Post-Detection Turbulence Compensation

It is against the above background that the technique of `post-detection turbulence compensation' has been developed (Primot et al. 1990; Gonglewski and Dayton 1992). This method uses a wavefront sensor to find the instantaneous phase corruption imposed by the atmosphere (on the usual time scales of about 10 msec) but instead of using the corrections to drive a deformable mirror, they are used within the data reduction process to aid imaging from data that are acquired as speckle interferograms. This has the advantage of eliminating the costly and difficult construction of a deformable mirror system; there are a number of other advantages as well. On the debit side, the `science detector' must (like the wavefront sensor) use numerous short exposures, and the data reduction system is more complex. Provided that there is enough signal for the wavefront sensor to operate, the science detector data can be treated in a manner analogous to a phase stable interferometer, avoiding the and dependencies.

Several systems incorporating post-detection turbulence compensation have been used or are under development (see Andersen 1992; Marais et al. 1992). They have used full 2-dimensional coverage of the telescope aperture. The fully-filled aperture results in 2-dimensional speckle interferograms on the science detector. While this is optimum in respect of the baseline coverage of the interferometer, it has a drawback due to limitations of the currently available detectors for 2-dimensional images. These either have low quantum efficiency (image intensifiers) or a low duty cycle and significant read noise for such short images (CCDs with a shutter).

The instrument proposed here, called `MAPPIT 2', uses the same principle, but in a 1-dimensional array. By going to one dimension, the data takes a form which can be read out by CCD detectors with a tolerable level of read noise and in a sufficiently short period that a 100% duty cycle is possible. The penalty is that observations must be taken at a number of different position angles on the sky, which increases the overall duration of the observation of one object. The different position angles are easily obtained using optical beam rotation. The novel features of MAPPIT 2 are the use of post-detection turbulence compensation in combination with aperture masking and 1-dimensional operation which allows a 100% duty cycle.

It is proposed to implement the MAPPIT 2 system at the coudé focus of the AAT. It would be complementary to the AO system being constructed by a consortium of Australian institutions, in that the AO system will perform infrared imaging, whereas MAPPIT 2 will aim for the full diffraction-limited resolution in the visible range (a FWHM as small as mas (milliarcsec) for objects that can be observed successfully at nm and at the maximum baseline of 3.9 m).

© Copyright Astronomical Society of Australia 1997