WFI Calibration Overview

A Calibration Plan is used by the 2p2 Team for monitoring and calibrating the Wide Field Imager (WFI) at the 2.2 m telescope. The plan we have adopted has two components: Instrument Calibration and Standard Calibration.

PIs of Service Mode Programmes should read the section below on Standard Calibration, to help them to decide whether they need to specify calibrations as part of their programme.

The notes on this web page are based on (but supercede) the short document WFI Calibration Plan of February 2001.

Notes for 2p2 Team Members (only) describing the execution of the plan and analysis of the data, can be found on this internal access web page.

Instrument Calibration

Instrument Calibration aims solely to provide data to quantify instrument behaviour for the information of the Team. It does not aim to supply calibrations for use in reduction. However, these Instrument Calibration data are available to users through the links below. The analysis of Instrument Calibration data is undertaken mostly by the 2p2 Team.

Instrument Calibrations are grouped in terms of Short, Medium and Long-Term Calibrations.

Short-Term Calibrations Status Frequency Contact Results Summary
CCD Bias Levels,
Gains, Read-Noises
(Daily Health Check)
Running Daily F. Selman Plots showing the variation in read-noise, gain, bias and flat-field levels during the past two months. Data are also shown on a CCD-by-CCD basis.
Image Quality Database Running Hourly (at night) L. Germany Scatter plots of FWHM (in both X and Y) against julian day, wind speed and direction. Data encompassing either the current, previous or a range of months can be plotted, as required.
Pointing Reproducibility
(Nightly Health Check)
Running Nightly L. Germany Scatter plot of pointing offsets from a predefined zero position over the past month, or since observations commenced on the current template. (Template fields are followed across the sky for the 3 - 4 months that they are visible.)
Image Quality Across Mosaic
(Nightly Health Check)
Running Nightly L. Germany Diagram showing the latest map of distortion vectors across the eight CCDs of the mosaic. There are also scatter plots of mean ellipticity as a function of time, over the current month, a previous month, or over the entire year.
Differential Throughput Measurements
(Nightly Health Check)
Running Nightly L. Germany Scatter plot of magnitude difference from a predefined zero value for a set of stars from our own calibration fields (not Landolt), as a function of time. Plots show the variation for individual stars over the current month, a previous month, or over the entire year. There is also a scatter plot showing the mean magnitude difference, (as averaged over all stars at a given instant in time), over the current month, a previous month, or the entire year.
Shutter and Linearity Errors,
CCD Transfer Function,
Sensitivity to Beta-light,
CCD Bias, Gain and RON
(Weekly Health Check)
Running Weekly (Tuesdays) F. Selman Plots of bias level, beta-light response gain, shutter error, linearity error, gain and RON over the past three months. Table of CCD transfer function values also given. There are also plots available showing the bias frame statistics, unedited transfer and linearity, and filtered linearity and non-linearity over the range of exposures used for each test. Data are presented on a CCD-by-CCD basis.

Status Frequency Previous
Contact Results Summary
Badpix mask for WFI Running 3 monthly . September 2002 L. Germany All masks found here.
Absolute Photometric
UBVRI Zeropoints
Running 6 monthly 31 May 2001 (FK) December 2001 F. Kerber,
F. Selman
F. Kerber took calibration data in May which are currently under analysis.
Astrometric Solution In Prep 6 monthly . September 2001 . .
Sky Concentration
Running 6 monthly April 2001 (JM, FS) November 2001 F. Selman J. Manfroid and F. Selman took independent measurements and the results are available in a short paper by Manfroid, Selman and Jones.
F. Kerber took measurements in May 2001 and is analysing the results.

Status Frequency Previous
Contact Results Summary
Telescope Pointing Model
Running after each
M1 removal
March 1999
June 2001
R. Castillo R. Castillo measured and analysed the pointing model coefficients. There are plots showing the RMS scatter in each coefficient, and a table summarising the newly-measured values.
Focus Temperature Change Running Yearly November 2000 November 2001 J. Cortes J. Cortes measured the focus change with temperature in four filters (ESO nos. 842-845). There are plots showing the change in both the X and Y pixel directions, along with the linear best fit and RMS residuals.
Filter focus offsets In Prep Yearly November 2000 . . .
Tilt of CCD relative
to focal plane
Running Yearly January 2000 October 2001 . MIDAS script available for analysis.
This (and earlier measurements) need to be located.
Filter Image Quality
Running 3 yearly February 2001 February 2004 2p2 Team A. Gilliotte measured the imaging quality of the entire filter set, the results from which are summarised on the WFI Filter Page .
Filter page needs to be updated with AG's most recent emails

Standard Calibration

PIs of Service Mode Programmes take special note)

Information on the Standard Calibration Plan can be found on the dedicated web page.

Quality Control

Quality Control is the process of checking Service Mode data to ensure it meets the specifications of the programmes. This is done during the observations in a cursory way by MET at the telescope. We hope that in the long term it can also be done more comprehensively by DMD as the data enter the Archive (as is the case with VLT). Formally, it is not part of either calibration plan, although informally the calibration data will be inspected for obvious signs of bad data.

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