A Design Study of a Cryogenic High Accurate Derotator

A Design Study of a Cryogenic High Accurate Derotator

De Koe, M.R.

Spronck, J.W. (mentor)

Mechanical, Maritime and Materials Engineering

Precision and Microsystems Engineering

2013-12-17

The goal of this research was to develop a concept of a derotator for ATRON. The derotator is a device that counters the rotation of the field of view due to the rotation of the earth. The image of the European-Extremely Large telescope rotates around it’s own axis due to the diurnal motion of the earth. Because of the low signal to noise ratio of the detector it needs a longer integration time, so the image needs to stay still to prevent smearing of the image. The rotation of the image needs to be counter rotated by an optomechanical de-rotation. Because the measurements are done in the infrared spectrum the instrument is placed inside a cryostat with a temperature of 80 K and a pressure of 10-6 bar. The initial idea of ASTRON was to build a passive system where the mirrors and the housing were aligned manually. This proved an impossible task due to the desired accuracy (12,6 µrad for rotation and 5 µm translation RMS values). To get more insight in the optical configuration a study is done on how the rotations and translations of the mirrors and the derotator effect the science beam. With the use of this study, two concepts are devised and graded on their properties. A conceptual test setup is designed to test the main properties of the system and to test the feasibility of the concept. In this study it appeared that almost all the degrees of freedom influence the science beam. It was discovered that a rotation, of the derotator around a specific point had no significant influence on the science beam. With this discovery a big simplification could be applied to the design of the derotator. Within the concept study two concepts where compared. The first concept was actively steering the two degrees of freedom that came from the optical configuration study. The other concept was compensating the error with the chopper. One of the main reasons for choosing the first concept was that there would be a dedicated system to compensate the error. Compensation with the use of the chopper would create extra requirements on the chopper. A test setup was designed to test one degree of freedom on feasibility of the concept. The discovery that the error can be compensated by using the three rotations helped in simplifying the design of the derotator and it was designed as a pendulum. The feasibility was tested and proved to work within specifications.

cryogenic
derotator

http://resolver.tudelft.nl/uuid:6bc965d8-3e7d-4fb3-a4dd-11470d3e4659

2014-01-01

Student theses

master thesis

(c) 2013 De Koe, M.R.