Work is underway at the University of Chicago and Caltech Optical Observatories to implement a sodium laser guide star adaptive optics system for the 200 inch Hale telescope at Palomar Observatory. The Chicago sum frequency laser (CSFL) consists of two pulsed, diode-pumped, mode-locked Nd:YAG lasers working at 1.064 micron and 1.32 micron wavelengths. Light from the two laser beams is mixed in a non-linear crystal to produce radiation centered at 589 nm with a spectral width of 1.0 GHz (FWHM) to match that of the Sodium-D2 line. Currently the 1.064 micron and 1.32 micron lasers produce 14 watts and 8 watts of TEM-00 power respectively. The laser runs at 500 Hz rep. rate with 10% duty cycle. This pulse format is similar to that of the MIT-Lincoln labs and allows range gating of unwanted Rayleigh scatter down an angle of 60 degrees to zenith angle.The laser system will be kept in the Coude lab and will be projected up to a laser launch telescope (LLT) bore-sited to the Hale telescope. The beam-transfer optics, which conveys the laser beam from the Coude lab to the LLT, consists of motorized mirrors that are controlled in real time using quad-cell positioning systems. This needs to be done to prevent laser beam wander due to deflections of the telescope while tracking. There is a central computer that monitors the laser beam propagation up to the LLT, the interlocks and safety system status, laser status and actively controls the motorized mirrors.We plan to install a wide-field visible camera (for high flying aircraft) and a narrow field of view (FoV) IR camera (for low-flying aircraft) as part of our aircraft avoidance system.
Stellar scintillations are considered noise in adaptive-optics sensors and are measured for calibration purposes only. We propose to use scintillations to provide direct instantaneous information about the structure of the atmosphere. As a result it will be possible to increase the field of view provided by adaptive optics. The scintillation pattern is created when stellar light is diffracted by high-altitude turbulence. Alternatively, this pattern can be viewed as a Laplacian of this turbulence and can thus be inverted to estimate it. The measurement is limited by the intensity and the angular size of the reference star, by the height distribution of the atmospheric turbulence, and by the detector resolution and spectral response.
We have successfully used nulling interferometry at 10 wavelength to interferometrically suppress a star's radiation. This technique was first proposed by Bracewell 20 years ago to image extra-solar planets1 and is now the basis for proposed space-borne instruments to search for Earth-like extra-solar planets and their spectroscopic signatures of habitability and life2'3'4. In our experiment, the beams from two 1.8 m telescopes of the Multiple Mirror Telescope were brought into registration at a semi-transparent beamsplitter, and the images made coincident on an infrared array detector capable of taking rapid short exposure images. The atmospheric fluctuations caused the phase difference between the beams to fluctuate, changing the total flux of the star seen in the image plane. When the atmosphere caused the wavefronts to be exactly out of phase the entire stellar Airy pattern disappeared. For the unresolved star a Tauri the cancellation was such that a companion only 0.2 arcsec from the star and 25 times fainter would appear equal in intensity to the nulled star. The residual flux was spread into a wide halo suggesting the cause of this flux was imperfect cancellation of the aberrated wavefronts. To increase the precision of nulling beyond this first step several sources of error need to be addressed. We discuss the control of errors due to amplitude, polarization, chromatic differences, stellar leak, and sampling time. Improvements such as active phase tracking, adaptive optics, and cooled optics will increase the achievable gain of nulling interferometry and allow it to be used on fainter objects.
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