FIFI-LS (the Field Imaging Far Infrared Line Spectrometer for SOFIA) was successfully commissioned in 2014 during six flights on SOFIA. The observed wavelengths are set by rotating reflective gratings. In flight these gratings and their rotating mechanisms are exposed to vibrations. To quantify these vibrations, an acceleration sensor was placed on the exterior of the instrument. Simultaneously, the angle sensor of the grating was read out to analyze the movement of the grating. Based on this data, lab measurements were conducted to evaluate the effect of the vibrations on the image quality of FIFI-LS. The submitted paper will present the measured data and show the results of the analysis.
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.
The Field-Imaging Far-Infrared Line-Spectrometer (FIFI-LS) entered service on the Stratospheric Observatory for
Infrared Astronomy (SOFIA) on March 2014.
Exact pointing of the instrument is important. The SOFIA telescope provides an absolute pointing stability of 1” rms,
which is sufficient for FIFI-LS. The instrument boresight relative to the telescope reference system is established with
accuracy better than 1”. FIFI-LS has a built-in rotating K-Mirror to derotate the instrument field of view. Perfect
alignment of the optical axis of the K-Mirror and the optical axis of the optical systems in both instrument channels is
practically impossible. The remaining offsets result in a dependence of the instrument boresight on the K-Mirror
position. Therefore a boresight calibration model is established for each channel. With these models the instrument
boresight is calculated and transferred to the telescope control software. Achieving precise calibration of the boresight
has been an ongoing process including the first optical models of the instrument, measurements in different laboratories
and finally measurements during the commissioning flight series. In this paper, the approach used to calibrate FIFI-LS’s
boresight is explained. This includes the model used and an overview of the laboratory, as well as the in-flight
measurements leading to the calibrated boresight model.
We describe observational operations and data reduction for the science instrument FIFI-LS (Field Imaging Far Infrared
Line Spectrometer) onboard SOFIA (Stratospheric Observatory for Infrared Astronomy). First, the observation strategy
is explained, which plans all the various observing modes and parameters based on the targets and the limitations of the
observatory and instrument. Next, the observations must be created in a format readable by instrument control software,
via a system of algorithms. Once the observations have been planned and prepared, they must be scheduled, executed
and analysed, and this process is outlined. The data reduction system which processes the results from these
observations, beginning from retrieving raw data, to obtaining a FITS file data cube readable by analysis programs, is
described in detail.
The Field Imaging Far Infrared Line Spectrometer (FIFI-LS) obtains spectral data within two wavelength ranges. The observed wavelengths are set by rotating the two diffraction gratings to specific angles. This paper describes on the grating assemblies, designed to rotate and stabilize the gratings. First the assembly itself and its special environment inside FIFI-LS is explained. Then a method is layed out how to monitor the performance of the drive and how to detect upcoming failures before they happen. The last chapter is dedicated to first inflight measurements of the position stability of the grating.
FIFI-LS is the German far-infrared integral field spectrometer for the SOFIA airborne observatory. The instrument offers
medium resolution spectroscopy (R ~ a few 1000) in the far-infrared with two independent spectrometers covering 50-110
and 100-200 μm. The integral field units of the two spectrometers obtain spectra covering concentric square fields-of-views
sized 3000and 6000, respectively. Both spectrometers can observe simultaneously at any wavelength in their ranges making
efficient mapping of far-infrared lines possible.
FIFI-LS has been commissioned at the airborne observatory SOFIA as a PI instrument in spring 2014. During 2015,
the commissioning as facility instrument will be complete and the SOFIA observatory will take over the operation of
FIFI-LS. The instrument can already be used by the community. Primary science cases are the study of the galactic and
extra-galactic interstellar medium and its processes.
In this presentation, the capabilities of FIFI-LS on the SOFIA telescope will be explained and how they are used by the
offered observing modes. The remaining atmosphere and the warm telescope create a high background situation, which
requires a differential measurement technique. This is achieved by SOFIA’s chopping secondary mirror and nodding the
telescope. Depending on the source size, different observing modes may be used to observe a source. All modes use spatial
and spectral dithering. The resulting data products will be 3D-data cubes.
The observing parameters will be specified using AOTs, like the other SOFIA instruments, and created via the tool
SSPOT which is similar to the Spitzer Space Telescope SPOT tool. The observations will be done in service mode, but
SOFIA invites a few investigators to fly onboard SOFIA during (part of) their observations.