Study of Coronagraphic Techniques
Abstract
Direct imaging of extra-solar planets is important for determining the properties of individual planets, for studying multi-planet systems, and for observing the spatial structure of debris disks. Obtaining spectra of extra-solar planets enables us to constrain the composition of planetary atmospheres and surfaces, their climates, and their rotation periods. The techniques required to isolate and detect an extra-solar planet next to its host star are quite challenging and require significant improvement. SAO has set up a testbed to study coronagraphic techniques, starting with Labeyrie's multi-step speckle reduction technique.
The testbed consists of a classical coronagraph with high precision optics. A telescope is simulated by a 2 inch spherical mirror with lambda/1000 surface quality. The focal length (1 meter) of this mirror was chosen that spherical aberration can be neglected. A spatially-filtered laser simulates the host star and an optional attenuated second laser simulates the planet. As an additional option, we can incorporate apodizing masks to further improve the performance of the coronagraph. The output signal of the coronagraph is fed into a single Labeyrie correction stage. It consists of a mirror to relay the light onto a 140-element MEMS deformable mirror (DM) for the phase correction. The reflected light is then focused onto a second occulter to block most of the speckle light and finally imaged onto a CCD. The phase correction function and, thus, the drive signal for the DM, is derived from images taken in and slightly out of the focal plane using phase diversity. The expected performance improvement is about one order of magnitude. An advanced concept utilizing phase and amplitude correction promises an even higher degree of speckle light suppression. In addition, we are using the testbed to characterize occulter masks developed in collaboration with Harvard University and Lockheed Martin Corp. At Harvard University we are developing a method to shape occulter masks out of dye-doped PMMA using a focused ion beam (FIB) system. Using dye-doped PMMA should enable us to manufacture masks working at any wavelength from the visible to the near-infrared. It should also be possible to manufacture masks for the IR if a suitable mask material can be found. In order to test the absorption profile of these masks, we are developing a high-precision mask scanner. This work is supported by NASA through grant NNG04GC57G, SAO IR&D funding, NSF REU program and Harvard College.- Publication:
-
The Advanced Maui Optical and Space Surveillance Technologies Conference
- Pub Date:
- 2006
- Bibcode:
- 2006amos.confE..16T