General
OSCA is mounted between the AO system NAOMI and the infrared camera INGRID. It is mounted on a
deployable table moving OSCA from below into the light beam. It focus the converging beam coming
from NAOMI via mirrors onto the focal plane mask. These masks are vaporized onto transmissive substrates
and are mounted in a wheel enabling the selected mask to move quickly into the beam.
The substrates avoid diffraction and ghost patterns in the final image which would be apparent if the
masks would be mounted on e.g. wires. The beam is then collimated and sent to a pupil conjugate Lyotstop.
The Lyotstop holders rotate (offset angle 348.75 degrees) with the same rate as the spiders of the telescope secondary. After
passing the Lyotstop the beam is refocussed via a set of mirrors onto the INGRID camera.
Available masks and their properties
The following table shows the currently available masks with their nominal and measured properties.
Mask
nominal size(arcsec) |
measured size*
(arcsec) |
Opacities (ND values)**
vis / J / H / Ks |
0.25 |
- | 4.5 / - / - / - |
0.65 |
0.68 | 3.5 / 2.1 / 2.1 / 1.8 |
0.8 |
0.77 | 3.5 / - / 1.9 / - |
1.0 |
- | - / - / - / - |
1.6 |
1.71 | 3.5 / - / - / - |
2.0 |
1.95 | 4.5 / 2.1 / 2.36 /1.9 |
fwhm=0"5 *** |
- | 4.5 / - / - / - |
fwhm=0"6 *** |
- | 4.5 / - / - / - |
- not characterized yet.
* As the edges of the masks extend over more than 1 pixel the measured sizes contain
an error of ~2-3 px corresponding to 0.08-0.09 arcsec.
** Measurements in the visible were done at UCL using a 630nm laser. H band opacities
were measured using an aritificial star on INGRID. Opacities in J and Ks band were estimated
from flatfields and show a higher error (pinhole measurements pending).
*** These masks have radial gaussian density profiles.
