Speckle Sensitivity

As was alluded to above, r0 strongly influences the sensitivity of any speckle observation. The "faster" the seeing is, the shorter the camera exposure time has to be to get enough non-overlapping speckles to process. A shorter exposure time implies a brighter magnitude limit. Also, to first order, a larger collecting aperture does not increase sensitivity. A larger telescope creates a physically smaller Airy disk as well as more speckles. In order to be able to use the techniques of speckle

Figure 17.4. A

background subtracted directed-vector autocorrelogram of the binary star 21 Oph. The central peak has been truncated for clarity. Adapted from Bagnuolo et al. Printed by kind permission of the American Astronomical Society

Figure 17.4. A

background subtracted directed-vector autocorrelogram of the binary star 21 Oph. The central peak has been truncated for clarity. Adapted from Bagnuolo et al. Printed by kind permission of the American Astronomical Society interferometry, one has to magnify the image and/or shorten the exposure time to get enough non-overlapping speckles, thereby offsetting the increased light-gathering power of the larger aperture size. A larger aperture merely gives better resolution. In 20 years of using speckle interferometry for binary star research, Hartkopf11 finds that the best results occur when the number of detector pixels across the central peak of the Airy disk is between 10 and 30. For wider systems (those where the speckles of the individual components barely overlap, and wider), better sensitivity can be gained by pushing the number of pixels below 10. However, it is not a good idea to go below 2 -one runs the risk of losing a significant amount of light to the space between the pixels. There is nothing special about these values, and the curious user is encouraged to experiment with them. They were determined empirically - autocorrelograms of the same collection of binary star systems were taken over a wide range of image scales and the results compared. Chapter 10 describes how to determine the size of the central peak of the Airy disk, and using that size to design a system will be described in the earlier section on Eyepiece Projection.

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