Thomas A. Bida

HIPO: a high-speed imaging photometer for occultations

HIPO is a special purpose instrument for SOFIA, the Stratospheric Observatory For Infrared Astronomy. It is a high-speed, imaging photometer that will be used for a variety of time-resolved precise photometry observations, including stellar occultations by solar system objects and transits by extrasolar planets. HIPO has two independent CCD detectors and can also co-mount with FLITECAM, an InSb imager and spectrometer, making simultaneous photometry at three wavelengths possible. HIPOs flexible design and high-speed imaging capability make it well suited to carry out initial test observations on the completed SOFIA system, and to this end a number of additional features have been incorporated. Earlier papers have discussed the design requirements and optical design of HIPO. This paper provides an overview of the instrument, describes the instruments features, and reviews the actual performance, in most areas, of the completed instrument.

Status and performance of the Discovery Channel Telescope during commissioning

Lowell Observatorys Discovery Channel Telescope is a 4.3m telescope designed for optical and near infrared astronomical observation. At first light, the telescope will have a cube capable of carrying five instruments and the wave front sensing and guider system at the f/6.1 RC focus. The corrected RC focus field of view is 30’ in diameter. Nasmyth and prime focus can be instrumented subsequently. Early commissioning work with the installed primary mirror and its support system started out using one of the wave front sensing probes mounted at prime focus, and has continued at RC with the recent installation of the secondary mirror. We will report on the on-sky pointing and tracking performance of the telescope, initial assessment of the functionality of the active optics support system, and tests of the early image quality of the telescope and optics. We will also describe the suite of first light instruments, and early science operations.

HIPO data products

HIPO is a special purpose instrument for SOFIA, the Stratospheric Observatory For Infrared Astronomy. It is a high-speed, imaging photometer that will be used for a variety of time-resolved precise photometry observations, including stellar occultations by solar system objects and transits by extrasolar planets. HIPO will also be used during the test program for the SOFIA telescope, a process that began with a series of ground-based tests in 2004. The HIPO requirements, optical design, overall description, and an early look at performance and planned data acquisition modes have appeared in earlier papers (e.g. Dunham, et al., Proc. SPIE 5492, 592-603 (2004)). This paper provides an update to the instrument description, final lab measurements of instrument performance, and a discussion of the data produced by the various observing modes.

Site testing for the Discovery Channel Telescope

We present results of an extended campaign to test astronomical and environmental qualities of the intended site for the Discovery Channel Telescope, located at 2361m elevation near Happy Jack, AZ. A semi-permanent test station has been in operation since January 2003, consisting of a Differential Image Motion Measurement (DIMM) system and a weather station. Median seeing derived from DIMM measurements for January 2003 - May 2004 on 117 separate nights was 0.84 arcsec, with a first-quartile average of 0.62 arcsec. A wind sensor array deployed on a 12.2m tower is used to characterize air flow over the site. We find that ground induced turbulence becomes more prevalent below the 7.3m level. The Lowell DIMM system has also been run adjacent to the WIYN telescope for simultaneous comparative seeing measurements. Absolute correlations of DIMM seeing with WIYN image quality were good over two nights observing under a range of environmental conditions.

Design, development, and testing of the DCT Cassegrain instrument support assembly

The 4.3m Discovery Channel Telescope delivers an f/6.1 unvignetted 0.5° field to its RC focal plane. In order to support guiding, wavefront sensing, and instrument installations, a Cassegrain instrument support assembly has been developed which includes a facility guider and wavefront sensor package (GWAVES) and multiple interfaces for instrumentation. A 2-element, all-spherical, fused-silica corrector compensates for field curvature and astigmatism over the 0.5° FOV, while reducing ghost pupil reflections to minimal levels. Dual roving GWAVES camera probes pick off stars in the outer annulus of the corrected field, providing simultaneous guiding and wavefront sensing for telescope operations. The instrument cube supports 5 co-mounted instruments with rapid feed selection via deployable fold mirrors. The corrected beam passes through a dual filter wheel before imaging with the 6K x 6K single CCD of the Large Monolithic Imager (LMI). We describe key development strategies for the DCT Cassegrain instrument assembly and GWAVES, including construction of a prime focus test assembly with wavefront sensor utilized in fall 2011 to begin characterization of the DCT primary mirror support. We also report on 2012 on-sky test results of wavefront sensing, guiding, and imaging with the integrated Cassegrain cube.

First-generation instrumentation for the Discovery Channel Telescope

The 4.3m Discovery Channel Telescope (DCT) has been conducting part-time science operations since January 2013. The f/6.1, 0.5° field-of-view at the RC focus is accessible through the Cassegrain instrument cube assembly, which can support 5 co-mounted instruments with rapid feed selection via deployable fold mirrors. Lowell Observatory has developed the Large Monolithic Imager (LMI), a 12.3 FOV 6K x 6K single CCD camera with a dual filter wheel, and installed at the straight-through, field-corrected RC focal station, which has served as the primary early science DCT instrument. Two low-resolution facility spectrographs are currently under development with first light for each anticipated by early 2015: the upgraded DeVeny Spectrograph, to be utilized for single object optical spectroscopy, and the unique Near-Infrared High-Throughput Spectrograph (NIHTS), optimized for single-shot JHK spectroscopy of faint solar system objects. These spectrographs will be mounted at folded RC ports, and the NIHTS installation will feature simultaneous optical imaging with LMI through use of a dichroic fold mirror. We report on the design, construction, commissioning, and progress of these 3 instruments in detail. We also discuss plans for installation of additional facility instrumentation on the DCT.

SOFIA observatory performance and characterization

The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently concluded a set of engineering flights for Observatory performance evaluation. These in-flight opportunities have been viewed as a first comprehensive assessment of the Observatorys performance and will be used to address the development activity that is planned for 2012, as well as to identify additional Observatory upgrades. A series of 8 SOFIA Characterization And Integration flights have been conducted from June to December 2011. The HIPO science instrument in conjunction with the DSI Super Fast Diagnostic Camera (SFDC) have been used to evaluate pointing stability, including the image motion due to rigid-body and flexible-body telescope modes as well as possible aero-optical image motion. We report on recent improvements in pointing stability by using an Active Mass Damper system installed on Telescope Assembly. Measurements and characterization of the shear layer and cavity seeing, as well as image quality evaluation as a function of wavelength have been performed using the HIPO+FLITECAM Science Instrument conguration (FLIPO). A number of additional tests and measurements have targeted basic Observatory capabilities and requirements including, but not limited to, pointing accuracy, chopper evaluation and imager sensitivity. This paper reports on the data collected during these flights and presents current SOFIA Observatory performance and characterization.

HIPO in-flight performance aboard SOFIA

HIPO is a special purpose science instrument for SOFIA that was also designed to be used for Observatory test work. It was used in a series of flights from June to December 2011 as part of the SOFIA Characterization and Integration (SCAI) flight test program. Partial commissioning of HIPO and the co-mounted HIPO-FLITECAM (FLIPO) configuration were included within the scope of the SCAI work. The commissioning measurements included such things as optical throughput, image size and shape as a function of wavelength and exposure time, image motion assessment over a wide frequency range, scintillation noise, photometric stability assessment, twilight sky brightness, cosmic ray rate as a function of altitude, telescope pointing control, secondary mirror control, and GPS time and position performance. As part of this work we successfully observed a stellar occultation by Pluto, our first SOFIA science data. We report here on the observed in-flight performance of HIPO both when mounted alone and when used in the FLIPO configuration.

Discovery Channel Telescope: progress and status

The Discovery Channel Telescope (DCT) is a 4.2-m telescope being built at a new site near Happy Jack, in northern Arizona. The DCT features a 2-degree-diameter field of view at prime focus and a Ritchey-Chretien (RC) configuration with Cassegrain and Nasmyth focus capability for optical/IR imaging and spectroscopy. Formal groundbreaking at the Happy Jack site for the DCT occurred on 12 July 2005, with construction of major facility elements underway.

HIPO in-flight performance improvements

The High-speed Imaging Photometer for Occultations (HIPO) is a special purpose science instrument for SOFIA. HIPO can be co-mounted with FLITECAM in the so-called FLIPO configuration for stellar occultation or extrasolar planet transit observations. We gained some flight experience with HIPO and FLITECAM in 2011 as described in a previous publication (Dunham, et al., Proc SPIE, 8446-42, 2012). Since that time a number of improvements to HIPO have been made and a deeper understanding of the airborne environments impact on photometric precision at optical wavelengths has been obtained. The improvements to HIPO include an improved beamsplitter for the FLIPO configuration, adding deep depletion CCDs as a detector option, expanding the filter set to include a Sloan Digital Sky Survey filter set as well as two custom filters for transit work, and an ability to guide the SOFIA telescope using HIPO data being acquired for science purposes. We now understand that variations in PSF size due to varying static air density has a noticeable impact on photometric stability while the related effect of Mach number is unimportant. The seriousness of ozone absorption in the Chappuis band is now understood and an approach to avoid this has been found. Finally we present demonstration transit data to illustrate our current transit photometry capability.