CAFOS - Focal Reducer and Faint Object
Spectrograph for the 2.2m Telescope

CAFOS ( = Calar Alto Faint Object Spectrograph ) is a focal reducer which
changes the 2.2m telescope's f-ratio from f/8 to f/4.4.

In its standard configuration CAFOS is equipped with a 2048 × 2048 pixel blue sensitive CCD, either with a SITe chip with 24 mm pixels (Id Site1d) or a LORAL chip with 15 µm pixels (Id Loral8o):

 

CAFOS with SITe 2048 × 2048 pixels (24 µm)

Imaging scale: 45.3 µm/" or 0.53"/pix
Field: 1040" diameter = 16'.0
Read-out-noise: <7 e¯

 

 

CAFOS with LORAL 2048 × 2048 pixel (15 µm):

 

Imaging scale: 45.3 µm/" or 0."33/pixel
Field: 650" × 650" = 10.'8 × 10.'8

 

 

Observing modes:

 

1. Direct imaging

For direct images, standard filters (Johnson, Cousins, Gunn) and (on request) a large set
of interference filters (l / Dl = 30 ... 100) are available.

 

 

2. Spectroscopy

Three grism sets (400Å/mm, 200Å /mm, 100Å /mm) are available for spectroscopy.
A long slit of variable width (b=50 ... 1000µm, corresponding to 0.6" ... 12") is used
as entrance aperture.

The following details again refer to the SITe chip with 24µm/pixel, i.e. 2pix/"). The
given limiting size is a preliminary estimate (± 0.5 mag).

 

 

¹ For the lower resolution grisms (B-400 to R-200) the wavelength range is set by the
efficiency of the grism + CCD response: l_min, l_max refer to 50% of the
maximum response.
For the higher resolution grisms (B-100 to R-100) the wavelength range refers to the
2k×2k chip.

²

 S/N = 10 per resolution element (1."2 slit) at the blaze wavelength

 

3. Polarimetry

 

CAFOS 2.2 is equipped with a Wollaston prism with an effective beam separation of
~20", plus a rotatable l/2 plate. This combination provides the following possibilities to
measure linear polarization:

 

3.1 Imaging Polarimetry:

 

Without a mask in the aperture unit imaging polarimetry of pointlike or marginally
resolved objects (d<~10") can be performed in the entire CAFOS 2.2 field. By
inserting a stripe mask in the aperture unit, imaging polarimetry of objects
extending over the entire field is possible (but you have to take 2 exposures for
every rotation of the l/2 plate).
Note that the masked mode improves the S/N of any transmitted object by root 2. The
strip mask is provided.

 

3.2 Spectropolarimetry:

 

Combining the polarimetric unit (Wollaston + l/2 plate) with any of the grisms
listed above provides full spectropolarimetric capabilities. Currently a
"broken" longslit with a slit width of 150 µm (=1.8") and 9 aligned slitlets
(width: 18", separation betweeen the apertures: 24") is available.

 

4. Narrow band imaging using a Fabry Pérot etalon

 

A Fabry Pérot etalon is available; this will enable narrow band images in the 600
< l < 1000 nm region at a resolution of R = l/Dl = 500.

Tuning of the etalon enables imaging spectroscopy in the entire field (note that
maximum of the etalon transmission is shifted by >2 nm towards the blue near the edge
of the field). Make sure that the pre-filter, required for your observation is available
(Dl/l >~3%).

 

5. Present status

 

It is strongly recommended to refer to further updates of this document
(available under WWW: http://www.mpia-hd.mpg.de/welcome.html) before your observing run.