Telescoping Edges

Aluminum or Kirksite can be used to construct prototype molds. However, molds from these materials will never produce close tolerance parts with finished edges since telescoping edges cannot be produced in these soft metals. Flash-free... 

The infeed taper should not be steeper than 5 deg. Also, all surfaces of the telescoping edges should be polished or at least finished to a maximum 16-microinch roughness. Close clearances of the land are required to form flash-free parts. Some recommended tolerances are" ... 

Edge effects must be considered carefully since there are so many interior edges with a segmented mirror telescope. Again several approaches have been considered. The approach used for the Keck segments was to pohsh the mir-... 

The original bend and polish idea is due to Bernard Schmidt who built what has become known as a Schmidt telescope with a spherical primary and an aspheric corrector plate that introduces exactly the right amount of spherical aberration to cancel the spherical aberration that would be introduced by the spherical primary (Schmidt). The corrector has to be thicker at the edge in proportion to the radial distance in the aperture to the fourth power to provide the needed correction. 

A hnal application of bend and polish may be applied to new telescopes with hundreds of segments. Plane mirrors are economically made using a continuous polisher, or CP machine. The machine consists of an annulus of pitch about 2.5 times the diameter of the maximum size workpiece to be polished. The work is placed face down on the pitch and constrained from rotating with the pitch annulus by rollers touching the edge of the work. A large flat mirror can be fully polished out in one eight hour shift and the machine holds three pieces at a time (Kodak-CP). 

It can be derived from the transport of intensity equation that this signal has two terms at the edges of the pupil it is proportional to the local wavefront gradient in the direction normal to the pupil edge (i.e. radial in the case of a circular pupil) and elsewhere in the pupil it is proportional to the local wave-front curvature (Roddier, 1988). The signal is more intense when the planes are nearer the telescope focus, but diffraction will limit the spatial resolution more. Thus there is a trade-off between resolution and signal-to-noise (see Ch. 24). 

Summing up the dynamical argument, telescopes reveal the architecture and motions of the cosmos on every scale. Planets, stars, galaxies, and clusters of galaxies are nested one within the other like Chinese boxes. But then the need for dark matter suddenly arises. Without it, stars at the edge of our Galaxy would fly off and the swarms of galaxies in clusters would scatter like birds. 

Figure 1.5 Left panel the young, accreting star-disk system HH30 seen edge-on at visible wavelengths. The optically thick disk occults the star and the scattered light image shows the flaring disk surface. The system also drives a powerful jet (NASA/Space Telescope Science Institute, Burrows et al. 1996). Right panel debris disk around the 12 Myr-old low-mass star AU Mic. The disk is geometrically flat, optically thin and depleted in gas (NASA/ESA/STScI).

---