GR1 Instrument & performance summary


Telescope Aperture

50 cm

Optical Design

Modified Ritchey-Chrétien with 4 channels: FUV & NUV Imaging, FUV & NUV Spectroscopy. FUV & NUV obtained simultaneously using dichroic beam splitter also acting as a field aberration corrector.

Field of View

~1.25 degrees, circular

Focal Length

3 m

Telescope coatings


Imaging/Grism Modes

Optics wheel with

 (1) CaF2 Imaging window, (2) CaF2 transmission grism; (3) Opaque position.

Grism Rotation

Grism position angle may be selected with a resolution of 0.4 degrees, independent of S/C roll


Aspheric astigmatism corrector

Ion-etched fused silica (aspheric surfaces on both sides)

Dichroic beam splitter with dielectric multilayer coating on input side


FUV Channel

NUV Channel

Band (>10% of peak band sensitivity)

1344-1786 Ĺ

1771-2831 Ĺ

Beam path

Reflected from dichroic

Transmitted through dichroic


Blue edge filter

(blocks OI, Ly a - transmission)

Red block filter/Fold mirror

 (blocks red Zodiacal light)


Sealed tube Z-stack microchannel plate with crossed delay-line anodes

Detector Window

MgF2, includes power for field flattering

Fused Silica

includes power for field flattering

Detector Photocathode

CsI, opaque, repeller grid on window

Cs2Te, semitransparent

300 mm proximity gap

Detector peak QE



Detector maximum local countrate w/ 10% temporary rolloff within PSF

89 c/s

471 c/s

System angular resolution

(current measured value, including image reconstruction, is ~20% larger)

6.0 arcsec (80% EE diam)

4.0  arcsec (FWHM)

8.0 arcsec (80% EE diam)

5.6 arcsec (FWHM)

Spectral Resolution



Imaging Effective Area

25 cm2

44 cm2

Photometric Zero Point [1 ct/s] mAB



Spectroscopy Effective Area

20 cm2

39 cm2