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 µm pixels (Id Site1d)


 
CAFOS with SITe 2048 2048 pixels (24 m)
Imaging scale: 45.3 m/" or 0.53"/pix
Field: 1040" diameter = 16'.0
Read-out-noise: <7 e
 

Quantum efficiency (SITe):

>85% for λ 400 - 700 nm
~45% at λ 300 nm
~50% at λ 900 nm


 
Observing modes:
 
1. Direct imaging
 
For direct images, standard filters (Johnson, Cousins, Gunn) and (on request) a large set
of interference filters ( λ/ Dλ = 30 ... 100) are available.
 

General guideline for the limiting magnitude (pointsources, seeing < 1.5")

Filter λ/Dλ = 650/160 nm,

Dt = 1 min: mlim > 22.5
Dt = 20 min: mlim = 24.0

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

When changing the slitwidth, only one of the blades of the slit is moving. This means that the center of the slit moves, towards the blue, too! So we recommend you to take arc lamps with the same slitwidth(s) than the one(s) you plan to use for your targets.

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

CAFOS grisms

Grism

R
[/mm]

λmin, λmax
[nm]

D
[nm/px]

mlim (1h)

B-400

388

320, 800

0.976

B=21.5

R-400

398

475, 1100

0.965

R=21.0

B-200

185

320, 700

0.470

B=21.2

G-200

187

400, 850

0.458

V=21.0

R-200

177

630, 1100

0.435

R=20.8

B-100

88

320, 580

0.200

B=20.5

G-100

87

490, 780

0.212

V=20.3

R-100

84

590, 900

0.204

R=20.0

 
 For the lower resolution grisms (B-400 to R-200) the wavelength range is set by the
efficiency of the grism + CCD response: lmin, lmax refer to 50% of the
maximum response.
For the higher resolution grisms (B-100 to R-100) the wavelength range refers to the
2k2k chip.
 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 λ/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 λ/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 + λ/2 plate) with any of the grisms
listed above provides full spectropolarimetric capabilities. Currently a
"broken" longslit with a slit width of 150 m (=1.8") and 9 aligned slitlets
(width: 18", separation betweeen the apertures: 24") is available.
 
4. Narrow band imaging using a Fabry Prot etalon
 
A Fabry Prot etalon is available; this will enable narrow band images in the 600
< λ < 1000 nm region at a resolution of R = λ/Dλ = 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 (Dλ/λ >~3%).