Scientific/Technical Abstract:

HIPO is a special-purpose science instrument for SOFIA that is designed to provide simultaneous high-speed time resolved imaging photometry at two optical wavelengths. It will be possible to mount HIPO and FLITECAM on the SOFIA telescope simultaneously to allow data acquisition at two optical wavelengths and one near-IR wavelength. HIPO has a flexible optical system and numerous readout modes, allowing many specialized observations to be made. The instrument characteristics required for our proposed scientific pursuits are closely aligned to those needed for critical tests of the completed SOFIA Observatory, and HIPO will be used heavily for these tests. The general design and performance goals of the instrument are described in the 2004 paper in the references below.

Our main scientific interest is in the use of HIPO for observing stellar occultations. In a stellar occultation, a star serves as a small probe of the atmospheric structure of a solar system object or the surface density structure of a planetary ring or comet. Such observations provide information at high spatial resolution that would otherwise require a space mission to obtain. This work makes use of SOFIA's mobility, freedom from clouds, and near-absence of scintillation noise to provide the best possible occultation data.

The low atmospheric scintillation in airborne photometry gives HIPO the potential to detect P-mode stellar oscillations in sunlike stars and will provide excellent photometry of stellar transits by extrasolar planets. HIPO will be available for Guest Investigator use on a collaborative basis, and potential Guest Investigators should contact the PI prior to proposing to insure that the proposed observations are feasible and make the best use of HIPO 's capabilities.

HIPO Performance Summary:

The instrument sensitivity and resolution summaries are provided to permit estimating feasibility of scientific investigations. The HIPO performance summaries show the expected system performance for Full Operational Capability, which may differ from that during commissioning.

HIPO Optical Design:

The HIPO optical system is reconfigurable to meet its varied requirements. It incorporates two dichroic beamsplitters, one to divert the infrared beam to FLITECAM (if mounted) and one to split the red and blue sides of the HIPO optical paths. Either or both of these may be removed if desired. It is also possible to move either CCD so it is placed directly at the optimal SOFIA focal plane for highest spatial resolution and throughput. The optical design is described in detail in the 2003 paper in the references below.

The 8-position filter wheels are located near the pupil image formed by the collimator optics. Two positions in the red CCD's filter wheel are normally used for Shack-Hartmann lenslet arrays, but these may be replaced if necessary for a particular observation. We have contemplated adding grism capability to HIPO but have not yet carried this out.

The region between the mounting flange and the gate valve on the telescope can be evacuated to reduce image degradation due to density fluctuations in this region of the optical path.

HIPO provides a number of CCD readout modes as described in the 2008 paper in the references below. The most commonly used are the single frame mode and a frame transfer time series (basic occultation) mode with readout frequency up to 50 Hz. A variety of readout rates are available allowing the observer to optimize the subframe size, speed, noise, full well, and linearity tradeoff for a particular event.

HIPO Angular Resolution

The primary HIPO detectors are e2v CCD47-20 1024 x 1024 pixel frame transfer CCDs with plate scales of 0.33" x 0.33" pixels at low resolution and 0.05" x 0.05" pixels at high resolution. The HIPO field of view (FOV) is a 5.6' square, the 8' diagonal of which corresponds to the 8' diameter SOFIA FOV. Pixels will normally be binned to best match the seeing blur size and to reduce the effect of read noise. The high resolution mode includes no reimaging optics. (It is possible to replace one or both of the CCD47's with CCD67's having half the field of view, twice the pixel size, and much faster imaging operation.

The figure below shows the expected instrument FWHM beam diameter as a function of wavelength. It is expected to be dominated by seeing and image motion effects. The red curve in this figure is the nominal image quality expected at first light for SOFIA, based on the expected shear layer seeing, the as-built optical performance, and 2" rms image motion. The blue curve represents the ultimate combined optical quality and image motion requirement (80% encircled energy in a 1.6" diameter circle) convolved with the expected shear layer seeing. Also plotted are representative photometry aperture diameters likely to be used for processing occultation frames under both conditions described above. The image motion assumed is larger than will be experienced when observing at high frame rates. Occultation photometry will be extracted from data frames using effective aperture sizes comparable to the 80% enclosed light diameter plotted here.

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HIPO Spectral Passbands and Filter Characteristics

Wavelength range: 0.3 - 1.1 µm. HIPO currently includes standard Johnson UBVRI filters that will be used primarily for facility performance testing. Occultation observations will normally be unfiltered for events involving faint stars or will use specialized filters such as the narrow-band methane filter (λ_c ~ 0.89 µm) for events with bright stars. Additional custom filters will be added for specific events.

HIPO uses a dichroic reflector to separate its blue and red channels. Two dichroics are currently available with transition wavelengths of 0.575 and 0.675 µm respectively. Other dichroics will be added as necessary for specific events.

Below is a plot of the HIPO total system throughput for each of the available bandpasses. The dichroic response in the figures shown below is only an example, assuming a transition wavelength of 0.62 µm. This figure assumes that the FLITECAM dichroic beamsplitter is not installed.

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HIPO Sensitivities

Below are shown HIPO first-light sensitivities for several representative cases. These figures assume that the FLITECAM dichroic beamsplitter is not installed. The upper figures correspond to occultations by Pluto or Triton while the lower two are for the case of a very faint occulting object. The left and right figures are for 0.5 sec and 50 ms integrations, respectively. Each figure shows S/N for no filter (dichroic only) and for the dichroic plus standard Johnson filters. The dichroic transition is assumed to occur from 0.57 and 0.67 µm.

The deviation of S/N from a square root dependence is mostly due to shot noise on the occulting object in the top two figures, mostly to shot noise on the sky in the bottom left figure, and mostly to read noise in the bottom right figure. The improved final SOFIA pointing stability will increase sensitivity for sky-limited events and improve discrimination from nearby bright objects (e.g. Neptune for a Triton occultation).

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HIPO Observation Preparation and Data Handling

Once the observatory has been fully commissioned, additional information will be provided, including a full accounting of overheads associated with particular instrument set-ups and observing strategies; information on preparing observations using the SPT; and details regarding data formatting, calibration, and reduction.

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Disclaimer

All sensitivity and resolution data are preliminary, and based on anticipated performance of the observatory and the instrument.  Actual performance of the SOFIA telescope and instrument combination will be established after flight operations begin.  Telescope performance is expected to be upgraded during the first two years, and instrument performance may be upgraded, or additional modes or capabilities may be added.

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Additional References:

Dunham et al., "HIPO data products," Ground-based and Airborne Instrumentation for Astronomy II, Ian S. McLean, Editor, Proc. SPIE 7014, 70144Z (2008), DOI: 10.1117/12.788040 [pdf]

Dunham et al., "HIPO: a high-speed imaging photometer for occultations," Ground-based Instrumentation for Astronomy, Alan F. M. Moorwood & Masanori Iye, Editors, Proc. SPIE 5492, 592 (2004), DOI: 10.1117/12.552152 [pdf]

E. W. Dunham, "The optical design of HIPO: a high-speed imaging photometer for occultations," Airborne Telescope Systems II, Ramsey K. Melugin & Hans-Peter Roeser, Editors, Proc. SPIE 4857, 62 (2003), DOI: 10.1117/12.458819 [pdf]

Dunham, et al., "SOFIA image motion compensation," Ground-based and Airborne Instrumentation for Astronomy III, Ian S. McLean, Suzanne K. Ramsay, & Hideki Takami, Editors, Proc. SPIE 7735, 77355X (2010), DOI: 10.1117/12.857731 [pdf]