The previous chapter examined wavefront sensor techniques that relied on measuring either local tilt (dWâdy) or the differential phase, dW, in the exit pupil. This chapter will concentrate on instruments that make measurements in the image plane. The generic name for this class of wavefront sensor is the Hartmann sensor. It is based on the classical Hartmann test. This was commonly used to measure the surface quality of astronomical telescope primary mirrors during polishing. For example, the 4 meter primary of the Mayall telescope atop Kitt Peak was tested in this manner.
6.2 Basics of Hartmann Sensing
An aperture plate with a tiny circular hole is placed in the entrance pupil of a âperfectâ lens. The plate can be moved around so that the hole can be positioned anywhere within the entrance pupil. Further, the aperture plate is mounted on a micrometer driven XY-translation stage so that the hole position relative to the optical axis can be monitored. When the micrometers read X=0 and Y=0, the hole is centered on the optical axis. This is illustrated in Fig. 6.1. The hole defines a âfatâ light ray. The data sampling interval is equal to the diameter of the hole as illustrated in Fig. 6.2. Every time the hole is moved a distance equal to its own diameter, a measurement is made.
Suppose a perfectly collimated beam is directed parallel to the optical axis. The hole passes a ray that will cross the optical axis at the paraxial focal point as shown in Fig. 6.3 (a). The transverse ray aberration, T, (Sec. 2.3) is zero. The quantity T will be measured with a position-sensitive detector (Sec. 3.3).