being considered by the design team. The first one is the "sub-aperture" method, which uses small calibration optics above the primary mirror to calibrate a small sub-aperture of the system. This calibration is then extended to the full aperture by means of actual observations. The second method is based on analyzing the polarization observed in a spectral line with a peculiar Zeeman pattern, such as the FeII 614.9 nm line, which does not produce any intrinsic linear polarization. Numerical simulations are presented that show the robustness of both techniques and their respective advantages and disadvantages are discussed.
The 4-m aperture Advanced Technology Solar Telescope (ATST) is the next generation ground based solar telescope. In this paper we provide an overview of the ATST post-focus instrumentation. The majority of ATST instrumentation is located in an instrument Coude lab facility, where a rotating platform provides image de-rotation. A high order adaptive optics system delivers a corrected beam to the Coude lab facility. Alternatively, instruments can be mounted at Nasmyth or a small Gregorian area. For example, instruments for observing the faint corona preferably will be mounted at Nasmyth focus where maximum throughput is achieved. In addition, the Nasmyth focus has minimum telescope polarization and minimum stray light. We describe the set of first generation instruments, which include a Visible-Light Broadband Imager (VLBI), Visible and Near-Infrared (NIR) Spectropolarimeters, Visible and NIR Tunable Filters, a Thermal-Infrared Polarimeter & Spectrometer and a UV-Polarimeter. We also discuss unique and efficient approaches to the ATST instrumentation, which builds on the use of common components such as detector systems, polarimetry packages and various opto-mechanical components.
We are developing a high-resolution cross-dispersed echelle spectrograph for installation at one of the coude foci of the new AEOS 3.67 meter telescope, operated by the Air Force Space Command on Haleakala, Maui, Hawaii. The spectrograph will consist of two major subsystems: an optical arm for the wavelength range 0.5-1.0 micrometers and an IR arm for the range 1.0-2.5 micrometers . Both arms of the spectrograph use a white- pupil collimator design to maximize grating efficiency and to limit the size of the camera optics. The optical arm of the spectrograph will use deep-depletion CCDs optimized for operation near 1.0 micrometers . The IR detector will be a 2048 by 2048 HgCdTe array that has bene developed by the Rockwell Science Center for this project. Both the optical and IR arms of the spectrograph will be equipped with slit-viewing cameras for object acquisition and control of a fast guiding tip-tilt mirror located in a pupil image in the spectrograph fore optics.
All existing night-time astronomical telescopes, regardless of aperture, are blind to an important part of the universe - the region around bright objects. Technology now exist to build an unobscured 6.5 m aperture telescope which will attain coronagraphic sensitivity heretofore unachieved. A working group hosted by the University of Hawaii Institute for Astronomy has developed plans for a New Planetary Telescope which will permit astronomical observations which have never before ben possible. In its narrow-field mode the off-axis optical design, combined with adaptive optics, provides superb coronagraphic capabilities, and a very low thermal IR background. These make it ideal for studies of extra-solar planets and circumstellar discs, as well as for general IR astronomy. In its wide-field mode the NPT provides a 2 degree diameter field for surveys of Kuiper Belt Objects and Near-Earth Objects, surveys central to current intellectual interests in solar system astronomy.
We describe and evaluate the performance of a wavefront sensor based on curvature sensing which can be used to detect static aberrations given an extended reference source. The description includes a full mathematical treatment of the sensor signal, as well as how this signal is relate to the Laplacian of the wavefront. Evaluation of the technique is performed with computer simulations. A Monte-Carlo simulation is utilized to evaluate the performance of the technique in the presence of noise. The sensor was found to provide accurate measurement of the wavefront coefficients on high-contrast extended objects. It behaves well in the presence of a field stop, and in the presence of additive Gaussian noise.
We are developing a high-resolution cross-dispersed echelle spectrograph for installation at one of the coude foci of the new AEOS 3.67 meter telescope, operated by the Air Force Space Command on Haleakala, Maui, Hawaii. The spectrograph will consist of two major subsystems, an optical arm for the wavelength range 0.5-1.0 microns and a SWIR arm for the range 1.0-2.5 microns. The optical arm will include a mosaic 4096 by 4096 thinned CCD array, providing coverage of the wavelength range in two settings at a resolving power of 50,000. The CCD camera will be operated in frame-transfer mode. The IR arm will consist of a compact, folded cross- dispersed cryogenic echelle spectrography. The SWIR detector will be a 2048 by 2048 HgCdTe array, based on the existing HAWAII 1024 by 1024 devices. The large-format detector will permit coverage of the entire J or H band in a single grating setting with a resolving power of 60,000, and the K band in two settings. The high resolution, coupled with careful attention to scattering and stray light in the optical system, will permit exploitation of the low sky background between the strong OH airflow lines. Adequate order separation will be maintained to permit work on moderately extended objects while still retaining sky subtraction capability. The spectrography is expected to be available for use in early 2000.
We present results of simulations involving a curvature-based wavefront sensor which uses an extended pattern as a reference source. The proposed sensor provides measurements of both symmetric and asymmetric aberration terms by comparing the Fourier transforms of two oppositely defocused images. Symmetric terms such as defocus and astigmatism can be measured without regard to the object distribution. The asymmetric terms, such as tip and tilt, rely on averaging the signal over many atmospheric realizations in order to determine the object phase, or on defining an arbitrary reference phase. Only after removal of the object Fourier transform phase can the asymmetric terms be identified. Although this paper reports on preliminary results, we believe the proposed sensor will be useful for both real-time compensation of atmospheric distortions while imaging the Sun, and post-facto compensation of optical misalignments in Earth-pointing satellites.