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PIL assembly mechanism design

The PIL assembly design (Figure 2) consists of a titanium arm that integrates three lenses and an opposing tungsten counterweight. The arm rotates around a titanium fixed base that is mounted to the optical bench assembly (OBA) by a hexapod type of mount. The flexible hexapods are required to accommodate the differential temperature changes in the PIL with respect to the OBA which is made from beryllium. This arm is allowed to make only a partial rotation, and uses a precision ball bearing. This bearing, which uses a thin film of Teflon for lubrication at cryogenic temperatures, provides low friction, smooth motion and helps to maintain the tight positioning repeatability.

The arm is actuated through its rotation by a redundantly wound, direct-drive, 3-phase servomotor that is mounted in the actuator body. The PIL assembly also incorporates a redundant inductive sensor system that provides position feedback to facilitate required motion smoothness as well as precise rotational position telemetry. The PIL assembly motors rotate the arm through about 120 degrees of rotation against a fail-safe spring until the arm makes a hard stop at the precise deployed position of the PIL optics. After completing the required operations with the optics deployed into the NIRCam short wavelength beam, the PIL arm is actuated back to the stowed position. The PIL is held in the stowed position, without power, by the fail-safe spring acting against a non-contact magnetic stowage stop. Through the use of the spring against the magnetic stowage stop, the PIL is held in the stowed position, unpowered, even through launch conditions. It is with this mechanism that the PIL optics are deployed with the required repeatability.


The PIL assembly optical design


The PIL optics goal

A different way of stating that the PIL optics must make an image of the JWST telescope primary would be to say that the PIL optics must image the telescope’s pupil. There are, however, a number of optical pupils throughout the JWST optical system. These pupils are the positions in space where chief rays from all field points “intersect”. Three of the pupils are as follows:

  • 1.

    Primary mirror – JWST entrance pupil

  • 2.

    Fine steering mirror (FSM) – intermediate pupil created by TMA

  • 3.

    NIRCam pupil wheel – exit pupil controlled by the pick-off-mirror (POM)

We recall the optical duality between images and pupils in general optical systems. In “normal” imagery we progress from object via entrance pupil to image via intermediate pupil to relay optics to final image surface. In the NIRCam “conventional” imaging process we have:

Object (Cosmos) – Entrance Pupil (Primary Mirror) – Image (POM) – Pupil Wheel – Focal plane array. In the NIRCam pupil imaging process we have:

Object (primary mirror) – Image (FSM) – Pupil (POM) – Image (pupil wheel) added PIL to create an image on the FPA.

Therefore, the PIL optics have been designed into the short wavelength beam after the pupil wheel. After reviewing the NIRCam imager layout in Figure 3, it is obvious that the PIL optics should be positioned between the two fold mirrors just before the FPA.

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