Alignment with a combination of deflectometry and the sine condition test

Abstract

We propose a new alignment method that is based on relative measurements with an on-axis test setup composed of a pixelated camera and a monitor. By combining deflectometry and the sine condition test, the new method eliminates the necessity of moving a test instrument to multiple field points but still estimates the state of alignment by measuring both the off-axis and on-axis performances of the system. Additionally, it can be a very cost-effective option for certain projects as a monitor, and a camera may be substituted for the return optic and the interferometer required in a conventional interferometric method. We explain the concept of the new alignment method using a meter-class Ritchey–Chrétien telescope. Additionally, we present a new metric, the Metric for Misalignment Indicators (MMI), which represents the transmitted wavefront error caused by misalignment in the system. Then we demonstrate the validity of the concept using simulations where a poorly aligned telescope is the starting point to show that this method has a larger dynamic range compared to the interferometric method. Even considering realistic levels of noise, the new alignment method works successfully, as there is an improvement of two orders of magnitude in the final MMI after three iterations of alignment. The MMI of perturbed telescope models is about 10 µm but, after alignment, the MMI converges to one-tenth of a micrometer.