Practical info

Calcite plates

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.

Polaroids

The polaroid filters are polarisers which allow passage of light which is linearly polarised in some specific direction. There are currently two sets available:

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.