The Focal Plane Imager (FPI+) is the standard tracking camera for the SOFIA telescope, utilizing a science grade CCD sensor. Since the FPI+ is a subsystem of the SOFIA tracking system, it is permanently installed on the telescope. Therefore, it can be operated on every observing flight, either stand-alone or in parallel with any science instrument that is mounted on the telescope.
As a science instrument, the FPI+ is intended to be used as a fast framerate imaging photometer in the visual wavelength range. The highly configurable readout modes of the camera can be adapted to the proposed observation needs. Examples for the scientific use of the FPI+ include observations of stellar occultations and exo-planet transits. The observations of stellar occultations benefit from SOFIA's mobility, e.g. the abilities to fly into the shadow path and to avoid cloud cover. The observation of exo-planet transits benefit from the much reduced scintillation noise at flight altitude, resulting in higher signal-to-noise ratios in the light curves compared to ground based measurements.
Most of the visual light passes SOFIA’s tertiary beam splitter (M3-1) before it is reflected into the Nasmyth tube by the fully-reflective tertiary (M3-2). A significant amount of visual light is not transmitted, but rather absorbed or reflected along with the longer, infrared wavelengths. However, in the range between 480 nm to 800 nm, where the visual-light CCD cameras are most sensitive, more than 50% of the light is transmitted to the FPI+. The visual light continues through a set of four silver-coated folding mirrors inside the so called delay line assembly of the telescope. This setup allows focusing the FPI+ independently from the instrument at the telescope science instrument flange. A pair of windows is installed between the Nasmyth tube and the delay line that create the boundary between the stratospheric conditions in the telescope cavity and cabin conditions inside the delay line assembly. Two eyepiece lenses are used to collimate the telescope beam. Close to the camera is a pellicle beam splitter made of a nitrocellulose membrane with 85% transmission. The beam splitter can be used to reflect a reticle into the light path for camera alignment purposes. The last optical element in front of the camera is an industrial ZEISS 1.4/85 mm Planar T* IR photo lens.
A double-carousel, filter wheel with six positions on each carousel is installed between the reticle beam splitter and the ZEISS lens.
The image quality at visible wavelengths on SOFIA is dominated by seeing and image motion. The major source of seeing is the turbulent shear layer across the telescope cavity, which causes scattering of the light from density fluctuations. These fluctuations are strongly dependent on the mean static air density and the Mach number. The resulting wavefront error is smaller at longer wavelengths. An average image size between 3.5 arcseconds FWHM and 4 arcseconds FWHM can be expected for the FPI+, depending on flight altitude and observed wavelength.
The CCD sensor of the FPI+ is an e2v CCD201-20 1024 x 1024 pixel frame transfer EMCCD with a plate scale of 0.51 arcsec/pix and a square field of view (FOV) of 8.7 x 8.7 arcmin. The unvignetted FOV is a circular beam of approximately 9 arcmin diameter centered on the FPI+ sensor. Pixel binning of 2x2, 4x4, etc. is available and can be used to increase the frame rate and reduce the effective readout noise. In flight, the seeing blur size of the observatory is at about 4 arcsec diameter. Therefore, a reduction of the angular resolution by binning up to 4x4 (2x2 arcsec2) still provides critical sampling of the seeing element.
The wavelength range of the FPI+ is 360 nm to 1100 nm. Six spectral filters are available within this range. These are five Sloan Digital Sky Survey filters u’g’r’i’z’ and a Schott RG1000 near-IR cut-on filter. Additionally, three neutral density (ND) filters can be used to attenuate bright stars. The ND filters are required for the tracking function of the FPI+ and the optical densities are chosen in such a way that stars within the brightness range of 0 < V mag < 16 can be imaged with an exposure time of 1 second. The Daylight filter is also a requirement for telescope tracking to be able to acquire bright guide stars in twilight.
Table 5-1: Filter Wheel Configuration
Table 5-1 shows the configuration of the FPI+ double filter wheel. Filters from carousel one and two can be combined freely with a few exceptions. The daylight tracking filter from carousel two can only be used with the OPEN position of carousel one to avoid non-overlapping wavelength bands. The Blocked position cannot be selected for observations, but instead is used for taking calibration data (bias frames, dark frames).
Figure 5-1 shows a plot of the FPI+ total system throughput, which includes a model for atmospheric extinction, the calculated SOFIA telescope throughput, and the instrument quantum efficiency. The filter spectral response has been measured and is added to the plot. At the wavelengths where the Sloan u’ filter is transparent, other elements in the FPI+ light path (dichroic tertiary mirror, protected silver coatings, ZEISS lens) are nearly opaque. This results in a very low throughput (~0.5%) for the selection of the FPI+ with the Sloan u’ filter.
Figure 5-1. Total system throughput for Sloan filters, the Schott RG1000 (daylight) filter and the OPEN FPI+ configuration.
Figure 5-2 is a plot of the neutral density filter transmittance vs. wavelength for the three installed ND filters. Over the entire wavelength range of the FPI+, the ND filters have the average optical density listed in Table 5-2. However, there is a wavelength dependence of the optical density of all ND filters that has to be considered when using the ND filters in conjunction with the Sloan filters. All filters are par focal despite their different thicknesses, because they are installed in the parallel beam in front of the Zeiss lens.
Figure 5-2. Transmittance curves of the FPI+ neutral density filters.
The instrument sensitivity and resolution is provided to analyze the feasibility of scientific investigations. The sensitivity of the FPI+ in its different Sloan filters was measured in-flight as part of the camera upgrade verification. The selected star field had targets with a wide range in V mag brightness (11.1 < V mag < 16.7). The full-frame images were acquired with an exposure time of one second without pixel binning. The SNR values in Figure 5-3 are calculated with the measured signal values and the known noise sources. Displayed are the results of the OPEN configuration and the Sloan filters u’, g’, r’, i’, z’ and DAYLIGHT.
Figure 5-3. Signal to Noise Ratio (SNR) for point sources imaged with FPI+ at texp = 1 sec. Displayed is the OPEN broadband configuration as well as the spectral Sloan filters u’, g’, r’, i', z’ and the FPI+ DAYLIGHT filter.
With the camera’s multi-stage thermo-electric cooler, it is possible to achieve sensor temperatures of 100˚ C below ambient temperature. The measured dark current rate at a sensor temperature of -85˚ C, the recommended setting, is 0.001 e-/pixel/second.
The frame rates listed in Table 5-4 are for the full frame. When sub-frames are used (without FPI+ tracking) the achievable rates can be increased. The frame rate then depends on the sub-frame size and its position on the sensor.
Table 5-4: Frame Rates (frames per sec) for the Acquisition of Full Frames
FPI+ is the standard tracking camera for the SOFIA telescope, and therefore does not require background ellimination techniques to be employed. The FPI+ science instrument offers one imaging configuration with three observing modes: FPI_TRACK_SLOW_STARE, FPI_TRACK_MEDIUM_STARE and FAST_STARE. There is no slit for spectroscopy available. The observing modes are described below.
FPI_TRACK_SLOW_STARE (1 MHz) Allows image acquisition while tracking on the same images. The parallel tracking requires a reasonably bright guide star in the field of view and exposure times should not exceed 4000 milliseconds.
FPI_TRACK_MEDIUM_STARE (5MHz) The higher readout rate results in a higher associated readout noise but also in faster frame rates. The parallel tracking requires a reasonably bright guide star in the field of view and exposure times should not exceed 4000 milliseconds.
FPI_TRACK_FAST_STARE (10 MHz) The fastest readout currently possible with FPI+. This mode does not allow FPI+ tracking on the science images--however FFI tracking can be used.
The two slower observing modes offer, but do not require, tracking in the FPI+ in parallel to the acquisition of science data. However, simultaneous FPI+ tracking does impose certain restrictions on the camera acquisition setting. Acquisition setting restrictions for simultaneous FPI+ tracking are as follows:
Image frame size: Full frame
Pixel binning: 1x1, 2x2, 4x4
Exposure time: between 100 and 4,000 ms for best tracking performance
Target: Track star available in FPI+ field of view
With simultaneous FPI+ tracking, a target position accuracy of 0.17 arcsec rms has been measured over a two hour time period. There is no positional drift of the target evident.
Alternatively, all of the three observing modes can be used with tracking in the Fine Field Imager (FFI; the tracking camera on the telescope front ring) or without any tracking at all. This allows the selection of an arbitrary sub-frame and binning factors along with the choice of very short (or longer) exposure times in the FPI+. With FFI tracking, positional drifts of the target of 3.9 arcsec per hour have been observed.
The sensitivities of the FPI+ with all spectral filter options have been implemented into the SOFIA Instrument Time Estimator (SITE). This online tool allows the estimation of exposure times to achieve a desired signal to noise ratio.
Generally, observations can be set up very efficiently and overheads are small. For each observation, bias frames and dark frames will be acquired for image calibration. These calibration acquisitions will result in additional overheads.
The overhead is the instrument set-up time per observation in minutes. It is calculated with:
Overhead = A * Repeat + B
where A = 0.013 sec, B = 200 sec, and Repeat is the total number of acquisitions with the specified exposure time.
The entire duration of the observation is:
Duration = ExposureTime * Repeat + Overhead
Observations are done in the frame transfer mode. This means parameter A represents the time for the charge transfer on the sensor and has a value of 1 to 3.4 µs.
The setup time is less than 20 seconds and approximately 3 minutes should be planned for the acquisition of bias and dark calibration frames.