The slow wavefront sensor, if used with starlight, could have the following functions:
A: Slow WFS Reconstructor Calibration
B: Slow WFS only Lock
C: Reconstructor Calibration for Fast WFS
D: Dual Wavefront Sensor Matrix Calibration
E: Beacon Calibration
This produces the following operation modes…
In order to get good sky coverage of reference stars, we will assume that the AO calibration is done on a 4th magnitude A star. Assuming B-band and half of V-band (0.4 to 0.55 microns), 80,000 photons arrive at the top of the atmosphere in every 5 millsec for a 0.2 m aperture. Assuming that we need at least 100 photons for a lock, this puts a minimum system throughput requirement of 0.13%. With a CHARA throughput in the blue of about 2% and 65% WFS quantum efficiency, this means that an IR dichroic throughput of at least 10% is needed over the 0.4 to 0.55 micron band in order to lock the slow WFS on calibration stars.
With the same arguments as above but a 5 second integration time, B magnitudes of 11.5 should enable slow WFS lock on sky. With a minimum 25% transmission requirement for the IR dichroic on average between 0.4 ant 0.55 microns, this becomes B=12.5. Red YSOs with B=14th magnitude will not have enough flux for slow WFS lock with starlight, and will require a beacon.
This is currently impossible to compute as we don't know the power spectrum or general amplitude of internal seeing ???
Is this possible? Or is starlight always needed? Could the calibration be done with one dichroic and the dichroic changed? How do the small wavefront errors in the dichroic (transmission and reflection) as well as alignment tolerances translate to modifications of the calibration algorithm?
Laszlo's email from 7 Sep states that the existing system has beams are arranged along a R=70 mm circle, 45 deg apart. The parabolic mirror is 8'' F/6. The SPIE paper describing the system is … (MJI had errors trying to upload it).
For an initial design, we will work with a pupil position 20m upstream from this mirror, meaning that the pupil-plane is 42mm behind the common focal-plane, where the pupils are 0.88mm in diameter and on a 3.3mm circle. Each sub-pupil in the F/6 paraboloid is F/47, meaning that the coma-free field of view is very large. By angling the mirrors immediately prior to the paraboloid, the pupil separation can be arbitrary, and could even be set so that the sub-pupils on the F/6 paraboloid overlapped on the system pupil-plane. However, the simplest option is definitely to maintain the 8-way arrangement.
MJI can not figure out how do get this to work with the 8-way arrangement as it stands. However, things could work with a 6-way arrangement where the 20cm sampled hexagons are arranged on a lenslet array like this: