SUMER Performance Characteristics

The optical design is based upon two parabolic mirrors, a plane mirror and a spherical concave grating, all made of silicon carbide. The first off-axis telescope parabola mirrors the Sun on the spectrometer entrance slit. The second off-axis parabola collimates the beam leaving the slit. This beam is then deflected by the plane mirror onto the grating. Two detectors, located in the focal plane of the grating, collect the monochromatic images of the spectrometer entrance slit. Coverage of the full spectral range of the instrument requires a wavelength scan performed by rotating the plane mirror. A baffle system, consisting of an entrance aperture, light traps, an aperture stop, a pre-slit and a Lyot stop, completes the design.

The Optical Design in Numbers

(1 arcsec at L1 corresponds to 715 km on the Sun.)

The Telescope:
   Focal length                 1302.77 mm at 75°C
   Off-axis angle               4.5°
   Equivalent f-number          10.67
   Plate scale in slit plane    6.316 µm/arcsec
   Total dynamic field-of-view  64 × 64 arcmin²
   Smallest step sizes          0.38 arcsec
      (N-S and E-W)

The Slits:                      1   × 300,
                                1   × 120,
                                0.3 × 120,
                                4   × 300 arcsec²

The Rear-Slit Camera:
   Wavelength range             5770 - 6520 Å
   Array size                   512 (spatial) pixels
   Pixel size                   25 µm

The Spectrometer:
   Wavelength ranges
      Detector A                390 -  805 Å (2nd order) *
                                780 - 1610 Å (1st order)
      Detector B                330 -  750 Å (2nd order) *
                                660 - 1500 Å (1st order)
   Collimator focal length      399.60 mm
      Off-axis angle            7°
   Grating radius               3200.78 mm
   Grating ruling               3600.42 lines/mm
   Magnification factor **      4.092 at  800 Å
      in detector plane         4.407 at 1600 Å

The Detectors:
   Array size                   1024 (spectral) × 360 (spatial) pixels
   Pixel size (mean value)      26.5 × 26.5 µm²
   Angular scale                1.03 arcsec/px at  800 Å
                                0.95 arcsec/px at 1600 Å
   Spectral scale               22.3 mÅ/px at  500 Å (2nd order) *
                                21.0 mÅ/px at  800 Å
                                45.2 mÅ/px at  800 Å (1st order)
                                41.9 mÅ/px at 1600 Å
* In the range below 500 Å the sensitivity is very low, because of three normal-incidence reflections. Strong lines have, however, been observed in this regime during the calibration phase and under operational conditions. Some lines have been observed in 3rd order.

** The magnification factor is the ratio of the effective grating focal length and the collimator focal length.

IED, 16.Dec.98

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