Calcite (CaCO3) is one of the most commonly found birefringent
materials. The degree of birefringence, i.e. the difference between the
refractive index of the principal extraordinary ray and the one of the
ordinary ray, is high (-0.172). Calcite has only one symmetry axis and is
called a uniaxial crystal.
At the upper boundary of the calcite plate, incoming light is divided into two
orthogonally polarized components, and while the ordinary ray is refracted
according to normal refraction laws, the extraordinary ray is deviated by an
angle of 6.2°. At the lower boundary the extraordinary beam is refracted
back to its incident direction. This gives a parallel separation of the two
beams by a distance:
d = h tan (6.2°),
where h is the thickness of the calcite plate. In our case, hence,
the o- and e- beams are separated by about 2 mm or 15" in the focal plane
of the telescope, and the calcite plate modes
should therefore be used only for targets which are pointlike or only marginally
extended. The calcite plates produce a vignetted field of about 140" in
diameter. The telescope focus offset for both calcite plates is +710 steps.
Both calcite plates are installed and aligned in holders appropriate for
the aperture wheel of ALFOSC. Per default the calcite used for imaging is
mounted in ALFOSC. Another calcite used for spectropolarimetry can be mounted
in ALFOSC, three different slitlets are available (~1.0", 1.4", 1.8"). Note though
that they are only two calcites and the slits are mounted on top of the calcite. Thus
only one slit can be used at a time!
1/2 wave plate (lambda/2)
The 1/2 wave plate is a retarder which is used to rotate the plane of
linearly polarised light. The ordinary and extraordinary components
of a ray are shifted in phase by half a wavelength, i.e. the phase
delay is 180°. It is mounted in FAPOL and can be rotated in steps
of 22.5°, which then rotates the polarisation direction by 45°.
NB! Very expensive! To be treated with outmost care.
Specifications for the ALFOSC/FAPOL lambda/2 plate:
- from Bernhard Halle Nachfl., Berlin
- 50 x 50 mm square, thickness = 15 mm
- superachromatic, range 310-1100 nm
- made of quartz and MgF2, uncoated
- flatness of surface: lambda/6
- surfaces parallel to within 20 arcsec
- calculated retardance: 180° ± 2 °
1/4 wave plate
The 1/4 wave plate is a retarder which is used to convert elliptically
or circularly polarised light into linearly polarised light or vice
versa. The constituent orthogonal o- and e- components of a ray are
delayed in phase by 90°, i.e. a quarter of a wavelength.
The 1/4 wave plate for ALFOSC is of hexagonal shape, 50 mm in diameter.
It is stored in a drawer in the optics cabinet. It is mounted in FAPOL.
The polaroid filters are polarisers which allow passage of light which is
linearly polarised in some specific direction. There are currently two sets
- Four polaroid filters (glass), with the direction marked. These are
located in two drawers in the filter merry-go-round, marked "ALFOSC
polarizers". They should be mounted with the mark pointing at 0, 45, 90
and 135 degrees in normal ALFOSC filter holders. Can be installed either
in the filter or grism wheel.
- Three polaroid films are readily mounted at 3 different angles in
ALFOSC filter-holders, stored in their own drawer, called "Polarizers"
in the filter merry-go-round. This is the old set.