First astronomical application of postdetection turbulence compensation : images of α Aurigae

It is well known that atmospheric turbulence typically limits the angular resolution of ground-based astronomical images to ~1 sec of arc. Compensated imaging systems are one solution to the problem of obtaining higher resolution images through the atmosphere. Alternatively, a technique known as speckle interferometry was developed by Labeyrie who discovered that high resolution autocorrelations of an object could be produced from many short exposure image frames obtained with a narrow bandwidth filter. Modifica­ tions of the speckle interferometry technique were subse­ quently developed that allowed real images to be recon­ structed from these short exposure frames. One modification, referred to as speckle holography, uses an un­ resolved star near the object of interest as a reference source in much the same way a reference beam is used in conven­ tional holography recordings. In this Communication, we describe our implementation of another method: a postdetection turbulence compensa­ tion technique for obtaining high resolution imagery through the atmosphere. We then present preliminary results from experiments performed in Feb. 1990 at the University of New Mexico's Capilla Peak Observatory. For this technique, the turbulence degraded image is re­ corded at the same time as an estimate of the atmospheric transfer function. A compensated image is then produced through a digital deconvolution. We refer to the technique as self-referenced speckle holography (SRSH) since the ob­ ject under observation acts as the reference source for a turbulence estimate, eliminating the need for an unresolved star. We use a Shack-Hartmann wavefront sensor placed at a point conjugate to the entrance pupil of a telescope to construct a reference image for an estimate of the atmo­ spheric turbulence. The construction of a reference image using a ShackHartmann wavefront sensor has been described extensively in the literature. Briefly, atmospheric distortions can be modeled as a single layer of phase distortion provided scintil­ lation is not too great. These phase distortions act as localized tilts, and the wavefront at the entrance pupil of a telescope is sampled by an array of small lenses (lenslets). The displacement of the spot focused by each lenslet can be used to determine the local tilt of the wavefront. This information can then be used to estimate the optical transfer function of the atmosphere and telescope. Due to the small diameter of the lenslets, even objects of large angular extent appear unresolved. Thus, even an extended object can act as a point reference, eliminating the need for a reference source external to the object. The technique is similar to compensated imaging systems which use a Shack-Hartmann sensor to drive a deformable mirror; however, in our case the compensation is done via postdetection computer process­ ing. SRSH is not a new imaging method, although it has never been applied to astronomical objects until now. McGlamery measured point spread functions in the laboratory and used them to correct laboratory images in 1967. In 1974, he simulated deconvolution imaging on a computer for both predetection and postdetection compensation cases. Fried described a practical SRSH system in 1987, and there have been recent laboratory demonstrations in which a wavefront sensor has been used to obtain the turbulence esti­ mate. The theory of SRSH is simple. The intensity distribution I(x,y) of a short exposure image frame can be described as