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 P104

MATISSE is offered in period 104 based on commissioning results. A few  modes are not yet fully commissioned and will be offered later

  • Hybrid is the only observing mode offered.
  • L-band observations are offered with low (R=34), medium (R=506), and high (R=959), but not very high resolution in P104.
  • M-band observations are not offered in P104.
  • N-band observations are offered with low resolution (R=30) and high resolution (R=218).
  • The DIT values are fixed. The chosen DIT for L-band 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 freely.

Execution Times

Setup single OB CAL-SCI CAL-SCI-CAL
L-band low 20 min 40 min 60 min
L-band medium 20 min 40min 60 min
L-band high 25 min 50 min 75 min
N-band photometry +10 minutes +20 minutes +30 minutes

Sensitivity and Errors vs. Observing Conditions

General

  • Observing conditions are considered to be "good" when the seeing is 0.8 or better, and the sky is CLR or better.
  • Observing conditions are considered to be "bad" when the seeing is not above and 1.2 and the sky is at least THN.

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!

Absolute measurements

For closure phase, visibility, and coherent flux measurements, the limits are given to achive the following precisions per spectral channel (see above for definition):

  •  For visibilities a precision of 0.1
  • For coherent flux (N-band only) of a SNR of 10

Note that the limit in low resolution is mostly caused by uncertainty of the photometry and independent of the binning. This limits the precision of the visbilities, not of the detection of fringes themselves. If a science case requires only closure phase or relative visbility data in low resolution, i.e., just the fringes, please contact the instrument scientist for limits.

Observations in MR are recommended for closure phase only, not visibilities. Observations with the high spectral dispersions are not recommended to measure absolute quantities at all.

Setup AT UT
  Seeing < 0.8", CLR Seeing < 1.2", THN Seeing < 0.8", CLR Seeing < 1.2", THN
L-band low 5 Jy 7 Jy 0.3 Jy 0.5 Jy
L-band medium 8 Jy 10 Jy 0.6 Jy 0.7 Jy
N-band low, full visibilities 25 Jy 30 Jy 1 Jy 1.3 Jy
N-band low, correlated fluxes only 4 Jy 6 Jy 0.2 Jy 0.3 Jy

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
  Seeing < 0.8", CLR Seeing < 1.2", THN Seeing < 0.8", CLR Seeing < 1.2", THN
L-band medium 3 Jy 4 Jy 0.2 Jy 0.3 Jy
L-band high 10 Jy 15 Jy 0.7 Jy 1.1 Jy
N-band high 35 Jy 40 Jy 0.7 Jy 0.8 Jy