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 µm): Imaging scale: 45.3 µm/" 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: m_lim > 22.5 Dt = 20 min: m_lim = 24.0

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.
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 24µm/pixel, i.e. 2pix/"). The
given limiting size is a preliminary estimate (± 0.5 mag).

CAFOS grisms

Grism	R [Å/mm]	λ_min, λ_max¹ [nm]	D [nm/px]	m_lim (1h) ²
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

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
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: 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 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

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

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