Observing on the Stratospheric Observatory for Infrared Astronomy (SOFIA) requires a strategy that takes the specific circumstances of an airborne platform into account. Observations of a source cannot be extended or shortened on the spot due to flight path constraints. Still, no exact prediction of the time on source is available since there are always wind and weather conditions, and sometimes technical issues. Observations have to be planned to maximize the observing efficiency while maintaining full flexibility for changes during the observation. The complex nature of observations with FIFI-LS - such as the interlocking cycles of the mechanical gratings, telescope nodding and dithering - is considered in the observing strategy as well. Since SOFIA Cycle 3 FIFI-LS is available to general investigators. Therefore general investigators must be able to define the necessary parameters simply, without being familiar with the instrument, still resulting in efficient and flexible observations. We describe the observing process with FIFI-LS including the integration time estimate, the mapping and dithering setup and aspects of the scripting for the actual observations performed in flight. We also give an overview of the observing scenarios, which have proven to be useful for FIFI-LS.
PRAXIS is a second generation instrument that follows on from GNOSIS, which was the first instrument using fibre
Bragg gratings for OH suppression to be deployed on a telescope. The Bragg gratings reflect the NIR OH lines while
being transparent to the light between the lines. This gives in principle a much higher signal-noise ratio at low resolution
spectroscopy but also at higher resolutions by removing the scattered wings of the OH lines. The specifications call for
high throughput and very low thermal and detector noise so that PRAXIS will remain sky noise limited even with the
low sky background levels remaining after OH suppression. The optical and mechanical designs are presented. The
optical train starts with fore-optics that image the telescope focal plane on an IFU which has 19 hexagonal microlenses
each feeding a multi-mode fibre. Seven of these fibres are attached to a fibre Bragg grating OH suppression system while
the others are reference/acquisition fibres. The light from each of the seven OH suppression fibres is then split by a
photonic lantern into many single mode fibres where the Bragg gratings are imprinted. Another lantern recombines the
light from the single mode fibres into a multi-mode fibre. A trade-off was made in the design of the IFU between field of
view and transmission to maximize the signal-noise ratio for observations of faint, compact objects under typical seeing.
GNOSIS used the pre-existing IRIS2 spectrograph while PRAXIS will use a new spectrograph specifically designed for
the fibre Bragg grating OH suppression and optimised for 1.47 μm to 1.7 μm (it can also be used in the 1.09 μm to 1.26
μm band by changing the grating and refocussing). This results in a significantly higher transmission due to high
efficiency coatings, a VPH grating at low incident angle and optimized for our small bandwidth, and low absorption
glasses. The detector noise will also be lower thanks to the use of a current generation HAWAII-2RG detector.
Throughout the PRAXIS design, from the fore-optics to the detector enclosure, special care was taken at every step along
the optical path to reduce thermal emission or stop it leaking into the system. The spectrograph design itself was
particularly challenging in this aspect because practical constraints required that the detector and the spectrograph
enclosures be physically separate with air at ambient temperature between them. At present, the instrument uses the
GNOSIS fibre Bragg grating OH suppression unit. We intend to soon use a new OH suppression unit based on multicore
fibre Bragg gratings which will allow an increased field of view per fibre. Theoretical calculations show that the gain in
interline sky background signal-noise ratio over GNOSIS may very well be as high as 9 with the GNOSIS OH
suppression unit and 17 with the multicore fibre OH suppression unit.
GNOSIS has provided the first on-telescope demonstration of a concept to utilize complex aperioidc fiber Bragg
gratings to suppress the 103 brightest atmospheric hydroxyl emission doublets between 1.47-1.7 μm. The unit is
designed to be used at the 3.9-meter Anglo-Australian Telescope (AAT) feeding the IRIS2 spectrograph. Unlike
previous atmospheric suppression techniques GNOSIS suppresses the lines before dispersion. We present the
results of laboratory and on-sky tests from instrument commissioning. These tests reveal excellent suppression
performance by the gratings and high inter-notch throughput, which combine to produce high fidelity OH-free
We discuss the development of multi-core fiber Bragg gratings (FBGs) to be applied to astrophotonics, more specifically
to near-infrared spectroscopy for ground-based instruments. The multi-core FBGs require over 100 notches to reject the
OH lines in a broad wavelength range (160 nm). The number of cores of the fiber should correspond to the mode number
in the multi-mode fibers and should be large enough to be able to capture a sufficient amount of light from the telescope.
A phase-mask based technique is used to fabricate the multi-core FBGs.
Ground based near-infrared observations have long been plagued by poor sensitivity when compared to visible
observations as a result of the bright narrow line emission from atmospheric OH molecules. The GNOSIS instrument
recently commissioned at the Australian Astronomical Observatory uses Photonic Lanterns in combination with
individually printed single mode fibre Bragg gratings to filter out the brightest OH-emission lines between 1.47 and
1.70μm. GNOSIS, reported in a separate paper in this conference, demonstrates excellent OH-suppression, providing
very “clean” filtering of the lines. It represents a major step forward in the goal to improve the sensitivity of ground
based near-infrared observation to that possible at visible wavelengths, however, the filter units are relatively bulky and
costly to produce.
The 2nd generation fibre OH-Suppression filters based on multicore fibres are currently under development. The
development aims to produce high quality, cost effective, compact and robust OH-Suppression units in a single optical
fibre with numerous isolated single mode cores that replicate the function and performance of the current generation of
“conventional” photonic lantern based devices. In this paper we present the early results from the multicore fibre
development and multicore fibre Bragg grating imprinting process.
GNOSIS is an OH suppression unit to be used in conjunction with existing spectrographs. The OH suppression
is achieved using fibre Bragg gratings (FBGs), and will deliver the darkest near-infrared background of any
ground-based instrument. Laboratory and on-sky tests demonstrate that FBGs can suppress OH lines by 30dB
whilst maintaing > 90% throughput between the lines, resulting in a 4 mag decrease in the background.
In the first implementation GNOSIS will feed IRIS2 on the AAT. It will consist of a seven element lenslet
array, covering 1.4" on the sky, and will suppress the 103 brightest OH lines between 1.47 and 1.70 μm. Future
upgrades will include J-band suppression and implementation on an 8m telescope.