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CIAO coronagraph
In direct imaging mode, CIAO has a Lyot stop the
same size as the pupil image. When used with an occulting mask, a slightly
undersized Lyot stop (90%) is used to suppress light diffracted by the
mask to the edge of the field.
The choice of mask size depends on your
scientific targets and seeing condition. See the Notes
for applying CIAO time page for more details.
There are various advantages to using a cold coronagraph in the infrared,
which include:
-
reduction of halo,
-
reduction of ghosting effects because the bright central light is blocked,
-
reduction of noise because of a longer integration time without detector
saturation,
-
reduction of detector memory effect.
Our limited test runs could not cover a wide parameter space for performance
evaluation. A short explanation of (a) is described below.
The above figure shows the effect of the 0.2'' occulting mask when used
with the Adaptive Optics unit.
The above figure shows the effect of the 0.4'' occulting mask when used
with the Adaptive Optics unit.
Without the AO unit, larger masks must be used, but the suppression
at large radial distances is significant. |