MUSE (Multi Unit Spectroscopic Explorer) is a second generation instrument built for ESO (European Southern
Observatory). The MUSE project is supported by a European consortium of 7 institutes.
After the finalisation of its integration in Europe, the MUSE instrument has been partially dismounted and shipped to
the VLT (Very Large Telescope) in Chile. From October 2013 till February 2014, it has then been reassembled, tested
and finally installed on the telescope its final home. From there it collects its first photons coming from the outer limit
of the visible universe.
This critical moment when the instrument finally meets its destiny is the opportunity to look at the overall outcome of
the project and the final performance of the instrument on the sky. The instrument which we dreamt of has become
reality. Are the dreamt performances there as well?
These final instrumental performances are the result of a step by step process of design, manufacturing, assembly, test
and integration. Now is also time to review the path opened by the MUSE project. What challenges were faced during
those last steps, what strategy, what choices did pay off? What did not?
MUSE (Multi Unit Spectroscopic Explorer) is a second generation instrument built for ESO (European Southern
Observatory) to be installed in Chile on the VLT (Very Large Telescope). The MUSE project is supported by a
European consortium of 7 institutes.
After the critical turning point of shifting from the design to the manufacturing phase, the MUSE project has now
completed the realization of its different sub-systems and should finalize its global integration and test in Europe.
To arrive to this point many challenges had to be overcome, many technical difficulties, non compliances or
procurements delays which seemed at the time overwhelming. Now is the time to face the results of our organization, of
our strategy, of our choices. Now is the time to face the reality of the MUSE instrument.
During the design phase a plan was provided by the project management in order to achieve the realization of the
MUSE instrument in specification, time and cost. This critical moment in the project life when the instrument takes
shape and reality is the opportunity to look not only at the outcome but also to see how well we followed the original
plan, what had to be changed or adapted and what should have been.
The foundation of the MUSE instrument with its high multiplexing factor of twenty-four spectrographs is formed
through its central main structure that accommodates all instrumental subsystems and links them with the telescope. Due
to instrument's dimension and complexity, the requirements on structural performance are demanding. How its
performance was tested and optimized through reverse engineering is addressed. Intimately mated with this central
structure is an optical relay system that splits the single telescopic field into twenty-four subfields. Each of those is
individually directed along three dimensions across the structure through a folding and imaging setup of an optical relay
system that at the end feeds one of the twenty-four spectrographs. This opto-mechanical relay system was tested when
mounted onto the main structure. The results obtained so far are given here.
The Multi Unit Spectroscopic Explorer MUSE is a second-generation VLT instrument. With its high multiplexing factor
of twenty-four individual spectrographs, it requires rather complex opto-mechanics to split the field of 1x1 arcminute on
the sky into twenty-four sub-fields and guide them along the central instrument structure to the feeding point of each
spectrograph. The requirements on the underlying mechanical structure are quite demanding in terms of opto-mechanical
stability under thermal loads and thermal mismatch, warping of its basement and excessive earthquake loads. In total
seven individual load cases and combinations of them have been analyzed in extensive finite-element analyses (within
Nastran) with subsequent optical analyses (within Zemax). These two types of analyses will be addressed here and their
combined output will be set into relation with the requirements.
The Multi Unit Spectroscopic Explorer MUSE is an integral field device containing 24 spectrographs at the Nasmyth
focus of the VLT unit telescope. The total field size of 1'x1' needs to be split and separated into 24 sub-fields which are
relayed along a central structure into the entrance aperture of the individual spectrographs. The realization of the optics
for field splitting and separation as well as the relay optics to direct the light of the individual fields to the spectrographs
is described here. A very tight link exists between the relay optics system layout and the mechanical arrangement of the
spectrographs in the common central structure. A compact mounting is essential due to the restricted space for such a
large instrument even on the VLT Nasmyth platform. A suitable arrangement of vertical and horizontal stacking of the
spectrographs was found enabling their feeding from the unobstructed front side of the instrumental structure. The
central instrument mount was designed as a stiff structure absorbing print-through effects due to thermal mismatch with
the telescope platform but rigid enough to withstand earthquakes.
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