9.6.5 Example 5: Coronagraphic imaging of the Beta-Pictoris Disk
In the final example we shall consider the case where we are trying to determine the S/N achieved on the Beta Pictoris disk, assuming a disk surface brightness of R magnitude of 16 arcseconds2 at a distance of 6 arcseconds from the central star with a V magnitude of 3.9, for an exposure time of 1000 seconds with an F435W filter. Assume that the star and disk have an A5 V-type spectrum. Using the ACS Exposure Time Calculator
and considering the case for the 3.0arcseconds occulting mask:
- Disk count rate = 4.98 e-/second for a 2 x 2 aperture (including 47.5% throughput of coronagraph) Sky count rate = 0.010 e-/second/pixel, Detector dark rate = 0.015 e-/second/pixel
- In 1000 seconds, this gives 4,980 e-/2 x 2 aperture in the disk region.
- Central star count rate = 3.63x108 e-/second for a 101 x 101 aperture (101 x 101 aperture used to estimate total integrated flux)
- At a distance 6 arcseconds from the central star, the fraction of flux per square arcsecond in the PSF wings is 2.6 x 10-6.
BPSF = 3.63 x 1011 * 2.6x10-6 = 943.8 e- per square arcsecond. The counts collected in 4 pixels are 4 x 0.0272 x (943.8 = 2.752.
- The S/N in a 2 x 2 box is then
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