SoXS: Son of X-Shooter

SoXS is a medium resolution high‐efficiency wide band spectrograph for the ESO-NTT telescope on LaSilla. It will feature a Resolution‐Slit product of 3500 to 6500 (depending on wavelength) with a simultaneous wavelength coverage from 350‐2000 nm. SoXS stands for Son of X-Shooter and will be a scaled down copy of this VLT-instrument. SoXS is being built to provide an unique specialized facility to follow up any kind of transient events with the best possible response time paired with high efficiency and availability. It consists of a central structure (the common path, CP) which supports two spectrographs optimized for the Visible and Near‐IR wavelength ranges. The infrared arm is a more classical design, while the optical spectrograph will use a set of dichroics and VPH-gratings to produce an Echelle like spectrum, albeit with higher efficiency. Attached to the CP are the two spectrographs and all necessary calibration and acquisition facilities, dichroic and relay optics. The slit viewer camera, equipped with a filter wheel, lends itself also to some science grade imaging and moderate high speed photometry.

SoXS will come with a pipeline and all tools to provide science operations and data flow at the ESO standard level. SoXS will basically be the only instrument at ESO’s NTT which promises a high success rate in early follow-up of transient events as the telescope will be available for this kind of programs with a very high duty cycle. The SoXS consortium will be reimbursed for building the instrument and operating it with ~50% of the available time. The rest, however, it is available to the ESO community for standard open time proposals. As to operations, the project will follow the practices of PESSTO (Public ESO Spectroscopic Survey for Transient Objects) further described here. SoXS will replace SOFI.

General SoXS view (on NTT platform and SOFI derotator). The reference system shown is centered on the NTT Nasmyth focus.
General SoXS view (on NTT platform and SOFI derotator). The reference system shown is centered on the NTT Nasmyth focus.



•    Istituto Nazionale di Astrofisica (INAF‐OA), Osservatorio Astronomico di Brera, Merate, Italy

other participating institutions

•    INAF‐OA Capodimonte Napoli, Italy

•    INAF‐OA Padova, Padova, Italy

•    INAF‐OA Catania, Italy

•    INAF‐TNG (telescopio nazionale Galileo), San Antonio de Breña Baja, Spain

•    INAF‐OA Roma, Monte Porzio Catone, Italy

•    University of Turku and FINCA - Finnish Centre for Astronomy with ESO,Turku, Finland

•    Millenium Institute of Astrophysics, Chile

•    Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot, Israel

•    Niels Bohr Institute, Copenhagen, Denmark

•    Astrophysics Research Centre, Queen’s University Belfast, UK

The project team is composed of:

Principle Investigator:

•    Sergio Campana, INAF‐OA, Osservatorio Astronomico di Brera, Merate, Italy

SoXS Science Board:

•    Enrico Cappellaro, INAF‐OA Padova, Italy

•    Massimo Della Valle, INAF‐OA Capodimonte, Italy

•    Avishay Gal‐Yam, Weizmann Institute, Israel

•    Seppo Mattila, Turku Univ.,Finland

•    Giuliano Pignata,  Millenium Institute of Astrophysics, Chile

•    Stephen J. Smartt, Queen's University Belfast, UK

•    Iair Arcavi, Tel Aviv University, Tel Aviv, Israel

•   Johan Fynbo, Niels Bohr Institute, Copenhagen, Denmark

Project Management:

•    Pietro Schipani, INAF‐OA Capodimonte, Napoli, Italy

ESO represented by:

•    Ulli Kaeufl, ESO, Germany



Planned date to be operational on sky: end 2020

Past milestones:

- Kick-off:   

January 2017

- Preliminary design review

July 2017


Status: Passed Final Design Review (pending closure of actions)


Future milestones (planned):

- AIT & Test in Europe

June 2020

- Instrument in Chile

August 2020

- End of Commissioning

December 2020


Instrument Description

SoXS general characteristics


slit viewer

spectral range

360-970 nm; set of standard science filters in wheel


Andor iKon‐M 934, 1k x 1k

scale and field

0.205 arcsec per pix:  ~3.5 x 3.5arcmin^2

operation modes

pierced mirror: field around object, all light to spectrograph pellicle: allows for monitoring while taking spectra

mirror: all light to camera

slit viewing: special mode to align the slits and the camera

optical arm

spectral range

350‐880 nm (red edge tbd by dichroic)

resolution (for 1”slit)

>3600 across the entire band, >4000 for at least 75% of the quasi‐order

slit length



e2V CCD44‐82  2k x 4k



infrared arm

spectral range

780-1833 nm (blue edge tbd by dichroic)

resolution (for 1”slit)


slit length



2k x 2k HgCdTe, 2.5µm cut-off (Teledyne Hawaii 2RG)



SoXS overall mechanical design at PDR
SoXS overall mechanical design at PDR, attached to the NTT Nasmyth adaptor (grey). The green part is the Common Path optics, holding the dichroics and the guide camera (violett). The light from the NTT is spectrally split and makes U-turns to be fed from the back into both spectrometers. The vacuum enclosure of the fully cryogenic IR-spectrograph is shown in red, while the optical spectrometer is shown in blue.

Optics in the Common Path (CP)

The figure opposite shows the common path of SoXS, in the same orientation as in the sketch above. The optical path in this sketch starts at the NTT Nasmyth focus. The focal plane of the NTT is at some 900 mm from the adaptor flange, rather generous as compared to the 250 mm backfocal distance at the VLT. For that reason, the CP bends the light in a U-turn back towards the telescope so that both spectrographs are fed “from the back”.

In the focal plane there is either an open position, a pierced mirror or a pellicle beamsplitter, all mounted on a motorized precision slide. The pellicle beam splitter allows to take spectra and simultaneously images from the field for photometry.

To the right of the CP structure the kinematic mirror feeding the cal-unit, again on a motorized slide, can be seen. The calibration system is a separate unit which contains all light sources necessary, controllers and injection optics. The unit contains standard arc lamps and flat field lamps.

The acquisition camera will be equipped with a 4-stage Peltier cooled CCD (Andor iKon‐M934, BEX2-DD) and a filter-wheel with standard filters. It will be quasi science grade and may also allow to do, within limits fast, photometry.

An Atmospheric Dispersion Compensator (ADC) is part of the path feeding the optical spectrograph (top of sketch).


The Optical Spectrograph

SoXS UV-VIS Spectrograph
SoXS UV-VIS Spectrograph. Above: feed level; Below: the disperser+camera level

The sketch of the spectrograph shows the principle. In the top panel one sees the light coming from the optical path will be separated by a dichroic ( C ). It is then sent to prisms (E) and (F) which are penta prisms, again with a dichroic function. The light is sent to the other side of the optical bench (lower panel of figure) where it is being diffracted by two pairs each of volume phase holographic gratings (VPH). For each optical path there is then a camera lens and a set of fold mirrors so that all four spectra are being imaged onto the same CCD (e2V CCD44‐82, 2k x 4k).

Apart from a systematically higher throughput compared to a classical x-dispersed spectrograph there are other interesting aspects: the quasi orders are almost straight and the wavelength intervals, in which the light is detected on two different locations of the detector are much smaller than in classical Echelle spectrographs.

The expected image quality is such that all light is collected in an aperture corresponding to 2x2 pixels.

Overlap regions for the quasi orders in wavelength
Overlap regions for the quasi orders in wavelength. Left: u+g dichroic surface efficiency; Right: r+i dichroic surface efficiency
Spot diagrams for a field set at the bottom edge of the slit (0.66mm/6").
Spot diagrams for a field set at the bottom edge of the slit (0.66mm/6"). 2x2 pixels are shown (30um x 30um). The wavelength for each spot is written above each spot.

The Infrared Spectrograph

Here a classical design has been chosen. The figure shows the lay out and lists some characteristic parameters. The detector will be a classical Teledyne 2k x 2k 2.5μm cut-off HgCdTe detector. The entire instrument will be cooled by a standard compressed Helium  expansion cooler.

SoXS-NIR Spectrograph Layout
SoXS-NIR Spectrograph Layout
NIR Spectrograph main parameters
NIR Spectrograph main parameters
NIR spot diagrams
NIR spot diagrams (order 11)

Also for this design the image quality is very good, as can be seen from the NIR spot diagrams for the worst case (order 11).


It is planned, that the SoXS consortium will be entitled to some 50% of the available time of the NTT as re-reimbursement for their effort to build the instrument for five years. In return the SoXS consortium will operate the instrument, also wrt all data flow aspects. SoXS will make some of their data obtained in guaranteed time available to the general public immediately. All open time programs will be executed by SoXS personnel and distributed via the ESO archive. All science operations aspects from observation preparation to delivery of pipeline reduced spectra and images will conform to ESO standards.

Primary Science Case and Competition

As far as we know there is no comparable instrument on a 4m class telescope existing or under way. Naively one could say, SoXS can do all science cases which are too bright for X-shooter at the VLT. While such kind of programs certainly will be handled in their own right, this is not the primary reason why SoXS is being built.

SoXS will be ESO’s tool to cater to the scientific needs and to the strong response from the community for a medium resolution (R~4,500) wide wavelength coverage (350-2000 nm) spectrograph specifically taylored for transient and otherwise time variable objects. Having a telescope like the NTT effectively earmarked for this kind of science enables the systematic followup of triggers and hence open a new window to observe the universe. Also a truly rapid response mode may offset to some extent that SoXS is mounted to a 3.5 m and not to an 8 m class telescope.

The SoXS follow-up is instrumental to exploit the cornucopia of transient and variable sources which will be discovered among others by wide field photometric surveys (e.g. PanSTARSS  or the, LSST in the future), alerts from space born instruments (e.g. Gaia, SVOM, EUCLID in the future), triggers from gravitational wave (ALIGO/VIRGO) and neutrino (KM3NET) experiments, which will require an intense collaboration with ground-based facilities in order to maximize their scientific output.