An AO-free coronagraph experiment in vacuum with a binary-shaped pupil mask
We present our first results from laboratory experiments on a binary-shaped checkerboard
mask coronagraph that was fitted inside a vacuum chamber for the development of
skills to the direct observation of extra-solar planets. The aim of this work
was to utilize a vacuum chamber for our coronagraph experiments in order to achieve
an environment with higher thermal stability and which is free from air turbulence.
We also aimed to evaluate and improve the performance of such a system consisting
of a vacuum chamber with a coronagraph set inside the chamber. Both the raw contrast
and the contrast after point spread function (PSF) subtraction are evaluated.
We sited the vacuum chamber in a clean room, and we installed an optical fiber
coupled to a visible He–Ne laser, appropriate coronagraph optics, a temperature
sensor and heaters in the chamber. This provided a vacuum environment and a temperature-controlled
environment with a visible light source, and was shown to improve the stability
of the coronagraph. A contrast of 1.7x10-7
was achieved for the raw coronagraphic images by analyzing the areal mean of all
of the observed dark regions. A contrast of 7.3x10-9
was achieved for the PSF subtraction by areal variance (1σ) of all
of the observed dark regions. Speckles were a major limiting factor throughout
the dark regions of both the raw images and the PSF subtracted images. The application
of PSF subtraction for the Space Infrared telescope for Cosmology and Astrophysics
(SPICA) and for other platforms is discussed.