The camera unit of the WSO-UV is named FCU: the Field Camera Unit Instrument. The main requirements for the FCU design are:

  • High radiometric efficiency in the FUV range, minimizing the number of optical elements involved.
  • Low degradation of the spatial resolution.
  • Adequate PSF sampling to guarantee accurate astrometric and photometric measurements. The optical design shall provide the necessary magnification for each detector.
  • Wide field of view to increase the scientific outcome of the mission.

FCU outline

The original FCU design evolved from Phase A of the instrument development, following the constraints in dimensions of the platform, mass budget, environment conditions:

  • The Far Ultraviolet Channel (FUV), providing the optimum sensitivity in the 1150-1750 Å interval, and equipped with a MCP detector.
  • The Ultraviolet and Optical Channel (UVO) covering wavelegnths in the 1740-3100 Å range; the detector is a MCP.

The FCU will be mounted onto the Optical Bench (OB) top base. Similarly, the three spectrometers UVES, VUVES, and LSS are joined into a single monoblock structural unit (WUVS), and then mounted onto the OB bottom base, by fastening in three points.

The sensor of the FGS is located on the case of each spectrograph near to its input slit. The recording equipment on the OB is mounted in such a manner that the input devices are located in a working zone of the Focal Surface limited by the 30’ field of view of the Telescope.

The layout is arranged as a telephoto system (positive elements are followed by negative elements), to enlarge the overall focal length. A refocusing mechanism (RM), located at the coupling stage between the telescope and FCU, changes the distance between the intermediate image from the telescope and the first mirror of FCU. The different operation modes for the two channels (imaging, spectroscopy, or calibration) are selected trhough the mode selection mechanism:

  • Imaging: the MSM uses flat mirrors aligned to direct the light into the selected channel (FUV or UVO). The optical filters are accomodated on two wheels for each channel: there are long-pass filters and narrow band filters for specific investigations, as well as two neutral filters for each channel, to increase the dynamical range.
  • Spectroscopy: the beam is directed to the dispersive elements, which are reflection gratings located at the MSM; in the nominal spectroscopy mode, the filter wheels are positioned into a hole configuration (no filter). The FUV diffraction grating has peak efficiency at 1400 Å and groove density of 450 lines/mm, while the NUV one has the peak at 2300 Å and 250 lines/mm.
  • Calibration: a shutter is used to block the light from the telescope, and the beam is directed to the calibration subsystem in order to take flat field images on a pixel-to-pixel basis. The calibration unit includes the lamp, optics and shutter.
  • Coronographic mode: is defined to alleviate the detector fatigue when observing faint structures close to bright sources. These dead areas on the detector could be implemented with masks in the photo-cathode.
Schematic view of FCU instrument. The letters indicate the main opto-mechanical elements.

FCU features

The next table summarizes the main FCU characteristics:

FUV Channel UVO Channel
Spectral range 1150-1750 Å 200-600 nm
Peak throughput (imaging) 1400 Å 2300 Å
Field of View: imaging 70 arcsec x 75 arcsec 70 arcsec x 75 arcsec
Field of View: spectroscopy 36 arcsec x 65 arcsec 36 arcsec x 65 arcsec
Detector type CsI MCP CsTe MCP
Detector diameter 40 mm 40 mm
Detector format (equivalent) > 2048 x 2048 pix > 2048 x 2048 pix
Pixel scale 0.036 arcsec 0.036 arcsec
Scale ratio < 7 % < 7 %
Number of reflections 4 4
Temporal resolution 40 ms 40 ms
Slitless spectroscopy resolution R=500 R=500