Atmospheric emission from OH molecules is a long standing problem for near-infrared astronomy. PRAXIS is a unique spectrograph, currently in the build-phase, which is fed by a fibre array that removes the OH background. The OH suppression is achieved with fibre Bragg gratings, which were tested successfully on the GNOSIS instrument. PRAXIS will use the same fibre Bragg gratings as GNOSIS in the first implementation, and new, less expensive and more efficient, multicore fibre Bragg gratings in the second implementation. The OH lines are suppressed by a factor of ~1000, and the expected increase in the signal-to-noise in the interline regions compared to GNOSIS is a factor of ~ 9 with the GNOSIS gratings and a factor of ~ 17 with the new gratings. PRAXIS will enable the full exploitation of OH suppression for the first time, which was not achieved by GNOSIS due to high thermal emission, low spectrograph transmission, and detector noise. PRAXIS will have extremely low thermal emission, through the cooling of all significantly emitting parts, including the fore-optics, the fibre Bragg gratings, a long length of fibre, and a fibre slit, and an optical design that minimises leaks of thermal emission from outside the spectrograph. PRAXIS will achieve low detector noise through the use of a Hawaii-2RG detector, and a high throughput through an efficient VPH based spectrograph. The scientific aims of the instrument are to determine the absolute level of the interline continuum and to enable observations of individual objects via an IFU. PRAXIS will first be installed on the AAT, then later on an 8m class telescope.
The accurate characterization of the field at the output of the optical fibres is of relevance for precision spectroscopy in astronomy. The modal effects of the fibre translate to the illumination of the pupil in the spectrograph and impact on the resulting point spread function (PSF). A Model is presented that is based on the Eigenmode Expansion Method (EEM) that calculates the output field from a given fibre for different manipulations of the input field. The fibre design and modes calculation are done via the commercially available Rsoft-FemSIM software. We developed a Python script to apply the EEM. Results are shown for different configuration parameters, such as spatial and angular displacements of the input field, spot size and propagation length variations, different transverse fibre geometries and different wavelengths. This work is part of the phase A study of the fibre system for MOSAIC, a proposed multi-object spectrograph for the European Extremely Large Telescope (ELT-MOS).
We here report on recent progress on astronomical optical frequency comb generation at innoFSPEC-Potsdam and
present preliminary test results using the fiber-fed Multi Unit Spectroscopic Explorer (MUSE) spectrograph. The
frequency comb is generated by propagating two free-running lasers at 1554.3 and 1558.9 nm through two dispersionoptimized
nonlinear fibers. The generated comb is centered at 1590 nm and comprises more than one hundred lines with
an optical-signal-to-noise ratio larger than 30 dB. A nonlinear crystal is used to frequency double the whole comb
spectrum, which is efficiently converted into the 800 nm spectral band. We evaluate first the wavelength stability using
an optical spectrum analyzer with 0.02 nm resolution and wavelength grid of 0.01 nm. After confirming the stability
within 0.01 nm, we compare the spectra of the astro-comb and the Ne and Hg calibration lamps: the astro-comb exhibits
a much larger number of lines than lamp calibration sources. A series of preliminary tests using a fiber-fed MUSE
spectrograph are subsequently carried out with the main goal of assessing the equidistancy of the comb lines. Using a
P3d data reduction software we determine the centroid and the width of each comb line (for each of the 400 fibers
feeding the spectrograph): equidistancy is confirmed with an absolute accuracy of 0.4 pm.