Calibration unit system

The CUBES calibration sub-system will provide the light sources to perform different calibration procedures, i.e., flat fielding, wavelength calibration, simultaneous calibration, alignment and flexure correction. CUBES calibration unit will provide means for daytime calibrations and for simultaneous calibration (Sim-Cal).

These functions will be provided through the presence of a calibration mirror that injects the light in the system (at the pre-slit optics level), and through the presence of fibers that inject light (for the simultaneous calibration) at slit level (above and below science spectra). Independent simultaneous calibration shutter will be available in the calibration unit to control the level of light during these calibration procedures.

For the CUBES low resolution mode, we intend to use the same lamps as for the high-resolution mode, and no Sim-Cal is provided. The ThAr lamp has enough unblended lines in the 300-400nm region to be used as wavelength calibration, provided we use a custom line list. Such a list was already developed for X-Shooter and is available and the results obtained at X-Shooter encourage the use of the ThAr lamp. On the Fig. 1 an X-shooter ThAr frame is shown.

Fig. 1 Example of the Photron ThAr lamp spectrum on X-shooter UV arm.

Calibration system shall be mounted in a light and heat-proof housing and will be remotely controllable. Input beam at the spectrograph will be provided in two ways: a) light input before the ADC for daytime calibrations through a Sliding mirror, and b) through optical fibers at the slit level for simultaneous calibration.

The calibration unit architecture can be summarized in the following subsystems (Fig. 2):

a) Light sources bench

b) “Daytime” fiber bundle (the calibration unit can technically also be used in at night)

c) Sim-Cal fiber bundle

d) Electronics (power sources, control)

Fig. 2 First level calibration scheme.

Fig. 3 Mechanical concept for the calibration bench (without cover).