A fast movable and very accurate positioned mirror working in cryo-vacuum conditions.
One of the most important elements within METIS is a small mirror that should be able to move very fast, the chopper. To make things even more complicated it should also be able to hold a random position very stable, operate at a temperature of -190 degrees Celsius and in vacuum.
The first step was to make a mechanical design, which would be able to achieve all our strict requirements. Than all critical components were first tested in separat breadboards before used in the demonstrator. An extensive test program is part of the project.
JPE, RUG-DTPA, NOVA, ASTRON and TNO.
One of the most crucial elements within the METIS is a small and fast tip-tilt mirror, the chopper. To complicate matters, this mirror has to perform in a high vacuum environment at a temperature of 77 Kelvin (-196 degrees Celsius). Even though similar mirrors have been made before the required positional accuracy and stability are beyond the capabilities of current ‘state-of-the-art’ technologies. Therefore, a development project has been initiated with the goal to demonstrate the feasibility of the chopper.
After investigating several concepts, the preferred solution was used to create a mechanical design which could meet the stringent requirements. At the same time an extensive test plan, describing how to verify the performance of the demonstrator, was devised. Besides realizing and qualifying a chopper demonstrator, the test plan also includes component level qualification using various bread boards.
All bread board tests have been finalized and reviewed. Based on the test results a minor modification has been made to the actuators. Moreover, the initial inductive displacement sensors were rejected and replaced by a fiber-coupled laser InterFeroMeter (IFM) system with customized retro-reflectors as targets. The lessons learned during the bread board tests of the IFM system were used to modify the demonstrator design by adding a sensor alignment mechanism. After a successful (critical) design review, the chopper design was approved and the demonstrator realized. A control strategy has been implemented and the chopper has undergone an extensive performance test program, including a 100 milion cycle lifetime test. Almost all initial requirements were met (including the lifetime test). Only the stability requirement in one direction was not met. Solutions to improve the performance in that respect have been identified and give enough confidence in the overall design. We can only conclude that this development program was a big success and the acquired knowledge very valueable.
Inspection of the chopper. Photo taken by Rob Doolaard.
Tip-tilt mirrors are regularly found in optical systems and with increasing demands on the optical performance the requirements on the tip-tilt mechanism also increase towards the specifications of the METIS chopper. Possible spin-offs can be found in ground-based and space telescopes, optical instruments for environment monitoring or earth observations, scientific instruments, and lithography or optical inspection tools for semi-conductor manufacturing (JPE designed and realized two prototypes of a chopper-like mechanism to an undisclosed industrial customer.
The design and realization of the MCC hardware (mechanics and electronics) as well as the breadboard, qualification and lifetime testing are the responsibility of JPE BV. The controller design and the performance test program of the mechanism is a combined effort of the University of Groningen and SRON and RUG-DTPA. The other project partners are NOVA, ASTRON and TNO.
"Cold chopping of baby starlight",
Mikroniek 5, p19-23, 2014
Sander Paalvast, Robert Huisman, Bernhard Brandl, Huub Janssen, Bayu Jayawardhana, Frank Molster, Maurice Teuwen, Lars Venema,
Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series 9151, Advances in Optical and Mechanical Technologies for Telescopes, 2014
Set-up to test certain aspects of an instrument or item
Name of the movable mirror