Imaging
The OSIRIS optical design is based on a focal reducer with a collimator-camera configuration, which provides an internal pupil of ~ 80 mm in diameter (for a single field) (see Figures 1 and 2)
The OSIRIS collimator is an off-axis ellipsoidal mirror of conic constant -0.94 and a focal length of 1240 mm. Its vertex is located 1290.64 mm away from the telescope focal surface and off-axis displaced 228 mm relative to the telescope optical axis. An all-refractive camera is used to focus the collimated beam on a CCD detector. The camera consists of 9 lenses defining 6 optical elements, all spherical surfaces. The last lens is the dewar window. The camera effective focal length of 181 mm allows to obtain the required detector scale (~0.125 arcsec/pixel) on a flat focal plane tilted 1.83º. With the present element apertures, the camera does not vignette a telescope FOV of 8 arcmin in diameter.
A flat mirror is located between the collimator mirror and the camera to fold the optical path in order to fit the instrument to GTC envelopes. The dimensions and shape of this flat mirror were designed not to vignette or obstruct the beam of an 8.53 x 8.53 arcmin FOV. Below is the optical concept.
Parameter |
Value |
EFL
(Telescope + Osiris) |
24752
mm |
Image
focal ratio |
~2.245 |
Plate
scale |
120
m m/arcsec
8.33 arcsec/mm |
Angular
Magnification |
140.27 |
FOV
(without vignetting) |
7
x 7 arcmin |
Wavelength
range |
365-1000
nm |
Collimator
focal length |
1240
mm |
Camera
EFL |
181
mm |
Magnification
Factor |
~0.146 |
Detector
scale (1 pixel = 15m m) |
0.125
arcsec/pixel |
Image
size (FWHM) 7'x7' FOV |
<
0.3 arcsec |
Last update August 8, 2005, by Héctor Castañeda