NTT Imaging Calibration Plan

(Minimal Version)
Authors: Dave Silva & Gautier Mathys
Document: LSO-PLA-ESO-40400-0001
Release: 0.2/1997-Jun-02


This document describes the minimal necessary calibration plan for NTT
imaging programs using SUSI2 and the EMMI imaging modes BIMG and RILD.

A more complete SUSI2 plan is described by Comeron (1997), which also
provides a complete description of the purpose of each kind of
calibration data.

General Remarks

(1) All calibration data described here will be taken using the
following "standard CCD configuration":

    Readout speed: normal
    Windowing: none
    Binning: none
    Exposure Time: > 10 secs to minimize shutter timing effects

EMMI red arm imaging (RILD) will be taken in dual-port "fast" mode.

(2) All calibration data described in this document will be acquired,
named, tracked, processed, and archived via standard Data Flow System
procedures and according to standard Data Flow System protocols.  A
set of calibration OBs will be developed jointly by DMD and the NTT

(3) All programs which require "non-standard" calibration data will
have to acquire and process that data independently.  "Non-standard"
data is defined to be any data which is windowed or binned and/or uses
special filters or read-out speeds.  If non-standard science
observations are made, non-standard calibration data is required.
Non-standard night-time calibration activity must be accomplished
within the time allocated to each program by the OPC. Time required to
complete on-standard daytime calibration activity will typically be
absorbed by ESO.

(4) Classical observers will be asked to adhere to this plan, although
they will not be required to adhere to it.

(5) It is assumed that the NTT Team will supply the following items at
least monthly as part of their normal preventive maintenance

    shutter timing correction 
    detector gain
    detector read-noise
    detector linearity
    bad/hot pixel masks

(6) It would clearly take too much time to provide adequate
calibration for all NTT imaging modes (let alone all imaging and
spectroscopy modes) on all nights.  This is especially true if the
nightly observation of photometric standard stars is required.  The
nightly observing schedule must take this into account.  In general,
only one (1) imaging mode should be supported per night.  The obvious
exception to this rule is when observing starts with either BIMG or
RILD but the delivered image size later falls below 0.5 arcsecs.  In
that instance, if a high priority SUSI program is in the queue, that
program should be executed.

(7) This plan will be in effect through the end of P60.  The
imposition of a calibration plan for P61 and beyond is currently at
the discretion of the NTT Team.

Required Calibration Data

Dark Frames:

    acquired monthly as part of normal NTT preventive maintenance

Bias Frames: 

    what: nine (9) frames per day
    when: TBD by NTT Team

Low count level flatfields:

    acquired monthly as part of normal NTT preventive maintenance

Dome Flats:

     level: >= 20K ADU but in linear range
     number: three (3) per filter used during previous night
     when: in early morning *after* a night of observations *but*
           before the filters are changed
     who:  NTT daytime operator

     Comment: During most service observing nights, the number of
              filters used will be constrained to four (4) filters.

Twilight Flats (UBVRI only):

     level: >= 10K ADU but in linear range
     number: five (5) per twilight period
             During each twilight period, acquire set of twilight
             flat frames for one (1) filter.  Rotate through UBVRI.

             Twilights should be acquired regardless of whether one
             of these filters was used on this specific night.

Photometric Standards (UBVRI only):

    General Characteristics

    Standard fields will typically be drawn from Landolt lists.
    A list of appropriate OBs will be developed jointly by DMD and
    the NTT Team.  

    There will be a minimum of two (2) standard stars per field.

    Color range: -0.2 <= B-V <= 1.5 -> try to achieve every night.

    OBs will be defined for these standard fields.

    During service observing, a maximum of four (4) filters per night
    should be used.

    The photometric flux calibration of narrowband filters is the 
    responsibility of the users.

    We will *not* provide the photometric transformation coefficients
    on a nightly basis.  If time permits, coefficients will be
    computed once per period and published.  In general, however,
    users are expected to compute their own transformations based on 
    data provided to them.

    Execution Options

    >>>> Option A: Official NTT Policy 
    The following policy was established by J. Spyromilio in his memo to 
    the DG describing the general NTT calibration plan:

    For each filter used during every clear night, 
    observe 20 - 30 standards at a variety of airmasses,
    regardless of whether programs being executed that night require
    photometric calibration.  For efficiency, these standards should
    be grouped into 5 - 7 standard fields which contain as many standards as
    possible.  Note that it is the number of standard stars not standard
    fields that is important.

    Advantages: All archived data has associated photometric
       calibration for nights when data was acquired.  Calibration is
       of varying precision.

    Disadvantages: Injects large calibration overhead into schedule (1
       - 3 hours per night every night imaging occurs). 

    >>>> Option B:  A Modest Proposal from Silva

    The following strategy was adopted by the WIYN Queue Observing


    (a) Standard star exposure times are short relative to CCD readout

    (b) It is impossible to assess how photometric a night really was
        until the standards are reduced.

    (c) If one takes many standard stars and many deep science
        exposures on a night which really was not photometric enough,
        all the standard data was wasted and must be thrown away and
        the science data must be calibrated in a different way.


    On a night which appears photometric, observe many standard stars 
    (30 - 40) while also acquiring short exposures (5 - 10 mins) of all fields
    of programs which requested accurate photometric calibrations.
    Repeat on different nights as necessary until reduction of
    standard stars demonstrates that user specifications for
    photometric accuracy have been achieved for all service observing programs.

    Acquire deep exposures of science targets on other nights.  Apply
    zeropoint from calibrated shallow exposures.

    On all other clear or semi-clear night, observe one (1) standard
    field through each filter used that night.  Estimate instrumental
    zeropoints for these nights using this one set of instrumental
    magnitudes and the extinction and color terms derived on the 
    "calibration nights".

       Uses much less telescope time at night than Option A.
       More definitively calibrates programs which need accurate
       photometric calibration.

       Archived data which was not directly calibrated
       have on average less accurate instrumental zeropoints.

    Unless otherwise directed by the NTT Team, Option A
    will be executed.