P2PP: MIDI Information

This page contains information specific to the creation of MIDI Observations Blocks (OBs) for Service Mode programs. For more general OB creation information, see the P2PP Web page.

Requirement Compliance Policy: Observing runs which do not adhere to the procedures and policies presented in this document will not be scheduled for execution. If you feel you must violate one of these procedures or policies, you must submit a Phase 2 Waiver Request before submitting any Phase 2 material.

This document has the following sections:

Read the Manuals
MIDI P2PP Tutorial
Target coordinates
Visibility Calculator
Calibrator Selection
Additional Requirements for MIDI README Files
Additional Requirements for MIDI Finding Charts
OB Naming Conventions
Matching of Science target and calibrator OB names
P2PP Constraint Set

Read the Manuals

It is essential that all users read the following manuals:

MIDI User Manual

MIDI Template Manual

P2PP User Manual

Note that the MIDI user manual has been updated for P81. Likewise, the MIDI template manual has been updated for Phase 2 of P81. It is also useful to check the MIDI Web pages for recent updates.

Further useful information and tools for the OB preparation can be found on the User Support Services and Tools Web page (for instance exposure time calculator, object visibility, catalogues, etc.)

MIDI P2PP Tutorial

An instrument-specific MIDI P2PP Tutorial has been prepared to guide you through the preparation of Observation Blocks (OBs) for successful MIDI observations.

A general README tutorial is available to guide you through the creation of README information.

A general Finding Chart Tutorial guides you through the attachement of Finding Charts. This tutorial includes also a part dedicated to attaching ephemeris files.

Target coordinates

Special care should be taken when entering the target coordinates in P2PP. We have made the best experience with 2Mass coordinates. Also, to ease fringe finding during the observations, proper motions of the target (if available) must be entered in P2PP).

Visibility Calculator

To prepare and plan your VLTI+MIDI observations, we recommend to use the Visibility Calculator VisCalc. The assessment of the feasibility of an observation requires an estimate of the expected visibility for the chosen VLTI configuration. Furthermore, the scientific goal of an interferometric observation campaign can often only be reached if visibility measurements at different projected baseline lengths and/or baseline angles are combined. VisCalc provides calculations of simulated visibilities as a function of hour angle based on software models of the VLTI instruments. The best suited hour angles can be identified using VisCalc, and the corresponding LST (Local Sidereal Time) interval for each OB, i.e. each instantaneous visibility measurement, must be specified in each OB. In case that, from a scientific point of view, there is no restriction for the LST range to be used, the LST range in the OB must still reflect the range when the target is higher than 30 degree above the horizon, and exclude the range when the observation is not feasible due to delay line restrictions and shadowing effects (see the section about the LST constraint below). See also plots of the VLTI pointing restrictions in the declination/hour angle space at http://www.eso.org/observing/p2pp/MIDI/vlti_pointing_restrictions.html.

Since we encountered in the past period that a few proposed targets were clearly larger than estimated, and thus over-resolved, we strongly encourage you to collect as much information on your target as possible. It may be advisable to organize imaging studies with single-telescope instruments before conducting MIDI observations if you have strong doubts about the extension of your proposed target.

Calibrator Selection

You are requested to submit a calibration star OB for each science target OB. This pair of science target/calibration star OBs will be executed in a row in order to monitor the atmospheric and instrumental transfer function. To select calibrator stars, it is recommended to use the CalVin tool developed by ESO. CalVin selects suitable calibrators from a fixed list based on different user criteria. The underlying list of calibrators is based on the list provided by the MIDI consortium (see Stecklum et al., in preparation, and Stecklum et al. VLTI calibrators Workshop) and lists preliminary values of the limb-darkened diameters for the calibrators as obtained by fitting photometric data to stellar atmosphere models.

The selection of the calibration star should be based on a reasonable compromise between different characteristics such as brightness (not much fainter than the target), distance on sky (close to the target), diameter (unresolved or at least clearly smaller than the target), and maybe spectral type or other characteristics (e.g. suitability as spectrophotometric standard) which are important for the respective observing mode.

VLTI observations are normally performed as pairs of science and calibrator OBs. The LST range of the calibrator OB of the pair must begin 30 minutes before the start of the LST range of the corresponding science OB and end 30 minutes after the end of the LST range of the corresponding science OB. In case that a single calibration star cannot be used for the full LST range of the science target, it is possible to use more than one calibration star OB to cover the required LST range. Only one of these calibration star OBs will be executed depending on the time of the observation of the science target. This ensures that a calibrator OB is available at any time when the science target observation may be attempted. Paranal Science Operations reserves the right to exchange a user submitted calibrator by another suitable calibrator from CalVin if there is a need to do so.

It is also possible to request additional calibration star OBs so that a sequence of [calibrator - science target - calibrator] is realized. OBs for the additional calibration star should also be submitted. In this case, the sequence must be described in the "Calibration Requirements" field of the OB and in the "Special Calibration" Section of the ReadMe file, as well as in the user comment field of the OB. The execution time for the additional calibrator is subtracted from the allocated time of the programme.

We recommend to select a calibrator from CalVin so that more detailed knowledge on these calibration stars will be acquired rapidly. However, the user has as well the option to use a calibrator which is not in the CalVin database. In this case, the following information is requested in the README file (Section "Special calibration requirements"):

Name
Coordinates
V magnitude
N magnitude
Spectral Type
Uniform Disk (UD) diameter at N band with error
Source of diameter estimate.

We remind that all calibration star data are public as soon as they are listed in the archive.

Additional Requirements for MIDI README Files

In addition to the general requirements for README files, the following MIDI specific recommendations should be followed:

Information about calibrator stars which are not from CalVin (see above), in the Section "Special Calibration information" of the README file.
Information on the variability of guide stars with variable V magnitude (minimum and maximum V magnitude expected during the period).
Acquisition images with MIDI are not usually obtained anymore since IRIS is used for alignment. If an acquisition image is desired for scientific exploitation, such a request should be included via the README information.

Additional Requirements for MIDI Finding Charts

In addition to the general requirements for finding charts, the following recommendations should be followed:

A 5 x 5 arcmin optical finding chart, centered on the target (whether it is a science object or a calibrator), showing objects brighter than V=17.
MIDI Service Mode observations will be performed on the brightest source in the 2 x 2 arcsec MIDI field of view. If a target is not a single isolated IR source in the MIDI FOV, please request visitor mode.

A description on the creation of Finding Charts using ESO tools can be found here. A tutorial to include the Finding Charts in the OBs is here.

OB Naming Conventions

Any MIDI OB must have its name beginning by one of the two following prefixes, in order to indicate the type of target (either science or calibrator):

SCI_ CAL_

It is recommended that the name of the OB, that follows the prefix, is the name of the target, as written in P2PP, without space characters.

If a scientific target or its calibrators shall be repeated at different hour angles (i.e., LST ranges), there must be as many OBs as desired observations (at different LST ranges). OBs can be duplicated (for scientific targets with the LST range in P2PP modified) and their names completed with a suffix like -sr-n, where sr is the subrun of the programme (A,B,C,...) and n (starting from 1) is the iteration of the observation.

If, instead, a certain observation shall be executed only once, but can be executed at either of several LST ranges, these alternative LST options for one observation must be indicated in one OB.

Matching of Science target and calibrator OB names

In each OB of a scientific target, the name of the corresponding calibrator OB (or OBs) must be indicated in the p2pp comment field labeled "Name of associated OB of SCI/CAL pair". Likewise, in each OB of a calibrator, the name of the corresponding scientific target OB must be indicated in this comment field.

If additional calibrators shall be observed instead of the regular cal/sci pair (sequence of cal/sci/cal), this information has to be inserted into the special calibration sections of the OB and the ReadMe file.

P2PP Constraint Set

Sky transparency

Observations in the thermal IR regime may require clear or photometric conditions, depending on the spectroscopic quality required. However, MIDI observations can even be done with thin cirrus. Sky transparency constraints are mostly relevant for MIDI in HIGH_SENS mode, where the photometry measurements are performed after the fringe exposure. Variability of water vapor quantity in THN condition may therefore affect the visibility accuracy obtained. In SCI_PHOT mode however (where fringe and photometry measurements are performed simultaneously), effect of THN condition should be strongly reduced. It is therefore recommended to set the constraint to THN in SCI_PHOT (to increase the probability to have the OB executed). CLR sky conditions may also be advisable for Coude guide stars with a V magnitude close to the limit.

Baseline choice

The choice of the baseline to be used must be indicated in the baseline field of the constraint set in each OB (this applies for UT as well as for AT baselines). Alternative baselines may be mentioned in the ReadMe file.

Intervals of local sidereal time (LST)

Constraints on the local sidereal time (LST) at which the OB has to be executed must be specified in the "Sidereal Time" section of each science OB. Desired constraints on the projected length, azimuth angle of the baseline, or the airmass must be translated into LST intervals, for example by using the ESO Visibility Calculator VisCalc. Furthermore, the specified LST interval must be limited to the time range when the target is above 30 degrees altitude and when the observation is not unfeasible due to the delay line restrictions or shadowing effects. The feasible HA range can be checked with VisCalc. See also plots of the VLTI pointing restrictions in the declination/hour angle space at http://www.eso.org/observing/p2pp/MIDI/vlti_pointing_restrictions.html.

The LST range of the calibrator OB of the pair must begin 30 minutes before the start of the LST range of the corresponding science OB and end no earlier than 30 minutes after the end of the LST range of the corresponding science OB. Alternatively, in case that a single calibration star cannot be used for the full LST range of the science target, it is possible to use more than one calibration star OB to cover the required LST range. This ensures that a calibrator OB is available at any time when the science target observation may be attempted.

We recommend to use LST ranges of at least 3 hours. Otherwise it might be difficult to schedule the observation. The minimum accepted LST range is 1 hour. In case that a desired observation shall be done only once, but can be executed at several alternative non-contiguous LST ranges (for instance either some hours before meridian or some hours after meridian), up to five alternative LST intervals can be specified for one OB (science OB and corresponding calibrator OB). The time separation between these intervals must be at least 1 hour. Please be reminded that multiple OBs are needed if your target shall be observed more than once (for instance at different LST intervals).

Other constraints (seeing and moon constraints)

MIDI observations in P81 will be executed by using the MACAO (multi application curvature adaptive optics) systems at all UTs, and using the tip-tilt corrector system called STRAP at all ATs. With the successful use of MACAO at the UTs and STRAP at the ATs, the image quality in terms of FWHM and positional stability on the MIDI detector remains very good, virtually independent of the regular (optical) seeing value. Also the sky background introduced by the moon does not significantly affect the data quality at mid-infrared wavelengths.

MACAO/STRAP guiding is mandatory in service mode observations. ESO assures that the MACAO systems are used at the UTs and the STRAP systems at the ATs, and that in both cases the adaptive optics loop is closed during the entire observation. Otherwise, the OB will be considered for re-execution.