A new-generation instrument for the VLT
ESPRESSO is a fiber-fed, cross-dispersed, high-resolution Echelle spectrograph which will be located in the Combined-Coudé Laboratory (CCL) at the incoherent focus, where a Front End Unit combines the light from up to 4 Unit Telescopes (UT) of the VLT. The telescope light is fed to the instrument via a so-called Coudé Train optical system and within optical fibers. The target and sky light enter the instrument simultaneously through two separate fibers, which together form the slit of the spectrograph.
The incoherent combined focus: a new facility for a larger telescope
Although foreseen since 1977 in the original VLT plan, the incoherent combined focus of the VLT has never been implemented. Only provision for it, in terms of space left in the UTs structures and ducts in the rock of the mountain, is what is actually available at VLT. As part of the project agreement, the ESPRESSO Consortium has been asked to materialize such a focus providing the necessary hardware and software as part of the deliverables. The implementation of the Coudé Train is requiring substantial changes in the Paranal Observatory infrastructure yielding to an elaborated interfaces management. This new facility will allow to use the four telescopes as a large 16 meter equivalent telescope.
Enhanced flexibility and power
ESPRESSO will be located in the VLT’s CCL and, unlike any other instrument built so far, will receive light from any of the four UTs, allowing for a more flexible usage of the observation time. The light of the single UT scheduled to work with ESPRESSO is then fed into the spectrograph (single-UT modes). Alternatively, the combined light of all the UTs can be fed into ESPRESSO simultaneously (multi-UT mode).
Figure 1. The ESPRESSO optical design
Several optical ‘tricks’ have been used to obtain high spectral resolution and efficiency despite the large size of the telescope and the 1 arcscec sky aperture of the instrument:
- At the spectrograph entrance the Anamorphic Pupil Slicing Unit (APSU) shapes the beam in order to compress it in cross-dispersion and splits in two smaller beams, while superimposing them on the echelle grating to minimize its size. The rectangular white pupil is then re-imaged and compressed.
- Given the wide spectral range, a dichroic beam splitter separates the beam in a blue and a red arm. Each arm is optimized for image quality and optical efficiency.
- The cross-disperser has the function of separating the dispersed spectrum in all its spectral orders. In addition, an anamorphism is re-introduced to make the pupil square and to compress the order height such that the inter-order space and the SNR per pixel are both maximized. Both functions are accomplished using Volume Phase Holographic Gratings (VPHGs) mounted on prisms.
- Finally, two optimised camera lens systems image the full spectrum from 380 nm to 780 nm on two large 92 mm x 92 mm CCDs with 10-um pixels.
A sketch of the optical layout is shown in Figure 1. The spectral format covered by the blue and the red chips as well as the shape of the pseudo slit are illustrated by Figure 2.
Figure 2. Spectral format and shape of the pseudo slit.