NICI Pupil Masks

Pupil masking is accomplished with two mechanisms: the Spider Mask wheel and the Pupil Mask wheel.

The Spider Mask wheel provides a fixed-size mask for the telescope central obscuration, as well as the telescope spider vanes. The mask rotates automatically to maintain vane alignment as the telescope tracks. The mask's central obscuration radius is 0.270 x the telescope pupil radius, compared to 0.164 for the telescope central obscuration. The mask spider vanes are 30 times the thickness of the imaged telescope vanes.

The Pupil Mask wheel provides a selection of masks to stop down the outer edge of the telescope pupil. In the optical path, this wheel lies behind the spider rotator and ahead of the beam splitter. The Lyot masks expected to be available in the initial instrument configuration are listed below. Since the Spider Mask includes the central obscuration, the initial Pupil wheel masks have none; they are simply holes cut in metal plates with no substrate in order to eliminate one transmissive element.

Created 15 July 2005, B. Rodgers and T. Hayward