Technique Links

Glass Loading

Casting

Lifting

Cleaning

Grinding

Polishing

Testing

Integration

Shipping

Mirror Interferometric Testing Techniques

	The figure of the mirror's surface is measured using a technique known as phase-shifting interferometry. An interferometer illuminates the mirror with laser light, forming a template wavefront that has the same shape as an ideal mirror. Light reflected from the mirror is captured and analyzed to determine the difference between the actual surface and this ideal template. The result of the measurement is a contour map, as shown here, with contour intervals of about 1/100 of a wavelength, or 6 nm. This topographic map is used to guide the next polishing run, and to evaluate the quality of the surface and the optical images the final mirror will produce.
	These two images (at left and below left)  show the null lens used to measure the LBT's 8.4-m mirrors at 10.6 microns. The greatest challenge in measuring a fast mirror, i.e., one with a short focal length, is to create a template wavefront that matches the shape of the mirror surface. A laser source naturally creates a spherical wavefront, one that has the same curvature everywhere. A set of lenses known as a null lens transforms this spherical wavefront into an aspheric wavefront that matches the ideal mirror surface. The null lens changes the shape of the template wavefront by up to 1.4 mm -over 2000 waves, and it must make this transformation with an accuracy of 0.01%.
	Initial measurements of the rough ground surface of the mirror are made in the infrared at a wavelength of 10.6 microns.  Light at this wavelength effectively reflects off the ground surface of the mirror and provides a good measurement when the surface errors range from a few hundred nm to a few tens of microns. Once the surface is polished, it is illuminated with red laser light at a wavelength of 633 nm for the measurement, and a different null lens is used to produce the template wavefront. This measurement is sensitive enough to see ridges and valleys of order 6 nm above or below the average surface.