SOFIA had three acquisition/guider/tracking cameras, the Wide Field Imager (WFI), the Fine Field Imager (FFI),and the Focal Plane Imager Plus (FPI+), which were designed to determine where the telescope was pointed on the sky and provide telescope pointing corrections via optical tracking on guide stars. The WFI and FFI were mounted on the telescope head ring and can be seen in the photograph below. The FPI+ imaged the telescope focal plane via a dichroic and a second tertiary mirror. All three cameras operated at optical wavelengths. The properties of the imagers are given in the table below.

Telescope showing location of Fine Field Imager and Wide Field Imager

Properties of Imagers
Parameter FPI+ FFI WFI Notes
Optics description main 2.5m TA (Cassegrain) + 8x reduction Schmidt-Cass + field expander F/2 Petzval lens FPI+ has ±60 cm focus range, reticle, reimaging optics
aperture diameter 2500 mm 254 mm 68 mm
FOV 9’ diam. 67’ x 67’ 6.0° x 6.0°
image scale (arc—(arc-sec/pixel) 0.51 ”/pxl 4.1 ”/pxl 21 ”/pxl FPI+ scale varies 10% over focus range
star image size
(80% Enc Energy)
6 ” 13 ” ~1.2 ’ FPI+ seeing-limited; FFI, WFI optics limited
R-band sensitivity
(S/N = 10 in 1 s)
>16 11 9
centroid precision
(S/N = 10)
~0.05 ” ~1 ” ~8 ”
Filters available
  • 3 ND’s
  • red (day)
  • clear
  • blankoff
  • Sloan U'
  • Sloan G'
  • Sloan R'
  • Sloan I'
  • Sloan Z'
  • 3 ND’s
  • red (day)
  • clear
  • blankoff
  • 3 ND’s
  • red (day)
  • clear
  • blankoff
ND (FFI/WFI) T: 8% - 75%
ND (FPI+) T: 0.001% - 5%
"red" = Schott RG1000
( l > 900 nm)

The FFI and WFI used the same model Proscan "High Speed Slow-Scan" camera, containing a Thomson 7888A frame transfer CCD. During the Focal Plane Imager Upgrade in early 2013, the FPI+ camera was replaced with a Andor iXon DU-888 camera with an E2V CCD201-20 EMCCD. With this upgrade, the FPI+ had a higher sensitivity, lower noise and faster readout speeds. An upgrade of the front-ring cameras FFI and WFI was under development. The basic (nominal) properties of the old and upgraded CCD are given in the table below.

CCD Properties
Parameter Value or range Notes
Imager/CCD FFI/WFI Thomson 7888A FPI+ E2V CCD201-20
Array dimensions 1024 x 1024 image area 1024 x 1024 frame transfer readout
Pixel size 14 microns 13 microns
Binning options 1x1, 2x2, 4x4 1x1, 2x2, 4x4 4x4 done off chip for FFI/WFI
Integration time 10 - 10,000 ms 1 - 10,000 ms
Data format 14-bit 2 MHz or 8-bit 5 MHz 16-bit 1 MHz or 14-bit 3 MHz or 14-bit 5 MHz
Maximum frame rate 8 frames / s 11 frames / s 4x4 binning, 5 MHz
Peak Q. E. 18% at 550 nm (front illuminated) 94% at 560nm (back illuminated)
Read noise 60 e - = 5 DN 6 e- at 1 MHz = 9 DN 36 e- at 5 MHz = 4 DN
Dark current rate
(-30 C)
50 e -/pixel/s 0.06 e-/pixel/s New FPI+ is thermo-electrically cooled, FFI and WFI stratospheric conditions
Gain 12 e - / DN (14 bit, 2 MHz) ~600 e- / DN (8 bit, 5 MHz) 0.7 e- / DN (16 bit, 1 MHz) 1.9 e- / DN (14 bit, 3 MHz) 8.9 e- / DN (14 bit, 5 MHz)
Saturation 170,000 -200,000 e - 80,000 e-

The imagers acquired star field frames and on-board control software correlated the star positions with star catalogues to determine the attitude of the telescope with respect to the sky. The guider camera images could also be captured during flight and saved as FITS files; they are archived along with the science instrument data. The two image pairs shown here provide a comparison between images acquired with the WFI during tests in the summer of 2004 and star charts for the associated fields.

For telescope diagnostic purposes and for science mission support it was possible to operate FPI+ from a dedicated laptop. When operated in this mode an additional 14-bit, 10 MHz readout rate was available as well as the readout of sub-frames which could increase the maximum frame rate to 400 frames per second (for science observations specifications, see the Observer's Handbook).

Star chart of Cepheus area compared to actual WFI image of same FOV

Star chart of Cassiopeia area compared to actual WFI image centered on Epsilon Cas.