Fabio Fumi

The SOFIA far-infrared spectrometer FIFI-LS: spearheading a post Herschel era

FIFI-LS (Field-Imaging Far-Infrared Line Spectrometer) is an imaging spectrograph for SOFIA comprised of two medium resolution (R~2200) grating spectrometers feeding two 16x25 pixel detector arrays, which enable simultaneous line observations across two wavelength ranges (42-110 μm and 110-210μm) each across a field of view of 5x5 pixel. FIFI-LS will be the extragalactic spectroscopic workhorse for SOFIA. FIFI-LS has enough sensitivity to observe a substantial sample of nearby galaxies. It also has the right combination of wavelength range and spatial resolution to carry out unique new observations beyond those possible with Herschel, Spitzer, ISO and IRAS. As the effective sensitivity of FIFI-LS is only about a factor of 3-5 lower than the PACS spectrometer onboard Herschel, mainly due to an enhanced multiplexing advantage, FIFI-LS will build upon recent exciting scientific results and spearhead the post- Herschel far-infrared era. FIFI-LS is scheduled for commissioning onboard SOFIA in early 2014. An account on the instrument and its current stratus will be presented.

FIFI-LS: the facility far-infrared spectrometer for SOFIA

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.

FIFI LS: a far-infrared 3D spectral imager for SOFIA

FIFI LS is a far-infrared integral field spectrometer for SOFIA that maximizes observing efficiency by spectrally imaging fields in two medium velocity resolution bands simultaneously and nearly independently. Although the two observing bands, Red (110-210 microns) and Blue (42-110 microns), share some common fore-optics, the Field-Imaging Far-Infrared Line Spectrometer (FIFI LS) can observe diffraction-limited spectra at R = 1400 to 6500, depending on wavelength, with two separate Littrow mounted spectrometers. To further increase the observing efficiency, we employ an integral field technique that allows multiplexing spatially. This is achieved by utilizing slicer mirrors to optically re-arrange the 2D field into a single slit for a standard long slit spectrometer. Effectively, a 5 × 5 pixel spatial field of view is imaged to a 25 × 1 pixel slit and dispersed to a 25 × 16 pixel, 2D detector array. The detectors are two large format Ge:Ga arrays, axially stressed in the Red channel to achieve a longer wavelength response and slightly stressed in the Blue channel. Overall, for each of the 25 spatial pixels in each band, the instrument can cover a velocity range of approximately 1500 km/s with an estimated sensitivity of 2 × x 10-15 W Hz1/2 per pixel. This arrangement provides good spectral coverage with high responsivity. With this scheme FIFI LS will have advantages over single-slit spectrometers in detailed morphological studies of the heating and cooling of galaxies, star formation, the ISM under low-metalicity conditions as found in dwarf galaxies, active galactic nuclei and their environment, starbursts, and merging/interacting galaxies.

FIFI-LS observation planning and data reduction

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.

Precise angular positioning at 6K: the FIFI-LS grating assembly

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.

Real-time operation without a real-time operating system for instrument control and data acquisition

We are building the Field-Imaging Far-Infrared Line Spectrometer (FIFI LS) for the US-German airborne observatory SOFIA. The detector read-out system is driven by a clock signal at a certain frequency. This signal has to be provided and all other sub-systems have to work synchronously to this clock. The data generated by the instrument has to be received by a computer in a timely manner. Usually these requirements are met with a real-time operating system (RTOS). In this presentation we want to show how we meet these demands differently avoiding the stiffness of an RTOS. Digital I/O-cards with a large buffer separate the asynchronous working computers and the synchronous working instrument. The advantage is that the data processing computers do not need to process the data in real-time. It is sufficient that the computer can process the incoming data stream on average. But since the data is read-in synchronously, problems of relating commands and responses (data) have to be solved: The data is arriving at a fixed rate. The receiving I/O-card buffers the data in its buffer until the computer can access it. To relate the data to commands sent previously, the data is tagged by counters in the read-out electronics. These counters count the systems heartbeat and signals derived from that. The heartbeat and control signals synchronous with the heartbeat are sent by an I/O-card working as pattern generator. Its buffer gets continously programmed with a pattern which is clocked out on the control lines. A counter in the I/O-card keeps track of the amount of pattern words clocked out. By reading this counter, the computer knows the state of the instrument or knows the meaning of the data that will arrive with a certain time-tag.

Spectral and Spatial Characterization and Calibration of FIFI-LS — The Field Imaging Spectrometer on SOFIA

The field-imaging far-infrared line spectrometer (FIFI-LS) is a science instrument for the Stratospheric Observatory for Infrared Astronomy (SOFIA). FIFI-LS allows simultaneous observations in two ...

Observing with FIFI-LS on SOFIA: time estimates and strategies to use a field imaging spectrometer on an airborne observatory

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.

FIFI-LS diffraction grating vibration on SOFIA

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.

Boresight calibration of FIFI-LS: in theory, in the lab and on sky

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.