Consejo Nacional de Investigaciones Científicas y Técnicas
The REOCS is an echelle spectrograph which belongs to the Institute d'Astrophysique de Liege in Belgium and is currently on loan at CASLEO to be used with the 2.15 m telescope. This instrument can operate in a range of 3500 Å to 7500 Å and in two operating modes: cross dispersion (CD) and simple dispersion (SD). For the latter, the echelle grating is replaced by a flat mirror. It is important to point out that the spectrograph can operate beyond 7000 Å; however, as it was not designed for that purpose, its efficiency can be reduced.
The overall efficiency of the spectrograph in combination with the telescope and the detector has not yet been measured. A StellaCam or an ST-7 camera is used to observe the field.
The detector currently used with the REOSC spectrograph is a 1024 × 1024 pixels CCD TEK. The chip in this detector is thinned, it has no coating, and is back-illuminated. The main features of the chip are listed in the table below:
|Rows × columns||1024 × 1024|
|Pixel size||24 × 24 µm|
|Chip size||25 mm × 25 mm|
|gain #1||7.97 e-/ADU|
|gain #4||1.98 e-/ADU|
|Read-out noise (gain #1)||0.4 e-|
|Read-out noise (gain #4)||7.4 e-|
|Dark current||0.4 e-/hour/pixel|
|Operating temperature||-120 °C|
The graph shows the chip's quantum efficiency.
The detector-REOSC-telescope efficiency has not yet been measured; however, it is possible to estimate the integration times for CD assuming 0.3 Å/2 px in 4500 Å resolution; i.e. with a slot opening of 273 µm, equivalent to 3" in sky and with a seeing lower than such value. In these conditions, it is possible to obtain a S/N relation of 50 in 30 minutes of integration for a B=10 magnitude.
In the case of SD and under the conditions mentioned above (unless the slot is opened 2 pixels which is equivalent to approximaltely 200 µm at the focal plane of the telescope and slightly over 2" on the sky) a 50 S/N relation in 30 integration minutes can be obtained for a 12 magnitude star.
|λ [Å]||m||d [Å/mm]||Fλ||C||λ [Å]||R|
(a): In SD this grating can be used in second order centered at approximately 3800 Å with a 22 Å/mm dispersion and a 500 Å spectral range.