8.1 Instrument: FPI+

The Focal Plane Imager (FPI) is the standard tracking camera for the SOFIA telescope. Since an imager upgrade in 2013, the FPI uses a science grade CCD sensor and is referred to as FPI+ (FPI_PLUS). 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’s SI flange.

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.

8.1.1 Instrument Overview

In this document, the instrument sensitivity and resolution is provided to allow estimation of the feasibility of scientific investigations. The performance summaries show the expected system performance based on data collected prior to Cycle 3. The values listed will be updated during further commissioning work indented to be performed during Cycle 3 and 4 SOFIA observations.

8.1.2 Instrument Design

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.

8.1.3 Angular Resolution

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’’/pix and a square field of view (FOV) of 8.7’x8.7’. The unvignetted FOV is a circular beam of approx. 9’ 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’’ diameter. Therefore, a reduction of the angular resolution by binning up to 4x4 (2x2 arcsec2) still provides critical sampling of the seeing element.

The image quality in visible wavelengths on SOFIA is dominated by seeing and image motion effects. A wavelength dependent analysis of the image quality for the visual wavelength range has been done by the HIPO team.

8.1.4 Filter Suite

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 8-1: FPI+ filter wheel configuration for the two filter wheel carousels

Carousel 1 Carousel 2
OPEN OPEN
Sloan u' ND 1
Sloan g' ND 2
Sloan r' ND 3
Sloan I' Daylight
Sloan z' Blocked

Table 8-1 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).

Filter Throughput

Figure 8-1 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.

 

Total system throughput for filters

Figure 8-1: Total system throughput for Sloan filters, the Schott RG1000 (daylight) filter and the "OPEN" FPI+ configuration.

Figure 8-2 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 8-1. 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.

 

Transmittance curves of the FPI+ neutral density filters

Figure 8-2: Transmittance curves of the FPI+ neutral density filters.

Table 8-2: FPI+ Neutral density filter properties

Filter name Glass Type Thickness Average Optical Density
ND1 Schott NG9 4.0mm 4
ND2 Schott NG3 3.5mm 2.6
ND3 Schott NG4 2.8mm 1.3

8.1.5 Imaging Sensitivities

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 3 are the result of aperture photometry with an exponential fit in the logarithmic plot. Displayed are the results of the OPEN configuration and the Sloan filters g’, r’, i’, and z’. The plot for the “daylight” filter is a calculation based on a filter transmission measurement done in the lab. The Sloan u’ filter is not displayed in this plot because it requires much brighter stars due to the low system throughput at the corresponding wavelengths. To achieve significant SNR values in the Sloan u’ filter, stars should be brighter than V mag=6.5.

Signal to Noise Ratio (SNR) for point sources imaged with FPI+

Figure 8-3: 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 g’, r’, i', and z’.

8.1.6 Camera Performance

Table 8-3: FPI+ Camera Modes and Performance

FPI+ Observing Mode Horizontal Clock Rate Bit Depth Gain [e-/DU] Minimum Read Noise FPI+ Tracking Possible
FAST_STARE 10 MHz 14 bit 10.7 45.9 e- rms No
FPI_TRACK_MEDIUM_STARE 5 MHz 14 bit 8.9 36.1 e- rms Yes
FPI_TRACK_SLOW_STARE 1 MHz 16 bit 0.7 6.0 e- rms Yes

Dark Current:

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.

Frame Rates:

The frame rates listed in Table 8-4 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 8-4: FPI+ frame rates in frames per second for the acquisition of full frames in the three observing modes

Size FAST_STARE MEDIUM_STARE SLOW_STARE
1x1 8.9 3.8 0.9
2x2 17.5 6.9 1.7
4x4 33.6 11 3.2