GREAT - German Receiver for Astronomy at Terahertz Frequencies

Basic Science Specific Information

Receiver Bands

During Basic Science, two bands, L #1 and L #2 will be operated simultaneously. The frequencies can be independently set in each band. The usable IF bandwidths of the receivers are typically 1.0-1.2 GHz, frequency dependent. The sky frequency ranges are:

Receiver noise temperatures have been measured to ~1500 K (L#1) and ~2500 K (L#2)

Backends

• 2 digital FFT spectrometers (FFTS): 1.5 GHz (8192 channels) or 750 MHz (16384 channels)
• 2 array-Acousto-Optical Spectrometers (AOS): 2x1 GHz bandwidth and 1.6 MHz spectral resolution
• TwoChirp-Transform-Spectrometer (CTS) spectrometers withh 220 MHz bandwidth and 47 kHz resolution

These backends record data simultaneously.

Observing Strategies

For Basic Science, there are three supported observing strategies:

1. position switching (PSW)
2. beam switching (BSW)
3. on-the-fly mapping in either position (PSW) or beam switching (BSW) mode.

Exposure Time Estimates

The exposure times for proposed GREAT observations should be estimated using the online tool SOFIA GREAT Time Estimator

Scientific/Technical Abstract

GREAT will offer opportunities for observations in (up to) three different frequency windows:

• The low frequency detectors, L#1 (1.25 - 1.50 THz; 240 - 200 µm) and L#2 (1.82 - 1.92 THz; 165 - 156 µm), will cover the important atomic fine-structure lines of ionized nitrogen and carbon [KOSMA].
• The mid-frequency detector, 2.4 - 2.7 THz (125 - 110 µm), is centered on the cosmologically relevant 1-0 transition of deuterated molecular hydrogen (HD) at 2.6 THz and the rotational ground-state transition of OH (²Π_3/2) [MPIfR].
• A high-frequency channel that targets at, e.g., the 63 µm transition of atomic oxygen [DLR].

GREAT is designed to investigate a wide range of astronomical questions, which ask for highest spectral resolution. A few examples:

• The 158 µm fine-structure transition of ionized carbon (CII) is the most important cooling line of the cold interstellar medium and therefore critical for its energy balance. KAO observations have demonstrated that the integrated emission provides a sensitive tracer of the star forming activity of a galaxy.On smaller scales, comparison with complementary observations of neutral atomic carbon [CI] and of carbon monoxide [CO] will constrain the physical conditions in the photon-dominated surface layers (PDRs) of molecular clouds.
• The 112 µm rotational ground-state transition of the deuterated hydrogen molecule, HD, will allow the derivation of the abundance profile of deuterium across the galactic disk and nearby galaxies, thereby providing unique information on the chemical evolution and star formation history of these systems. The ultimate goal is to better confine the cosmological deuterium abundance, which according to models of Big Bang nucleosynthesis, critically constrains the baryon density of the Universe.
• A series of rotational transitions of excited carbon monoxide CO will be accessible (J >13) for high-resolution excitation studies of, e.g., the inner shells of circumstellar envelopes and the physics of PDRs and shock layers.

GREAT Performance Summary

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

GREAT Angular Resolution

GREAT has four bandpasses centered at 63 µm, ~120 µm, ~160 µm, and 220 µm. The beam sizes for the four passbands are shown below, scaled to an image of Saturn. The beam sizes shown represent the FWHM for nominal operating conditions.

Shown below is a plot of the GREAT angular resolution (FWHM, arcsec) as a function of wavelength. The instrument resolution approaches the diffraction limit of the telescope. The difference is due primarily to telescope jitter.

GREAT Passbands and Spectral Resolution

Wavelength range: 60 - 240 µm (4.7 - 1.5 THz). GREAT has four bandpasses centered around prominent emission bands of astronomical interest. The available bandpasses are ~63 µm (~4.7 THz), 110 - 125 µm (2.7 - 2.4 THz), 156 - 165 µm (1.92 - 1.82 THz), and 200 - 240 µm (1.50 - 1.25 THz). For each flight, two of the three bandpasses will be installed. Frequency setting accuracy is better than 1 kHz.

Three backend options are available: FFTS, AOS and CTS providing different spectral resolutions and instantaneous bandwidths.

Below is plot of the spectral resolution versus wavelength. The plotted values correspond to the FWHM of the instrument line spread function for a monochromatic line from a point source. The upper curves are maximum resolutions; the lowest curve is for the AOS backend binned to a minimal resolution of 400 MHz (~30 - 70 km/s). The shaded regions indicate the effectively continuous range of resolution available between the curves.

GREAT Sensitivities

The table below presents the 1σ RMS Antenna Temperature (in Kelvin), which characterizes the noise level at the receiver. The values were calculated for the central frequency of each channel, 1.375 THz in L1, 1.86 THz in L2, and 2.55 THz in M, with receiver temperatures of 1600, 2000, and 3000 K respectively. Due to the high resolution of GREAT, the sensitivity is going to be heavily depended upon the local atmospheric transmission profile. To reflect this, the values below are calculated at transmission values of 90%, 75%, 50%, and 25%. Likewise, values are given both for lines of 1 km/s and 10 km/s width. These data are also presented graphically below.

In a similar manner to the data above, the table below presents the Minimum Detectable Line Flux (MDLF; W/m²), representing a 4σ detection in 900 seconds. The values were calculated for the central frequency of each channel, 1.375 THz in L1, 1.86 THz in L2, and 2.55 THz in M, with receiver temperatures of 1600, 2000, and 3000 K respectively. Due to the high resolution of GREAT, the sensitivity is going to be heavily depended upon the local atmospheric transmission profile. To reflect this, the values below are calculated at transmission values of 90%, 75%, 50%, and 25%. Likewise, values are given both for lines of 1 km/s and 10 km/s width. The MDLF scales roughly as (S/N) / sqrt(t), where t = net integration time.These data are also presented graphically below.

Plotted below are emission line sensitivities for line widths of 1, 10, and 100 km/s observed with resolutions indicated by q = (line width / resolution). The red curve is for a 1 km/s line observed at the full 1 MHz resolution of the AOS. The sensitivity is plotted as the Minimum Detectable Line Flux (MDLF), representing a 4σ detection in 900 seconds. The MDLF scales roughly as (S/N) / sqrt(t), where t = net integration time.

Line measurements in bright continuum sources may take longer to reach the same (S/N). Atmospheric transmission may preclude measurements at some wavelengths and reduce sensitivity at others.

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.

Additional References

Heyminck et al., "GREAT: a first light instrument for SOFIA," Ground-based and Airborne Instrumentation for Astronomy II, Ian S. McLean & Mark M. Casali, Editors, Proc. SPIE 7014, 701410 (2008), DOI: 10.1117/12.788273

Graf et al., "GREAT: the German first light heterodyne instrument for SOFIA," Infrared Spaceborne Remote Sensing and Instrumentation XV, Marija Strojnik-Scholl, Editor, Proc. SPIE 6678, 66780K (2007), DOI: 10.1117/12.768027