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WFS Pickoff Mirror
The pickoff mirror has a diameter of 5 arcsec, and sits on a small strut mounted perpendicular to a vertical glass plate at one end of the WFS unit. The glass plate moves the pickoff mirror horizontally (x) and vertically (y) in the field, to intercept light from the guide star (unvignetted field 2 arcmin), and direct it to the WFS. As the pickoff mirror moves in x and y, the WFS foreoptics move along the track to compensate for the change in focus.
The spots on the WFS begin to disappear when the telescope is moved 4 arcsec from nominal position (so maybe the diameter of the pickoff mirror is > 5 arcsec?).
The pickoff mirror lies close to a focal plane, but a few mm away, to minimise the effect of any dust particles on its surface.
Until ~ 3/2002, there was no stop in the focal plane near the pickoff mirror, which meant, unusually for an AO system, that the sky was not being properly baffled i.e. each WFS cell sees overlapping skies. E.g. if the pickoff feeds a 5-arcsec field, each 2-arcsec cell sees ~ 2.5 * 2.5 x true sky brightness. Tom Gregory noted this ~ 6/2001 and designed a stop which fits on the stem supporting the pickoff mirror. It is effective in reducing background light by the appropriate factor, and on-sky 5/2002 it did indeed appear easier to acquire faint stars. Objects subtending > 2 arcsec (e.g. Neptune) cannot currently be acquired on the WFS.
The pickoff mirror is preceded by a lens which makes the beam passing through the dichroic telecentric, to minimise the curvature of the focal plane near the pickoff mirror (the WFS compensates in focus for the residual curvature?).
The pickoff mirror is followed by another 45-deg feed mirror, above it, which directs the light horizontally into the WFS chassis. The combination of pickoff and feed mirrors may be acting as crossed polaroids if they are AR-coated with a dielectric, thus reducing throughput. Tom Gregory plans to replace them.
Light from the remainder of the field (i.e. except for a region of diameter 5 arcsec) passes through the glass plate and can be intercepted by a camera behind the WFS (e.g. JOSE, SLODAR, LGS WFS).
Cleaning: use a swab.
In most AO systems (including ELECTRA, NAOMI's predecessor), light from
the guide star is on-axis. Keck's AO system can also pick off light
from a guide star anywhere in the field, but uses two mirrors (one for
x, one for y).
Links contained introductory information to different NAOMI components are tabulated below.
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Created by Stephen Goodsell
sjg@ing.iac.es |