Optical Quality and Interferometric Test Protocol

1) Quality Assurance and Interferometric Measurements

To assure Spacewalk Telescopes' high quality standard, every manufactured mirror is measured and tested many times between the start of the manufacturing process and the finished product.

This continuous review process already starts with the surface grinding of the mirror blank´s back side. Here the planar surface needs to be achieved to ensure that the mirror will be evenly supported by the mechanical support elements, preventing the creation of astigmatism during the further grinding and polishing steps which would require a time intensive correction and is to be avoided.

During the grinding stages the optically used surface is controlled concerning small surface defects (so called pits) to make sure the current grain size has been used for a sufficient time before advancing to the next, finer grade. When polishing the mirror in the following phase, the surface condition and polishing progess is tracked via a reflected-light microscope so the number of pits is reduced to a minimum.

The final and very important phase of creating the parabolic shape of the mirror is constantly controlled using interferometric measurements. Errors concerning the parabolic form, for example astigmatism, steep zones or a central „hill“ can effectively be checked and ruled out as well as an over- or undercorrection.

In the end after finishing the form a thorough final interferometric test of the mirror surface is performed. Many interferograms of the mirror in different positions on the testing system are averaged and analysed. This leads to an exact and comprehensive representation of the mirrors optical performance. All the information collected during this final measurement is included in a test certificate. Such a certificate comes with every mirror manufactured by Spacewalk Telescopes.

A sample protocol can be downloaded under the following link:

Sample Protocol

In case you decided on a mirror set including not only the primary mirror made by Spacewalk Telescopes but also a matching secondary mirror you will also receive an interferometric certificate for the secondary created with a so called „Zygo“ interferometer that is provided by the supplier.