---
title: 'VIMOS: ESO’s Multi-Object Spectrograph'
url: https://www.emergentmind.com/topics/vimos
type: topic
---

# VIMOS: ESO’s Multi-Object Spectrograph

VIMOS, the Visible Multi-Object Spectrograph on ESO’s Very Large Telescope Unit Telescope 3 (“Melipal”), was a wide-field optical spectrograph built around high-multiplex slit spectroscopy and, in a distinct configuration, integral-field spectroscopy. In the literature represented here, it appears as the instrumental backbone of several major extragalactic programs—notably VVDS, GOODS/VIMOS, VIPERS, and VUDS—because it combined a four-channel focal-plane geometry, custom slit-mask operation, and multiple grism configurations with wavelength coverage extending from the near-UV to the red optical. Its scientific role was correspondingly broad: large redshift surveys at intermediate redshift, ultra-deep spectroscopy of galaxies at \(2<z<6\), environmental and cosmic-web reconstruction, targeted spectroscopy of compact stellar systems, and spatially resolved spectroscopy of nearby infrared galaxies [1611.07048][1403.3938][1001.1115][1009.0112].

## 1. Instrument architecture and observing modes

VIMOS was a four-channel imaging spectrograph whose focal plane was divided into independent quadrants. Across the survey papers, each quadrant is described as covering roughly \(7\times8\) arcmin\(^2\), with a single pointing spanning about \(218\)–\(224\) arcmin\(^2\); the quadrants were separated by an unobserved cross-shaped gap about \(2\) arcmin wide [1611.07048][1001.1115][1407.2969]. In slit-mask mode, the entrance focal plane accepted custom masks, allowing simultaneous spectroscopy of tens to over a hundred targets per quadrant, and in maximum multiplex configurations the instrument could deliver up to about \(1000\) spectra in one exposure [1611.07048][1310.1008].

The dominant use of VIMOS in the cited work is multi-object slit spectroscopy. Slit masks were prepared from direct imaging with the ESO mask-preparation software, and several programs adopted a \(1\) arcsec slit width as a standard operating point [1310.1008][1611.07048][1001.1115]. VIPERS exploited an aggressive short-slit strategy to raise the median number of slits to about \(87\) per quadrant, or about \(350\) per pointing [1310.1008]. VUDS likewise optimized slit lengths down to \(6\) arcsec to maximize multiplex [1403.3938].

A separate VIMOS integral-field mode is documented for nearby luminous and ultraluminous infrared galaxies. In that configuration the IFU field was \(27\times27\) arcsec with \(40\times40=1600\) spectra per pointing, sampled at \(0.67\) arcsec per fiber, and combined through a four-point dither into a \(44\times44\) “super-cube” containing \(1936\) spectra [1009.0112]. This established VIMOS not only as a survey spectrograph but also as a spatially resolved optical spectroscopy instrument.

## 2. Spectroscopic configurations and performance envelope

The instrument was used with several grisms and filters, and its scientific behavior was therefore configuration-dependent rather than fixed by a single resolution or wavelength range. The standard resolving-power definition used in the survey literature is \(R=\lambda/\Delta\lambda\), while redshifts follow the usual relation \(z=(\lambda_{\rm obs}-\lambda_{\rm rest})/\lambda_{\rm rest}\) [1303.3930][1410.0465].

A concise view of the principal configurations documented in these papers is given below.

| Configuration / program | Wavelength coverage | Resolution or related metric |
|---|---:|---:|
| VIPERS LR-Red | \(5500\)–\(9500\) Å | \(R=210\)–\(230\) |
| VUDS LRBLUE + LRRED | \(3650\)–\(9350\) Å | \(R\approx230\) |
| GOODS LR-Blue | \(\approx3700\)–\(6700\) Å | \(R\approx180\) |
| GOODS MR orange / Fornax MR | \(\approx4800\)–\(10000\) Å | \(R\approx580\) or \(R=580\) |
| VIMOS IFU HR-Orange | \(5250\)–\(7450\) Å | FWHM \(1.80\pm0.07\) Å |

In VIPERS, VIMOS was configured with the LR-Red grism, \(1\) arcsec slits, and wavelength coverage \(5500\)–\(9500\) Å; PDR-2 quotes \(R\equiv\lambda/\Delta\lambda=220\), while the main survey and filament-analysis papers quote \(R=210\) and \(R=230\), respectively [1611.07048][1303.3930][1611.07045]. In VUDS, the low-resolution LRBLUE and LRRED settings were combined to cover \(3650\)–\(9350\) Å at \(R\approx230\), and one ECDFS pointing used the MR grating at \(R\approx580\) over \(5000\)–\(9500\) Å [1403.3938]. GOODS/VIMOS split its program between LR-Blue and MR orange, targeting complementary redshift regimes [1001.1115]. The Fornax compact-stellar-system survey used the MR grating with the CG475 filter over \(4800\)–\(10000\) Å at \(R=580\) and \(2.5\) Å/pixel [1803.03275].

Exposure times were likewise survey-specific. VIPERS used \(2700\) s per pointing, split into five \(9\)-minute exposures [1310.1008]. GOODS/VIMOS used \(4\) h per mask in both LR-Blue and MR campaigns [1001.1115]. VUDS pushed to approximately \(14\) h per grism and per pointing, with the overlap region effectively receiving about \(28\) h in the combined spectrum [1403.3938]. This depth enabled continuum sensitivity around \(I_{AB}\approx25\) mag and detections of weak diagnostic lines in faint galaxies [1403.3692][1403.3938].

Detector state affected performance materially. During the 2010 upgrade, VIMOS received red-sensitive thicker CCDs and improved flexure compensation; VIPERS reports an increase in red efficiency by up to a factor of about \(2.5\) and a reduction in fringing, while VUDS notes that sensitivity at \(9000\) Å increased by about a factor of \(2\) and fringing above \(8000\) Å was significantly reduced [1611.07048][1403.3938].

## 3. Major survey implementations

VIMOS’s place in extragalactic astronomy is clearest through the surveys built around it. VIPERS used VIMOS to construct a magnitude-limited spectroscopic survey to \(i_{AB}<22.5\), with colour pre-selection to suppress \(z<0.5\) galaxies, over the CFHTLS-Wide W1 and W4 fields. The completed public release spans about \(23.5\) deg\(^2\), with \(288\) VIMOS pointings and a complete spectroscopic sample of \(86{,}775\) galaxies plus \(4{,}732\) other objects; the average effective sampling rate is about \(47\%\) over \(0.5<z<1.2\) [1611.07048]. Earlier VIPERS public releases already distributed \(57{,}204\) spectroscopic measurements and then the associated calibrated 1D and 2D spectra [1310.1008][1410.0465].

VUDS used VIMOS for a different regime: ultra-deep spectroscopy of very faint galaxies at early cosmic epochs. The survey targeted about \(10{,}000\) galaxies over about \(1\) deg\(^2\) in COSMOS, ECDFS, and VVDS-02h, with a main spectroscopic sample peaking at \(z\approx3\)–\(4\) and extending to reliable redshifts beyond \(z=6\) [1403.3938]. Its first CANDELS-area public release distributed spectra and spectroscopic redshifts for \(698\) objects, including about \(500\) galaxies at \(z_{\rm spec}>2\) and \(48\) at \(z_{\rm spec}>4\) [1602.01842].

GOODS/VIMOS used the instrument in a more targeted field-survey mode. The completed campaign comprised \(20\) masks and yielded \(5052\) spectra, with \(2242\) LR-Blue redshifts and \(976\) MR redshifts, complementing the FORS2 ESO/GOODS spectroscopy and extending spectroscopic coverage in the CDFS/GOODS-S region [1001.1115].

Earlier VVDS work demonstrates the same instrumental logic at somewhat lower sampling. In the VVDS-02h Deep field, VIMOS underpinned a purely flux-limited survey \(17.5\le I_{AB}\le24.0\), from which a group catalogue of \(318\) systems at \(0.2\le z\le1.0\) was constructed [0911.3740].

Outside the canonical galaxy-redshift surveys, VIMOS also supported targeted wide-field programs. In the Fornax cluster core, \(25\) masks produced about \(4500\) low-resolution spectra and a final catalogue of \(372\) globular clusters and \(15\) ultra-compact dwarfs [1803.03275]. In IFU mode, it produced a spatial atlas of continuum and H\(\alpha\) emission for \(38\) local luminous and ultraluminous infrared systems [1009.0112].

## 4. Calibration, extraction, and public data products

The reduction environments associated with VIMOS were themselves an important part of its scientific infrastructure. VIPERS reductions were automated within a pipeline derived from VIPGI and managed in the EasyLife environment; redshifts were measured with EZ and then visually validated [1310.1008][1611.07048]. GOODS/VIMOS also used VIPGI, with visual feature identification supplemented by EZ template cross-correlation in ambiguous cases [1001.1115]. VUDS DR1 used VIPGI together with EZ and extensive visual inspection of both 1D and 2D spectra [1602.01842].

Calibration chains were survey-specific but structurally similar. VIPERS PDR-1 describes tracing of 2D spectra, first-pass sky subtraction, wavelength calibration from arc lamps, rigid offsets based on sky lines when needed, co-addition, optimal extraction, and flux calibration through the instrument sensitivity function, followed by normalization to the CFHTLS \(i\)-band magnitude [1310.1008]. The later public releases delivered wavelength- and flux-calibrated 1D spectra, optional 2D spectra, and the ancillary arrays required for downstream analysis: wavelength, cleaned flux, flux uncertainty, sky intensity, original flux, and a cleaning mask [1611.07048].

The first VIPERS public spectra release is especially explicit about per-object products. Each FITS binary table includes columns labeled WAVES, FLUXES, EDIT, NOISE, SKY, and MASK; FLUXES is the wavelength-calibrated, sensitivity-corrected 1D spectrum normalized to CFHTLS \(i\)-band photometry, while EDIT is a cleaned version in which fringing residuals, poor subtraction of strong sky lines, and zero orders are mitigated by manual edits plus an automatic PCA-based reconstruction [1410.0465]. FITS headers include the VIPERS ID, sky coordinates, selection magnitude, spectroscopic redshift and reliability flag, normalization, and exposure time [1410.0465].

VUDS DR1 added a different calibration emphasis because of its blue coverage and very long integrations. The LRBLUE and LRRED spectra were resampled and joined through the \(5600\)–\(6700\) Å overlap region; atmospheric refraction and extinction were corrected by comparison with broadband photometry, with slit-loss estimates based on a refraction pseudo-spectrum and an elliptical model of each galaxy image through the \(1\) arcsec slit. After correction, spectroscopic fluxes agreed with broadband photometry to better than \(10\%\) rms for \(\lambda\le4500\) Å and better than \(5\%\) rms for \(\lambda>4500\) Å [1602.01842].

Several releases also documented noise models and flexure handling. VIPERS PDR-1 writes the sky-residual and noise estimators explicitly and notes wavelength-calibration residuals with a mode of \(0.771\) Å [1310.1008]. The Fornax survey found that time-dependent flexure could shift the wavelength zero-point across exposures; it therefore measured residual sky-line offsets after stacking and applied multiplex-by-multiplex corrections [1803.03275].

## 5. Cosmological and environmental measurements enabled by VIMOS

The combination of VIMOS multiplexing and VIPERS sampling density made intermediate-redshift large-scale-structure studies possible at a level previously associated mainly with local surveys. A prominent example is the reconstruction of the filamentary cosmic web at \(z\simeq0.7\) using DisPerSE, where VIPERS yielded the first quantitative detection of large-scale filamentary structure at that redshift and showed that massive or quiescent galaxies lie closer to filament axes than less massive or active galaxies [1611.07045].

Void science followed a parallel track. Using VIPERS, empty-sphere algorithms identified and characterized voids at \(0.55<z<0.9\), producing a catalogue of \(411\) maximal spheres with radii broadly spanning \(15\)–\(31\) Mpc and a void–galaxy cross-correlation function showing line-of-sight anisotropy consistent with outflow velocities [1407.2969]. A subsequent completed-VIPERS analysis measured the redshift-space distortion parameter around voids as \(\beta=0.423^{+0.104}_{-0.108}\) and inferred \(f\sigma_8=0.296^{+0.075}_{-0.078}\) at \(z=0.727\) [1611.07046].

VIMOS sampling geometry also motivated methodological work. The four-quadrant footprint, slit-collision constraints, TSR/SSR variations, and the unobserved cross imprint a non-trivial angular selection function on galaxy catalogues. VIPERS analyses addressed this in several ways: Bayesian Wiener-filter reconstruction of the redshift-space density field jointly inferred the anisotropic power spectrum, luminosity function, bias, and density field while modeling the inhomogeneous selection function, yielding \(f\sigma_8=0.38\) with \(18\%\) uncertainty at redshift \(0.7\) [1505.06337]; count-in-cell studies showed that at low sampling the underlying one-point density PDF is better represented by a Gamma expansion than by a Skewed Log-Normal, and explicitly corrected the “angular selection effect of the VIMOS instrument” [1505.00442]; and pairwise inverse probability weighting was shown to recover unbiased clustering from VIPERS slit assignment, confirming that earlier approximate corrections were already sufficient within VIPERS statistical errors [1807.05999].

Earlier VVDS work shows that these cosmological applications emerged from a longer VIMOS lineage. The VVDS group catalogue trained the Voronoi-Delaunay Method on Millennium-based mocks to recover group counts and velocity dispersions under VIMOS-like sampling, finding \(318\) groups at \(0.2\le z\le1.0\) with global completeness \(C=60\%\) and purity \(P=50\%\) [0911.3740].

## 6. Galaxy populations, diagnostics, and instrument-driven systematics

VIMOS datasets were also used to construct physically organized views of galaxy populations. In VIPERS, a PCA classification of \(27{,}350\) secure spectra in \(0.4<z<1.0\) used an iterative masked repair of noisy or gappy spectra and found that the first three eigenspectra contain about \(90.56\%\) of the total variance, enabling a two-parameter classification space that separates early, intermediate, late, and starburst galaxies [1207.4374]. This suggests that, despite low-resolution and imperfect spectra, a large VIMOS survey could support homogeneous spectral typing at scale.

At higher redshift, VUDS used the depth of VIMOS to identify \(31\) extremely compact, metal-poor, star-forming dwarf galaxies at \(0.2<z<0.9\), with \(R_{50}<1\) kpc, \(M_\star\lesssim10^8\,M_\odot\), high ionization conditions, and \(12+\log({\rm O/H})\) between \(7.5\) and \(8.3\) [1403.3692]. Another VUDS analysis classified the Ly\(\alpha\) line morphology of \(914\) Ly\(\alpha\) emitters from a parent sample of \(4192\) star-forming galaxies at \(2<z_{\rm spec}\lesssim6\), finding about \(45\%\) coincident, \(24\%\) extended, and \(11\%\) offset emitters relative to the UV continuum [2007.01322].

VIPERS spectroscopy also supported line-diagnostic studies of AGN and quenching. A survey of [NeV] \(\lambda3426\) emitters at \(0.62<z<1.2\) used the VIMOS wavelength range to isolate AGN in composite galaxies and found younger stellar ages and higher [OII] luminosities for active galaxies in the green valley and blue cloud than in matched inactive controls [1712.08168].

These uses make the instrument’s trade-offs explicit. VIMOS frequently operated at low resolution—\(R\approx210\)–\(230\) in VIPERS and VUDS—which was adequate for robust redshift determination and large statistical studies but limited detailed line-profile diagnostics [1303.3930][1403.3938]. Fringing, residual sky features, zero-order contamination, and slit-based incompleteness were persistent concerns, especially before the 2010 detector upgrade; the literature responded with PCA-based spectral cleaning, explicit TSR/SSR/CSR weights, mock-catalog validation, and gap-aware topology or void-finding methods rather than assuming ideal completeness [1410.0465][1611.07048][1407.2969]. A plausible implication is that VIMOS’s scientific legacy is inseparable from the correction frameworks developed around it: the instrument did not merely generate large spectroscopic samples, but also forced precise treatments of mask geometry, angular incompleteness, and redshift reliability that became part of the measurement itself.

Source: https://www.emergentmind.com/topics/vimos