Subject:	ESO VLTI calibrator program

Ladies and gentlemen--

With the coming of the ATs, and the slow movement of MIDI and AMBER into 
routine operation I think the time has come to improve the general level of 
interferometric calibrators at the VLTI and their accessibility to users.
The goal is to generate a database of calibrators that both indicates a propri 
to observers what their  choices of calibrators are, and also provides a 
postiori aid to people reducing data, by giving flux, diameter and spectral 
information for the standard calibrators.  Certainly the lack of well known 
spectrophotometric calibrators for MIDI is currently a problem for reduction.

I propose a multi-step improvement to the current situation, which involves 
several people at several institutes, although the center of gravity lies a 
ESO.  The database should contain at least N- and K-band data, and should be 
expandable to include data at other bands.

The steps I currently see, and suggestions for who might lead each step are:

1.  Define database formats to contain information on
    each calibrator.  The initial starting point would
    be the TARGET/SOURCE definitions in the Jaffe/Cotton
    format document, but a lot of additions and modifications
    are necessary.  Things that should be contained are
    at least standard names, aliases, positions(+errors),
    diameters(+errors), broad band fluxes, spectra(where known),
    and a bunch of quality flags and reference flags indicating
    where the data came from at which elements are reliable.

    I suggest that Rainer and I start on the format definitions 
    (in the form of FITS binary Tables) and after review by the 
    MIDI and AMBER teams and by ESO they in the end become DICB
    controlled.  They can then be integrated into MIA-EWS and
    the AMBER packages, and into WEB-based selection programs.

2.  An initial set of well understood calibrators be entered
    into the data base, after careful review of the available
    data.  (Isabelle, Rens, Olivier,.. someone from AMBER)

3.  Observing programs be set up with MIDI, AMBER, and
    possibly VISIR and ISAAC to establish a consistent set
    of spectrophotometric and visibility data.  These might
    involve dedicated time on the ATs, and filler programs
    on the UTs and the other instruments.

    For both spectrophotometry and visibility, the idea is
    to begin with sources whose characteristics are well
    known either from previous studies (e.g. ISO) or from
    theoretical calculations and then cross calibrate their
    (visibility) spectra against a much broader list of
    calibrators.  For the visibility measurements the
    interferometers must be used, for spectrophotometry,
    the single dish instruments are more sensitive, but
    (particularly at N-band) it may be true that the interferometric
    measurements are less sensitive to sky subtraction problems
    than the single dish instruments.

    Particularly for AMBER some decision must be made on which
    spectral resolution should be used.  For MIDI probably
    all measurements should be made in GRISM mode.

    It may be that ESO has some motivation to extend this program
    to ISAAC or VISIR IR calibrators in general.  In this case
    the spectral characteristics of these instruments should
    also be considered in the choice of observing modes.

    This part of the program should clearly be set up and operated 
    by the ESO Paranal team, after recommendations from the rest on the 
priority
    of various sources and observing modes.

4.  The observations must be reduced, checked for quality and consistency,
    and entered into the "best" version of the database.

    This can best be done at Garching by Isabelle or her group, since
    this type of data monitoring is one of their mandates.

5.  Once things get going, the role of the non-ESO participants will
    probably decrease.

I would appreciate hearing from you (all) if you agree that this is an 
important program, whether you want to participate, whether you have suggested 
changes to the above scenario, and whether specific other people should be 
encouraged to participate (particularly from the AMBER direction).

Walter