Mirror Making at Spacewalk Telescopes



1) Introduction:


At Spacewalk Telescopes parabolic mirrors with a diameter of 300... 635mm are manufactured and produced in-house. So besides the required mirror optics for the telescope series "Infinity NL" and "Horizon" also custom-made products for special projects can be offered.

For this purpose, a grinding machine is available for the rough work, such as surface grinding, milling of the blanks or rough grinding. The fine work, such as polishing or parabolizing, is then carried out on another machine. Both machines are designed for a maximum blank size of D=640mm. Different grinding and polishing tools allow the production of many different mirror sizes and focal lengths.

The control and measurement of the mirror surface is done with spherometers, a microscope and an interferometer.

2) The manufacturing process of a parabolic mirror


The following short introduction to the production of a parabolic mirror will give you a look behind the scenes of the mirror production at Spacewalk Telescopes.

At the beginning there is the processing of the back side. Since the blanks are not flat but slightly wavy, the backs of several blanks are ground flat against each other. This prevents any undesirable effects from occurring during fine grinding or polishing during later machining. The sides of the blank are then ground and the chamfers on the edges are traced.

Now the desired radius can be milled into the front side. The required depth can be easily calculated using a simple formula. The grinding tool is also roughly adjusted to the desired radius of curvature. Now everything is ready for the rough grinding of the blank. The grooves from milling out are smoothed with a coarse abrasive. The focal length is checked from time to time. Whenever a grit has been ground to the end, a change to the next smaller grit is made. This is done until the surface is very smooth, spherical and even slightly reflective when backlit.

The blank is then polished. The shape and surface are checked again and again. After a few hours, the mirror blank is polished over the entire surface and is spherical in the optimum case.

In an initial measurement, the focal length can now be determined to within a few millimeters. In addition, the interferometer is used to check the surface for larger deviations, which are then smoothed with another polishing tool before the final parabolization. When the mirror is sufficiently spherical, parabolization begins. A whole range of different tools with different diameters and pitch hardnesses are available for this purpose. In this phase, the degree of parabolization is checked again with the interferometer after each session so that the next polishing session can be derived from it.

After the end of the parabolization, the final measurement of the mirror is performed with the interferometer. For this purpose, a large number of interferograms are recorded in different mirror positions with the camera and averaged. Once the mirror has achieved the desired surface accuracy, the test report is generated and the mirror can be sent for coating.