Instrument Description

MATISSE in the VLTI context

The VLTI delivers four input beams to MATISSE, from either the four ATs or the four UTs. Each beam has been stabilized with an adaptive optics system in the telescope (NAOMI or MACAO, respectively), and an the residual tip-tilt due to effects downstream from the telescope is measured in the VLTI lab by IRIS and applied through the AO system as well. Since the target acquisition is performed with IRIS, the respective limits apply. For details see the VLTI manual.

The instrument is currently largely stand-alone from the infrastructure of the VLTI. As such it does not use external fringe-tracking, and only acts as a coherencer, meaning it will only center the entire fringe package after a number of exposures, but does not keep the individual fringes in position. A coherent adding up of indivdual exposures is therefore only possible if the fringes can be centered in each exposure, or if assumptions on the fringe movements are made. The use of GRAVITY as an external fringe tracker for MATISSE is being investigated.

The CIAO adaptive optics system for the UTs is currently not offered for MATISSE.

MATISSE Optical Elements

The following is a general description of the optical path, for more detail and numerical information see the instrument manuals.

On the warm optics bench, the beams pass through two commuting devices that can interchange the input beams 1 vs. 2 and 3 vs. 4, respectively. They are always used, and any observation consists of at least one BCD-IN and one BCD-OUT measurement. Comparing the BCD-IN with the BCD-OUT data enables correction of instrumental chromatic effects on the measured interferometric phases.

After the LM- and N-bands have been separated on the wam optics part, the beams enter the LM or N-band cryostat. The internal setup of each cryostat is comparable, so this general description is applicable to both cryostats. In an intermediate focus the beams pass through a spatial filter, either a slit or a pinhole, with defined size of the order of the point-spread function. ESO has chosen a standard setup for those, but expert users in visitor mode can make use of the alternative options.

When the photo-interferometric splitter is inserted into the beam, source photometry and fringes are observed simultaneously (SIPHOT mode), otherwise they have to be taken sequentially (HISENS mode). Currently the only offered option is to observe L-band in SIPHOT and N-band in HISENS, which is the so-called HYBRID mode.

Users experienced with N-band interferometry might wish to use the correlated flux measurement provided by MATISSE instead of the full visibility information. Since a correlated flux measurement does not require that source photometry is obtained, the execution time per OB is shorter, and correlated fluxes can be obtained for fainter sources than full visibility measurements. However, in that case the user must have information about at least the calibrator flux from an external source, and preferably the science target as well. Correlated flux measurements are not possible in the LM-band.

The dispersive optics provides a choice of several resolving powers. The detector pixels oversample the optical resolution. For this reason, the pipeline enables binning from spectral pixel to spectral channels, and the performance values are given per spectral channel. This  improves the performance limits without sacrificing any recorded astrophysical information. Binning any further would destroy such information, and is only recommended for users who are sufficiently experienced to judge the trade-offs and have a clear scientific justification for doing so. The oversampling (i.e., binning) factors are as follows:

          R        pixel/DIT       pixel/channel        
LR L-band 34 65 5
MR L-band 506 118 5
HR L-band 959 118 5
LR N-band 30 120 7
HR N-band 218 819 7

The "pixels per DIT" are the number of pixels read in spectral direction within the default DIT values.

Filters to reduce the total background are inserted according to the chosen spectral resolution and wavelength range. The polarizing filters are not available for scientific observations.

Calibrators and Calibration Strategy

MATISSE if offered with two observing sequences, either CAL-SCI or CAL-SCI-CAL.

Calibrator stars for N-band and L-band can be found, for instance, with the SearchCal tool provided by the jmmc (see links). However, finding a star that is suitable for both bands at the same time can be tricky. Users should make sure already at phase 1, i.e. for proposal submission, that their chosen calibration strategy is possible and suitable calibrators are available. In case no good L+N calibrators are nearby, the user should consider to use the CAL-SCI-CAL sequence with one calibrator for L-band and one for N-band. The following recommendations should be considered when choosing a calibrator star:

  • Prefer to use the JMMC-SearchCal Tool instead of ESO-CalVin, since the latter is currently not updated.
  • Mid-IR fluxes are hard to measure. It is recommended to compute median fluxes in L and N bands using for example the VizieR Photometry viewer provided by CDS-SIMBAD intead of relying on a single source.
  • Good calibrators should follow the Rayleigh-Jeans approximation, i.e., avoid IR excess. L/N-band flux ratio (at 3.3 and 8.5 microns) should best be within a range of 7 to 9.
  • A SIMBAD search on the calibrator should show "star" as object type.
  • A calibrator catalogue is in preparation by the MATISSE consortium. For further information please contact the consortium.

MATISSE in P105

MATISSE is offered in period 105 based on commissioning results. Please find below the list of mode offered:

  • Hybrid is the only observing mode offered. The hybrid mode consists of the following two steps: 1. acquisition of fringes in SiPhot mode LM + HiSens mode N; 2. SiPhot with chopping in LM + Photometry in N. Users interested only in closure and differential phases can skip step 2. All the other users are asked to include step 2 (i.e. the additional time needed to record the N-band photometry, see table "Execution Times" and Overheads)
  • L-band observations are offered with low (R=34), medium (R=506), and high (R=959), but not very high resolution in P105.
  • M-band observations are offered with low (R = 34) and medium (R = 506) resolution. The latter mode is not available for monitoring programs during P105. M-band measurements need the photometry step.
  • N-band observations are offered with low resolution (R=30) and high resolution (R=218).
  • The DIT values are fixed. The chosen DIT for LM-bands enables to observe a spectral window of about 0.1 micrometer in high resolution, and 0.2 micrometer in medium resolution, which the user can center following the table below. In visitor mode the user can center the spectral window freely.
Setup Central wavelength Spectral feature
Medium and high L-band 3.03 Pfund 10-5
  3.05 Water ice
  3.17 C2H2, HCN, CN
  3.3 PAH
  3.4 CH
  3.52 Nano-diamonds
  3.77 C2H2
  3.88 C2H2
  3.95 C2H2
  4.0 SiO
  4.05 Bracket Alpha
Medium M-band 4.65 Pfund 7-5
  4.78 CO

 

Execution Times

Setup single OB CAL-SCI CAL-SCI-CAL
Low L and M-band 20 min 40 min 60 min
Medium L and M-band 20 min 40min 60 min
High L-band 25 min 50 min 75 min
N-band photometry* +10 minutes +20 minutes +30 minutes

* Note: during the N-band photometry exposure the LM data (fringes + photometry) are simultaneously recorded with chopping. The user must include such observations in case of M-band measurements, visibility or correlated fluxes measurements in any band. 

Sensitivity and Errors vs. Observing Conditions

General

Starting from P105 a new atmospheric turbulence constraint replaces the current seeing constraint. The limiting magnitudes given below are listed for two turbulence regime. However, bad observing conditions do not only diminish the flux. If a science case is critically dependent to achieve the best possible error bars, it is strongly recommended to request good observing conditions regardless of the target brightness. Observations at seeing values worse than 1.2" are not recommended. All flux limits are given in Jansky and they correspond to coherent fluxes (Flux at a given band times Visibility).

Absolute measurements

For closure and differential phase, visibility, and coherent flux measurements the following limits will allow to achieve a precision per spectral channel at least as good as:

  • For visibilities a precision of 0.1
  • For coherent flux (N-band only) of a SNR of 10
  • 4 deg for the differential phase
  • 5 deg for the closure phase

The limit in low resolution is mostly caused by uncertainty of the photometry and independent of the binning. This limits the precision of the visibilities, not of the detection of fringes themselves. For specific science cases, usable information such as variation of some measurements with wavelength or baseline can be obtained at substantially lower magnitudes. Please contact the instrument scientist for such limits. 

Setup AT UT
 

T≤ 30%

Seeing  0.8"

τ0 > 4.1 ms

T≤ 70%

Seeing  1.15"

τ0 > 2.2 ms

T≤ 30%

Seeing  0.8"

τ0 > 4.1 ms

T≤ 70%

Seeing  1.15"

τ0 > 2.2 ms

Low L-band  1 Jy 1.5 Jy 0.1 Jy 0.15 Jy
Low M-band  5 Jy 9 Jy 0.5 Jy 1 Jy
Medium L-band 8 Jy 10 Jy 0.6 Jy 0.7 Jy
Medium M-band  30 Jy 40 Jy 3 Jy 6 Jy
Low N-band, full visibilities 25 Jy 30 Jy 1 Jy 1.3 Jy
Low N-band, coherent fluxes only* 4 Jy 6 Jy 0.2 Jy 0.3 Jy

Note: the N-band coherent fluxes < 10 Jy are offered for P105 only in Visitor mode.

Relative measurements

For medium and high resolutions, additional limits are given for measurements where only the differential phase is considered, since absolute measurements are better obtained with low resolution setting. We give the limiting magnitudes to achieve a precision of 4 degree per spectral channel in differential phase, and an SNR on the coherent flux in N-band (see above for definition of spectral channel):

Setup AT UT
 

T≤ 30%

Seeing  0.8"

τ0 > 4.1 ms

T≤ 70%

Seeing  1.15"

τ0 > 2.2 ms

T≤ 30%

Seeing  0.8"

τ0 > 4.1 ms

T≤ 70%

Seeing  1.15"

τ0 > 2.2 ms

Low M-band 1.5 Jy 1.5 Jy 0.1 Jy 0.1 Jy
Medium L-band 3 Jy 4 Jy 0.2 Jy 0.3 Jy
Medium M-band  7.5 Jy 7.5 Jy 0.7 Jy 0.7 Jy
High L-band 10 Jy 15 Jy 0.7 Jy 1.1 Jy
High N-band 35 Jy 40 Jy 0.7 Jy 0.8 Jy