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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