Integral Field Spectrograph Optical Design

As shown in the previous section, the image formed by the diffraction-limited coronagraph suffers from wavelength dependent speckles. The aim of the Integral Field Spectrograph (IFS) is to take the image plane of the coronograph, and form a spectrum for every pixel in this image plane. The coronagraph image plane is divided up using a grid of lenslets, which act as spatial filters forming a grid of pupil images.

For each lenslet, the beam diverges after these pupil images, where it is then collimated, then dispersed by passing through a prism, then refocussed onto the detector.

The optical design for the IFS was developed by Dave King. It consists of:

A lenslet array

A dioptric collimator of 160mm focal length, consisting of 5 lenses of different glasses, with only spherical surfaces

A prism/disperser element

A catadioptric camera of 400mm focal length, consisting of a meniscus corrector lens, a spherical mirror, and a field-flattening lens in front of the detector.

Two fold mirrors were included to help with packaging.

The optical design was modelled using the Zemax optical design software. The complete optical train will be built into a cryogenic enclosure, cooled to 78K (-195 degrees Centigrade). The effects of the thermal contraction are taken into account in the optical design.

A powerpoint presentation of the optical design is available here

Created by David King on 29 January, 2007