MUSE Overview

Work based on MUSE observations should cite the paper Bacon et al., SPIE 7735, 7 (2010).

Introduction

MUSE, the Multi-Unit Spectroscopic Explorer,  is an Integral Field Spectrograph located at the Nasmyth B focus of Yepun, the VLT UT4 telescope. It has a modular structure composed of 24 identical IFU modules that together sample, in Wide Field Mode (WFM), a near-contiguous 1 squared arcmin field of view. Spectrally the instrument samples almost the full optical domain with a mean resolution of 3000. Spatially, the instrument is designed to exploit the VLT AO Facility via the GALACSI AO system, sampling the sky with 0.2 arcseconds spatial pixels.

MUSE is currently offered in Wide Field Mode with natural seeing mode. In the future, once the AOF is commissioned, a Narrow Field Mode (NFM) will be made available, and will cover 7.5x7.5 arcsec2 field of view sampled at 0.025"/pixel, always with AO-correction. 

Science Objectives

  • Formation of galaxies
  • Nearby galaxies
  • Stars and resolved stellar populations
  • Solar system
  • Serendipity
UT4 with MUSE at sunset
View of UT4 with MUSE sitting on the Nasmyth platform. Photo Ghaouti Hansali (CRAL).

MUSE in a nutshell

The table below gives the basic parameters of the instrument.

Number of IFU modules 24
Wavelength range

480-930 nm (nominal)

465-930 nm (extended)

Detectors 24 x 4k x 4k MIT/LL CCD
AO type Ground Layer, 4x(5-10 W) lasers
Throughput WFM
14 % (480 nm) 35 % (750 nm) 14 % (930 nm)
   

Wide Field Mode (Currently offered)

Field of view 59.9"x 60.0"
Saptial Sampling 0.2" /pixel
Spatial resolution (FWHM) 0.3"-- 0.4"
Resolving power 1770 (480 nm) -- 3590 (930 nm)

Limiting magnitude

(1 hr, airmass=1.0, seeing 0.8"@V)


VAB = 22.64 mag (550 nm)
RAB = 22.70 mag (650 nm)

IAB= 22.28 mag (784.9 nm)

Wide Field Mode with AO (Not currently offered)

Gain in ensquared energy within one pixel with respect to seeing 2
Condition of operation with AO 70th percentile
Sky coverage with AO 70% at Galactic Pole

Narrow Field Mode (Not offered yet. To be offered with GALACSI)

Field of view 7.42" x 7.43"
Spatial Sampling 0.025" / pixel
Spatial resolution(FWHM) 0.03" -- 0.05"
Resolving power 1740 (480 nm) -- 3450 (930 nm)
Strehl ratio 10--30%
Predicted limiting flux in 1 hr 2.3 10-18 erg s-1 cm-2
Predicted limiting magnitude in 1 hr RAB = 22.3 mag
Predicted limiting surface brightness in 1 hr RAB = 17.3 mag arcsec-2

Calibration Plan

These are the main performed as part of the standard calibration plan. They will be performed with a frequency that will be adjusted based on the results from commissioning runs.

 

Calibration No. of exposures (per mode) Frequency Phase Purpose
Bias 5 Daily Day Master bias, bias characteristics

Dark

3 Monthly Day

Master dark, dark current

Geometrical calibration: Multi-Pinhole mask with arc lamps
80 Monthly Day Relative location of the slices
Flat-field with sky >3 On demand, validity period to be determined
Twilight Illumination correction

Flat-field with continuum lamp

5 1--3 nightly (a)
Day Illumination correction, Tracing solution, Background light, Instrument throughput.
Attached Flat-field with continuum lamp n On request Night

High-precision flat-fielding

Wavelength

1 per lamp (b)
Daily Day Wavelength solution, Dispersion characteristics

Attached Wavelength

n On request Night High-precision wavelength solution
Spectrophotometric calibration 1
(c) Night Absolute flux calibration. Optional: Telluric correction
Astrometry 1 Monthly Night Astrometric model

(a) The actual frequency of flat field exposures is currently under investigation, but it is forseen to happen between 1 to 3 times per night.

(b) The default is to obtain one exposure per lamp for each of the Ne, Xe, and HgCd lamps.

(c) During clear or photometric nights in which MUSE science OBs are executed.

Instrument overheads

The table below gives the instrumental overheads that can be used for the proposal preparation phase.

 

Acquisition
Telescope preset 360
Interactive acquisition loop
120
Slow guiding loop closure
90

Observation

Detector setup 15
Detector readout + file merging
80
Telescope offset < 2' 15
SGS loop closure for large ofsets 90
Derotator offset
0.11s/degree
Attached calibration
Attached flat (5 flats)
250s+112s/flat-field
Attached arcs (3 calibrations, 1 per lamp)

180s + 112s/exposure

(1 per lamp)

Examples of overhead computation

Example 1. Not using SGS

Ttotal = 360s (preset) + Tacq + 120s (acquisition, 1 iteration) + N*(15s+UIT+80s)

with Tacq the acquisition integration time, and UIT the integration time of each of the N exposures.

Example 2. using the SGS with small telescope offsets and no PA offsets

Ttotal = 360s (preset) + Tacq + 120s (acquisition, 1 iteration) + 90s (SGS loop closure) +N*(15s+UIT+80s+15s (telescope offset) )

Example 3. using the SGS with large telescope offsets and PA offsets

In that case we are assuming that the SGS has to close the loop after each offset/rotation because it has lost the current guide stars.

Ttotal = 360s (preset)  + Tacq + 120s (acquisition, 1 iteration) + 90s (SGS loop closure) +N*(15s+UIT+80s+15s (telescope offset) + 20s (PA rotation) + 90s (SGS loop closure))

 

MUSE Science show case 1: Kapteyn's Star

First light image of a M1 red dwarf of 9th magnitude, in the constellation of Pictor, named after the dutch astronomer Jacobus Kapteyn who discover it in 1898.

Kepteyn_recima