D. A. Harper

University of Chicago, Chicago, Illinois, United States

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Publications (123)308.73 Total impact

  • American Astronomical Society Meeting Abstracts; 01/2013
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    ABSTRACT: The Stratospheric Observatory For Infrared Astronomy (SOFIA) is an airborne observatory consisting of a specially modified Boeing 747SP with a 2.7 m telescope, flying at altitudes as high as 13.7 km (45,000 ft). Designed to observe at wavelengths from 0.3 μm to 1.6 mm, SOFIA operates above 99.8% of the water vapor that obscures much of the infrared and submillimeter. SOFIA has seven science instruments under development, including an occultation photometer, near-, mid-, and far-infrared cameras, infrared spectrometers, and heterodyne receivers. SOFIA, a joint project between NASA and the German Aerospace Center Deutsches Zentrum für Luft und-Raumfahrt, began initial science flights in 2010 December, and has conducted 30 science flights in the subsequent year. During this early science period three instruments have flown: the mid-infrared camera FORCAST, the heterodyne spectrometer GREAT, and the occultation photometer HIPO. This Letter provides an overview of the observatory and its early performance.
    The Astrophysical Journal Letters 03/2012; 749(2):L17. · 6.35 Impact Factor
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    ABSTRACT: Polarimetry at far-infrared wavelengths is a key tool for studying physical processes on size scales ranging from interstellar dust grains to entire galaxies. A multi-wavelength continuum polarimeter at these wavelengths will allow studies of thermal dust polarization in an effort to constrain the grains' physical properties and test grain alignment theory. High spatial resolution (5-30 arcsec) and sensitive observations will measure the influence of magnetic fields on infrared cirrus clouds, the envelopes and disks of YSOs, outflows from both low- and high-mass star forming regions, and the relative strength of magnetic, gravitational, and turbulent effects in star- and cloud-formation.
    EAS Publications Series 11/2011;
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    ABSTRACT: Polarimetry at infrared wavelengths is a promising method of investigation using SOFIA, allowing mapping of magnetic fields in dusty clouds and characterization of dust grain properties and alignment. We describe an instrument we are building to perform polarimetry at wavelengths from 50 to 200 microns early in the operating lifetime of SOFIA: a cryogenic polarimeter which will be inserted permanently into the facility camera HAWC. The simple polarimeter consists of a continuously-rotating quartz half-wave plate followed by a wire polarizer -- one monochromatic wave plate for each of the four HAWC photometric bands. The polarimeter is removed from the camera beam by rotating the pupil wheel assembly. We anticipate that the polarimeter will be available during the first year of operation of HAWC on SOFIA. Key science targets are expected to be the magnetic field strength and morphology within nearby molecular clouds, the detailed magnetic field configuration of the Galactic Center, and pioneering polarization mapping of nearby galaxies. This HAWC polarimeter is envisaged as a key step toward more powerful far-IR polarimeters on SOFIA and eventually in space.
    01/2009;
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    ABSTRACT: We have used the SPIREX telescope to conduct a wide-field thermal infrared imaging study of the star formation complex NGC 6334 in the southern Galactic plane. We imaged a 30' region along the main star-forming ridge of NGC 6334 with 06 pixel scale through broadband filters for L (3.5 μm) and M (4.8 μm) and through narrowband filters for the H2 v = 1-0 Q-branch (2.42 μm), polycyclic aromatic hydrocarbon (PAH) (3.3 μm), and Brα (4.05 μm) lines. The images reveal the spectacular, complex structure of the photodissociation regions (PDRs) that pervade the region, with enhanced line emission around each of the seven sites of massive star formation along the ridge. Bubbles and loops of PAH emission, typically 1-1.5 pc across, have been carved out of the parent molecular cloud by the intense UV radiation from the massive stars and surround H II regions (seen in Brα) typically 0.2-0.3 pc across. The PAH emission regions coincide with both [C II] 158 μm line emission, indicating that the PAHs are excited in PDR gas, and extensive H2 emission, which therefore must be fluorescent. However, the textures of the emission regions in PAH and H2 are different. This is attributable to variations in the physical environment in which the gas is excited. Several compact reddened objects are observed; these are likely to be massive protostars.
    The Astrophysical Journal 12/2008; 542(1):359. · 6.73 Impact Factor
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    ABSTRACT: We compare 160μm and 360μm photometric measurements of three normal spiral galaxies with theoretical models and observations at other wavelengths. Our observations indicate that on average spirals have few large, cold grains, in apparent discrepancy with the 1.3 mm measurements of Chini et al. (1986).
    09/2008: pages 262-265;
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    ABSTRACT: Multi-wavelength imaging polarimetry at far-infrared wavelengths has proven to be an excellent tool for studying the physical properties of dust, molecular clouds, and magnetic fields in the interstellar medium. Although these wavelengths are only observable from airborne or space-based platforms, no first-generation instrument for the Stratospheric Observatory for Infrared Astronomy (SOFIA) is presently designed with polarimetric capabilities. We study several options for upgrading the High-resolution Airborne Wideband Camera (HAWC) to a sensitive FIR polarimeter. HAWC is a 12 x 32 pixel bolometer camera designed to cover the 53 - 215 micron spectral range in 4 colors, all at diffraction-limited resolution (5 - 21 arcsec). Upgrade options include: (1) an external set of optics which modulates the polarization state of the incoming radiation before entering the cryostat window; (2) internal polarizing optics; and (3) a replacement of the current detector array with two state-of-the-art superconducting bolometer arrays, an upgrade of the HAWC camera as well as polarimeter. We discuss a range of science studies which will be possible with these upgrades including magnetic fields in star-forming regions and galaxies and the wavelength-dependence of polarization. Comment: 12 pages, 5 figures
    Proc SPIE 11/2007;
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    ABSTRACT: ANTe describe 12 x 32 arrays of senlicoladucting cryogenic bolometers designed for use in far-infrared and submillimeter cameras. These 12 x 32 arrays are constructed from 1 x 32 monolithic pop-up detectors developed at NASA Goddard Space Flight Center. The pop-up technology allows the construction of large arrays with high filling factors that provide efficient use of space in the focal planes of far-infrared and submillimeter astronomical instruments. This directly leads to a significant decrease in integration time. The prototype array is currently operating in the second generation Submillimeter High Angular Resolution Camera (SHAR( II), a facility instrument in use at the Caltech Submillimeter Observatory ((1S0). The elements of this array enlploy a bismuth absorber coating and quarter wave backshort to opthnize the bolometer alp sorption for a passbands centered at 350 and 450 microns. A second array is to be installed in the High-resolution Airborne Widebandwidth Caniera (HAW( ), a far-infrared imaging camera for the Stratospheric Observatory for Trifrared AstronOTTly (SOFIA). This array has heel] COMpleted and is now awaiting integration into the HAWC test cryostat. HAWC is scheduled for commissioning in 2005. The HAWC array employs titanium-gold absorbers and is optimized for uniforin absorption from 40 to 300 microns to accommodate all four of its far-infrared pa.ssbands. We describe the details of the HAWC array construction including the mechanical design and electrical characterization of the constituent linear arrays.
    15th International Symposium on Space Terahertz Technology; 04/2004
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    ABSTRACT: We describe 12 x 32 arrays of semiconducting cryogenic bolometers designed for use in far-infrared and submillimeter cameras. These 12 x 32 arrays are constructed from 1 x 32 monolithic pop-up detectors developed at NASA Goddard Space Flight Center. The pop-up technology allows the construction of large arrays with high filling factors that provide efficient use of space in the focal planes of far-infrared and submillimeter astronomical instruments. This directly leads to a significant decrease in observing time. The prototype array is currently operating in SHARC II, a facility instrument in use at the Caltech Submillimeter Observatory (CSO). The elements of this array employ a bismuth absorber coating and quarter wave backshort to optimize the bolometer absorption for a passband centered at 350 microns. However, this resonant structure also provides good bolometer performance at 450 and 850 microns, the two additional SHARC II passbands. A second array is to be installed in the High-resolution Airborne Widebandwidth Camera (HAWC), a far-infrared imaging camera for the Stratospheric Observatory for Infrared Astronomy (SOFIA). This array is currently in the final stage of construction, and its completion is expected in early 2004. HAWC is scheduled for commissioning in 2005. The HAWC array employs titanium-gold absorbers and is optimized for uniform absorption from 40 to 300 microns to accommodate all four of its far-infrared passbands. We describe the details of the array construction including the mechanical design and electrical characterization of the constituent linear arrays, comparing the SHARC II and HAWC cases. We also summarize the overall characteristics of the final two-dimensional arrays. Finally, we show examples of array performance in the form of images obtained with SHARC II.
    02/2004;
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    ABSTRACT: We report on the performance of the SHARC II detector, a 12×32 array of ion implanted Si pop-up bolometers. This 384 element detector array was built as a prototype for the High Angular Resolution Widefield Camera for the Stratospheric Observatory for Infrared Astronomy. We will discuss the design process, the characterization of the detectors, and the performance of the array in the SHARC II instrument. SHARC II is now a facility instrument on the Caltech Submillimeter Observatory, providing background-limited imaging at 350 and .
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2004; · 1.14 Impact Factor
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    ABSTRACT: We present KAO 60-micron, CSO SHARC II 350-micron and JCMT 450 and 850-micron data of the colliding galaxies NGC 4038/39 ("The Antennae"). These far-infrared and submillimeter data are the most complete set to date which fully sample the spectral energy distribution of the large grain dust component in this object. With a resolution of 17 arcseconds or better, they also represent the highest resolution data available at these wavelengths. Using these data, we construct maps which compare the distribution of large grains to the distributions of PAHs and very small grains imaged by ISOCAM. We also map the variation in temperature of the large grains, and investigate how this correlates with the burst of star formation which has been inferred in this colliding system.
    12/2003;
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    ABSTRACT: The High resolution Airborne Wideband Camera (HAWC) and the Submillimeter High Angular Resolution Camera II (SHARC II) will use almost identical versions of an ion-implanted silicon bolometer array developed at the National Aeronautics and Space Administration's Goddard Space Flight Center (GSFC). The GSFC "Pop-Up" Detectors (PUD's) use a unique folding technique to enable a 12 × 32-element close-packed array of bolometers with a filling factor greater than 95 percent. A kinematic Kevlar suspension system isolates the 200 mK bolometers from the helium bath temperature, and GSFC - developed silicon bridge chips make electrical connection to the bolometers, while maintaining thermal isolation. The JFET preamps operate at 120 K. Providing good thermal heat sinking for these, and keeping their conduction and radiation from reaching the nearby bolometers, is one of the principal design challenges encountered. Another interesting challenge is the preparation of the silicon bolometers. They are manufactured in 32-element, planar rows using Micro Electro Mechanical Systems (MEMS) semiconductor etching techniques, and then cut and folded onto a ceramic bar. Optical alignment using specialized jigs ensures their uniformity and correct placement. The rows are then stacked to create the 12 × 32-element array. Engineering results from the first light run of SHARC II at the Caltech Submillimeter Observatory (CSO) are presented.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    01/2003;
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    ABSTRACT: The High resolution Airborne Wideband Camera (HAWC) and the Submillimeter High Angular Resolution Camera II (SHARC II) will use almost identical versions of an ion-implanted silicon bolometer array developed at the National Aeronautics and Space Administration's Goddard Space Flight Center (GSFC). The GSFC "Pop-Up" Detectors (PUD's) use a unique folding technique to enable a 12 × 32-element close-packed array of bolometers with a filling factor greater than 95 percent. A kinematic Kevlar suspension system isolates the 200 mK bolometers from the helium bath temperature, and GSFC - developed silicon bridge chips make electrical connection to the bolometers, while maintaining thermal isolation. The JFET preamps operate at 120 K. Providing good thermal heat sinking for these, and keeping their conduction and radiation from reaching the nearby bolometers, is one of the principal design challenges encountered. Another interesting challenge is the preparation of the silicon bolometers. They are manufactured in 32-element, planar rows using Micro Electro Mechanical Systems (MEMS) semiconductor etching techniques, and then cut and folded onto a ceramic bar. Optical alignment using specialized jigs ensures their uniformity and correct placement. The rows are then stacked to create the 12 × 32-element array. Engineering results from the first light run of SHARC II at the Caltech Submillimeter Observatory (CSO) are presented.
    Proc SPIE 01/2003;
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    ABSTRACT: The Stratospheric Observatory For Infrared Astronomy (SOFIA) is a 2.5 m telescope which will fly in a Boeing 747SP, commencing operation in late summer 2004. SOFIA is a joint American/German project and for most of this decade it will be the largest far-infrared telescope. SOFIA will have a suite of instruments available to the astronomical community, including the High-resolution Airborne Wide-band Camera (HAWC) – its facility far-infrared camera.
    EAS Publications Series 01/2002;
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    ABSTRACT: The HAWC instrument on SOFIA is diffraction limited in four bands between 50-220mum , with background limited sensitivity. Its purpose is to provide sensitive and reliable facility-imaging capabilities for SOFIA during its first operational years. It is the first flight instrument to use a state of the art bolometric 12x32 pixel array of ion implanted silicon PUDs, a closed-packed 2D array with >95% filling factor. It will be cooled to ~0.2K, using an Adiabatic Demagnetization Refrigerator (ADR). Here we report on detector characteristics: Measured I/V curves for different temperatures are completely consistent with a four parameter bolometer model. The measured detector noise contribution to the measured noise is only ~1-2% of the sky background noise. In September, 2000 a prototype instrument operating at lambda =350mum using a single linear array of detectors was successfully deployed, and saw first light on the Caltech Submillimeter Observatory (CSO) on Mauna Kea.
    12/2000;
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    ABSTRACT: Measurements of the mid-infrared sky brightness at the South Pole throughout the winter of 1998 show that the sky background is extremely low and stable. For 50% of the time, the flux in the 8.78 to 9.09 μm window is below 50 Jy per square arcsecond. Typical background levels in this window during clear conditions are of the order of 20 Jy per square arcsecond. This is almost an order of magnitude better than any other site on earth. The lower limit to the sky background across most of the N window appears to be set by the aerosol content of the sky rather than by residual water vapor. These data were acquired remotely using an automated instrument housed in the AASTO (Automated Astrophysical Site-Testing Observatory).
    The Astrophysical Journal 05/2000; 535(1):501. · 6.73 Impact Factor
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    ABSTRACT: Measurements of the mid-infrared sky brightness at the South Pole throughout the winter of 1998 show that the sky background is extremely low and stable. For 50% of the time the flux in the 8.78 to 9.09 m window is below 50 Janskys per square arcsecond. Typical background levels in this window during clear conditions are of the order of 20 Janskys per square arcsecond. This is almost an order of magnitude better than any other site on earth. The lower limit to the sky background across most of the N-window appears to be set by the aerosol content of the sky, rather than by residual water vapor. These data were acquired remotely using an automated instrument housed in the AASTO (Automated Astrophysical Site-Testing Observatory). Subject headings: site testing; atmospheric effects -- 3 -- 1.
    03/2000;
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    ABSTRACT: We present the first results of the Abu instrument on the SPIREX facility at the South Pole. Abu uses a 1024x 1024 InSb detector array with a 10.2'*E.2' field-of-view and 0.6'' pixels. Using the 60 cm SPIREX telescope with the Abu camera, during the 1998 austral winter we detect L=13.5 mag (S/N=10 in 1 hour). Engineering improvements to reduce the thermal background should allow even better performance this season. Abu/SPIREX offers the best currently available wide-field 3-5 mu m imaging and pioneers a future capability which can effectively complement SIRTF, Gemini, and SOFIA. We have imaged NGC 6334, a southern massive star formation region, in Br alpha , the 3.3 mu m PAH feature, and in L-band continuum. The 3.3 mu m PAH emission matches the spatial distribution of the [C II] 158 mu m emission, a tracer of photodissociation regions (PDRs). The 3.3 mu m PAH imaging of NGC 6334 therefore reveals the structure of its various PDRs in unprecedented angular resolution ( ~ 1''), more than an order of magnitude better than traditional FIR ( ~ 1') tracers of the PDRs. The bubbles and loops carved out by the winds from the young stars show that PDRs are clumpy and filamentary. With this detailed structure, we can now compare the PDR distribution with the other components of the ISM (ionized gas, dust, molecular gas) at comparable angular resolution. A preliminary examination of these data shows an anti-correlation between the 3.3 mu m feature and the ionized gas (traced by radio and Br alpha ). The distribution of molecular gas (traced by the extinction), of photodissociated gas (traced by the 3.3 mu m emission), and of ionized gas (traced by the Br alpha emission), is precisely that expected for a photodissociation region: the ionized gas lies on the interior of the shell, the photodissociated gas just outside the ionized gas, and the molecular gas just outside the photodissociated gas. There are also a few remarkably red objects that suggest deeply embedded protostellar sources.
    01/1999;
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    ABSTRACT: One mission of the Center for Astrophysical Research in Antarctica is to determine the benefits of establishing observatories at the South Pole. We have shown that the South Pole sky is extremely dark in the thermal portion of the K band (called Kdark, 2.27-2.43 \micron), exhibiting average sky brightness of 16.7 mag arcsec(-2) \quad (Nguyen et al. 1996). Because the South Pole experiences weather not found at most observatories, like airborne ice crystals, we needed to determine if it is ever photometric. This past austral winter we made observations to characterize the near-infrared extinction and its variation over 8-hours. Our procedure typically consisted in measuring the brightness of 10 standard stars which spanned a range of magnitude (K ~ 7-12) and airmass (sec(z) ~ 1-2.5). Our program included measuring the extinction 2 or more times in 8 hours at least 3 days per month from May through August. We also measured the extinction once on another 5 days in each month. In order to characterize the hourly variation of the transparency, in mid-July we spent 6 days monitoring 3 standard stars. After a preliminary analysis of the observations, we draw the following conclusions. 1) The South Pole shows good to excellent K band transparency with most observing days having extinction of 0.12 mag airmass(-1) \quad or better. 2) Days with good extinction show good to excellent stability over 8 hours with an average change of 0.1 magnitudes. Days with poor to fair extinction show greater variation. Nguyen et al. 1996, PASP 108, 718 The National Science Foundation supports this research through Grant No. NSF DDP 89-20223.
    12/1997;
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    12/1996; 29:725.

Publication Stats

1k Citations
308.73 Total Impact Points

Institutions

  • 1970–2012
    • University of Chicago
      • Department of Astronomy and Astrophysics
      Chicago, Illinois, United States
  • 1979
    • Fermi National Accelerator Laboratory (Fermilab)
      Batavia, Illinois, United States