WFCAM On-Sky Characterization Tests

WFCAM On-Sky Characterization Tests

A. Adamson, P. Hirst (JAC), M. Irwin, D. Evans, J.Lewis, S. Hodgkin (CASU)

Last update by PH 20031006 – add tweaks from JRL


This document lists the various on-sky tests which will be used to characterize WFCAM as soon as possible after commissioning time, and in some instances during commissioning. 

I (PH) would like to use this document to define a list of what observations are necessary and to some extent who will be responsible for looking at the data and signing off each requirement.

Detector Noise Properties

The detectors will be used in a new mains power environment, attached to a telescope, and may have noise properties different from those measured in the lab. The difference needs to be characterized so that its impact on data reduction can be assessed. This will include: 

  1. Basic read noise measurements (can be done in the daytime)
  2. Re-measure the crosstalk matrix (could be done pre-final-alignment)
  3. Bad pixel stability (need bias and dark frames from the earliest stages of commissioning)
  4. Dark current stability (daytime, to some extent)
  5. Repeatability and level of reset anomaly (daytime)


Basically, check that it works. Verify 2×2, 3×3, and 4×4 sequences. Also, finalize which option is optimal in the cases where more than one option is available for a given mode in terms of the step sizes (i.e., where there’s more than one common denominator between the two pixel scales.)

Sky Emission

Note: In the case of measurement of sky properties, the measurement listed is to be carried out in every appropriate filter. Repeatability of filter placement should also be tested, and this can be done in a number of the tests below. 

  1. Brightness, time variation, spatial scales of the emission.
  2. Effect of large spatial offset to a “sky” position.
  3. Sky brightness vs. distance from Moon (check for additional contribution from scattered light).
  4. Sky brightness as a function of zenith distance.


Can probably mostly be done with the sky measurement data above.

  1. Fringe amplitude and stability during a change of sky brightness.
  2. Variation time and spatial scales, speeds of motion.


Need an early measurement of the sensitivity limit in the standard observing modes. Also should compare coadded images of the same field taken on widely-separated nights. Quantify overheads in standard observing modes. 

Background Limit

Early check of background-limited exposure times.

Cosmic Rays

Long exposure to pick up a lot of CRs – test whether DR rejects them. Probably daytime to some extent. Can be done in conjunction with dark current monitoring.


  1. Check in a field with a range of point-source brightnesses.
  2. Measure the decay profile of a latent image in terms of time and number of reads and resets.
  3. Repeat at intervals to check temporal stability.
  4. Look for and quantify and adjacency effects.

Flat Field

  1. Dome flats (lamp on/off), fading twilight flats – assess usability. Screen and illumination will be provided for this. (Will it? PH)
  2. Nonlinearity from dome flats.
  3. Vignetting function and its motion, if any (i.e., dependence on telescope attitude). It also includes taking a mesostep sequence.
  4. Assess the color dependence of the flat field.
  5. Assess the spatial thermal signature in the K band.
  6. Tests of different flat fielding algorithms.
  7. Assess flatfield stability (impacts on data taking and reduction strategy).

Scattered Light

Observe a field with large numbers of stars and one with a few bright stars. Quantify the scattered light and ghosting. 

Area Calibration

  1. Place a UKIRT faint standard on each chip independently (if possible, in each of the different channels on each chip).
  2. Observe a number of the predefined standard fields.


Dead-reckoning coordinate system – first go. Includes the following:

  1. Chip and readout orientation.
  2. Refine WCS constants.
  3. Refine radial distortion model (needed later for more accurate astrometry).
  4. Use the same data to check microstepping accuracy.


  1. Test guiding in crowded fields.
  2. Test guiding in the field of a large, bright object.
  3. Find the brightness of the faintest viable guide star as a function of sky conditions.


Require observations of standard fields as yet to be finalized.