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Register a new userThe case for OH suppression at near-infrared wavelengths
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S.C. Ellis
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We calculate the advances in near-infrared astronomy made possible through the use of fibre Bragg gratings to selectively remove hydroxyl emission lines from the night sky spectrum. Fibre Bragg gratings should remove OH lines at high resolution (R=10,000), with high suppression (30dB) whilst maintaining high throughput (~90 per cent) between the lines. Devices currently under construction should remove 150 lines in each of the J and H bands, effectively making the night sky surface brightness ~4 magnitudes fainter. This background reduction is greater than the improvement adapative optics makes over natural seeing; photonic OH suppression is at least as important as adaptive optics for the future of cosmology. We present a model of the NIR sky spectrum, and show that the interline continuum is very faint (~80 ph/s/m^s/arcsec/micron on the ecliptic plane). We show that OH suppression by high dispersion, i.e. `resolving out the skylines, cannot obtain the required level of sensitivity to reach the interline continuum due to scattering of light. The OH lines must be suppressed prior to dispersion. We have simulated observations employing fibre Bragg gratings of first light objects, high redshift galaxies and cool, low-mass stars. The simulations are of complete end-to-end systems from object to detector. The results demonstrate that fibre Bragg grating OH suppression will significantly advance our knowledge in many areas of astrophysics, and in particular will enable rest-frame ultra-violet observations of the Universe at the time of first light and reionisation.
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Ground-based near-infrared astronomy is severely hampered by the forest of atmospheric emission lines resulting from the rovibrational decay of OH molecules in the upper atmosphere. The extreme brightness of these lines, as well as their spatial and temporal variability, makes accurate sky subtraction difficult. Selectively filtering these lines with OH suppression instruments has been a long standing goal for near-infrared spectroscopy. We have shown previously the efficacy of fibre Bragg gratings combined with photonic lanterns for achieving OH suppression. Here we report on PRAXIS, a unique near-infrared spectrograph that is optimised for OH suppression with fibre Bragg gratings. We show for the first time that OH suppression (of any kind) is possible with high overall throughput (18 per cent end-to-end), and provide examples of the relative benefits of OH suppression.
The background noise between 1 and 1.8 microns in ground-based instruments is dominated by atmospheric emission from hydroxyl molecules. We have built and commissioned a new instrument, GNOSIS, which suppresses 103 OH doublets between 1.47 - 1.7 microns by a factor of ~1000 with a resolving power of ~10,000. We present the first results from the commissioning of GNOSIS using the IRIS2 spectrograph at the AAT. The combined throughput of the GNOSIS fore-optics, grating unit and relay optics is ~36 per cent, but this could be improved to ~46 per cent with a more optimal design. We measure strong suppression of the OH lines, confirming that OH suppression with fibre Bragg gratings will be a powerful technology for low resolution spectroscopy. The integrated OH suppressed background between 1.5 and 1.7 microns is reduced by a factor of 9 compared to a control spectrum using the same system without suppression. The potential of low resolution OH suppressed spectroscopy is illustrated with example observations. The GNOSIS background is dominated by detector dark current below 1.67 microns and by thermal emission above 1.67 microns. After subtracting these we detect an unidentified residual interline component of ~ 860 +/ 210 ph/s/m^2/micron/arcsec^2. This component is equally bright in the suppressed and control spectra. We have investigated the possible source of the interline component, but were unable to discriminate between a possible instrumental artifact and intrinsic atmospheric emission. Resolving the source of this emission is crucial for the design of fully optimised OH suppression spectrographs. The next generation OH suppression spectrograph will be focussed on resolving the source of the interline component, taking advantage of better optimisation for a FBG feed. We quantify the necessary improvements for an optimal OH suppressing fibre spectrograph design.
The principles and practice of astronomical imaging Fourier transform spectroscopy (FTS) at far-infrared wavelengths are described. The Mach-Zehnder interferometer design has been widely adopted for current and future imaging FTS instruments; we compare this design with two other common interferometer formats. Examples of three instruments based on the Mach-Zehnder design are presented. The techniques for retrieving astrophysical parameters from the measured spectra are discussed using calibration data obtained with the Herschel SPIRE instrument. The paper concludes with an example of imaging spectroscopy obtained with the SPIRE FTS instrument.
A search for the near-infrared water-ice absorption band was made in a number of very red OH/IR stars which are known to exhibit the 10um silicate absorption. As a by-product, accurate positions of these highly reddened objects are obtained. We derived a dust mass loss rate for each object by modelling the spectral energy distribution and the gas mass loss rate by solving the equation of motion for the dust drag wind. The derived mass loss rates show a strong correlation with the silicate optical depth as well as that of the water-ice. The stars have a high mass loss rate (> 1.0E-4 Msun/yr) with an average gas-to-dust mass ratio of 110. In objects which show the 3.1um water-ice absorption, the near-IR slope is much steeper than those with no water-ice. Comparison between our calculated mass loss rates and those derived from OH and CO observations indicates that these stars have recently increased their mass loss rates.
We perform a numerical analysis of mid-infrared photoluminescence emitted by praseodymium (III) doped chalcogenide selenide glass pumped at near-infrared wavelengths. The results obtained show that an effective inversion of level populations can be achieved using both 1480 nm and 1595 nm laser diodes. The rate of the spontaneous emission achieved when pumping at 1480 nm and 1595 nm is comparable to this achieved using the standard pumping wavelength of 2040 nm.
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