Aims

The principle aim of these additional observing runs was to establish the applicability of the Lucky Exposures method to general astronomical programs when using low-noise imaging detectors.

As discussed in Chapter 1.3, the range of faint astronomical targets to which the technique can be applied depends on the likelihood of finding a suitably bright reference star sufficiently close to the astronomical target, and on the limiting magnitude for the target which can be reached in a reasonable amount of observing time.

The likelihood of finding a suitable reference star depends on three principle factors:

1. the isoplanatic angle (the maximum separation angle on the sky between the reference star and a source of interest for which high image quality can be obtained);
2. the limiting magnitude of reference star which can give useful measurements of the Strehl ratio and position of the brightest speckle; and
3. the density of stars on the sky which are sufficiently bright to act as reference stars.

The observational data presented in this chapter address points 1 and 2 directly; galactic star counts from the literature are used to address point 3.

A further issue which can be explored is the astrometric potential of the method. The image quality which can be obtained in the vicinity of a reference star using the Lucky Exposures method was discussed in detail in Chapter 3.5.1. The compact stellar cores in images produced by the Lucky Exposures method should allow accurate stellar positions to be determined. The performance will be very competitive, particularly in crowded fields where other speckle imaging techniques give relatively poor performance. The high Strehl ratios for the selected exposures give high dynamic range as starlight from bright objects contributes less flux to surrounding parts of the field of view, allowing accurate astrometry on nearby faint objects. In this chapter I will undertake a simple experiment intended to assess the suitability of Lucky Exposures imaging for astrometry.

In order to obtain images with the highest possible Strehl ratios and signal-to-noise ratios, I will investigate Fourier filters which are designed to suppress the noise in the short exposure images. The performance of these filters will then be assessed using observational data.