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. Either
a long slit of variable width (b=50 ... 1000µm, corresponding to 0.6" ... 12") or
(quickly exchangeable) multi-object-masks can be used as entrance aperture. (MOS
masks have to be provided by the observer, measures and guidelines for mask
preperation on request).

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.