Sensitivity Limits

The wavefront sensor has a limit near 9th magnitude, as noted above. In general it is expected that the overall sensitivity limit will be imposed by the readout noise from the science detector, even given that coherent `blind' integration is possible. The principal reason for this is that the speckle or fringe data on the science detector must be narrow band in order to avoid smearing the fringes, and a large number of pixels () must be used in order to adequately sample all the fringe frequencies. The sensitivity limit depends on the complexity of the object structure, since a very high initial signal/noise of the combined fringe (or speckle) data is required to achieve even a modest final signal/noise for a baseline which has very low visibility due to resolved object structure. As an example, a uniform circular disc resolved into 10 pixels across the diameter has a theoretical fringe visibility that goes through 9 extrema beyond the central maximum, which have visibilities ranging from 10 to 1%. To achieve a signal/noise of 3 for the baseline with 1% visibility requires an initial signal/noise of at least 300. For an observing bandwidth of 20 nm, a CCD readout noise of 5 e, NRM with 5 holes each 10 cm square, and an observing time of 1000 sec per position angle, the limit will be at 4.5 - 5.5 mag. Using slit speckle and/or a wider wavelength band or larger holes would gain in photon rate but lose further fringe visibility due to fringe smearing. Trials and simulations will be needed to find the optimum strategy. The above limit is almost entirely due to CCD readout noise, and if essentially noise-free detectors became available (but retaining the quantum efficiency and count-rate ability of CCDs) then the limit would improve by 3 magnitudes.

Thus we expect that a system using presently realisable detectors would be limited to about 4.5 - 5.5 magnitude for the most complex feasible objects, viz. about 10 resolution elements along each axis. But for simpler, less well-resolved objects, the limit would extend to 7 - 8 mag, depending on the actual object structure and the dynamic range required.

© Copyright Astronomical Society of Australia 1997