Long Trace Profiler (LTP)

The determination as the optics substrate shape during the manufacturing phase and after is essential in order to evaluate the focusing performances. The presence of errors in the spatial low frequencies range (figure errors or slope errors) causes the rise of unwanted aberration that, in the worst cases, can degrade the focused spot in such a way that the obtained images are unusable for scientific purposes. An accurate evaluation of the effect due to these errors over the optics surface is obtained by using wavefront propagantion codes that simulate the spot at focus, as well as along the whole optical system.The Long Trace Profilometer (LTP) is a non-contact optical profiling instrument designed specifically for measuring the slope and figure errors of large optics mirrors (with planar, spherical and aspherical shapes) with an high resolution and repeatability. It is based on the original concept of the pencil-beam interferometer von Bieren (1982), and developed by P. Tackacs, Q. Shinan et al. (1986), which employs two Ne-He laser beam pencils scanning the surface of the mirror under test. The reflected beam direction changes according to the local surface slope at that position, and a Fourier transform lens converts the angle variation of the reflected beam in a variation of position in its focal plane. The focused laser beam position is recorded by a CCD detector and, after a proper fit, the local slope of the mirror under test is obtained (see figure 1). If the instrument is properly calibrated, thermo-isolated and the surrounding environment vibration-free and air-flow controlled, the LTPIV at Optical metrology Lab can reach resolutions below 100μrad with a scan length up to meters with a sub-mm lateral resolution. This kind of instrument covers a spectral band meters-millimetres and below.
 

Figure 1: Optical Head sketch of the LTPIV.

Figure 2: Example of figure error of a KB-bendable mirror measured using the LTPIV.