Project

General

Profile

Cal-R5

Time variability of Y-band fringing has been investigated sufficiently to see whether OBSTAC must be instructed to take a minimum number of consecutive images in Y band, such that a temporally local fringe frame can always be constructed.

This should also be investigated for z-band.

Cal-G6 goal is to bring fringe residuals to be invisible to the eye.

Prerequisites

  • Software for basic image processing and to mask bright objects.
  • Software to create median fringe frame and scale and subtract them from individual images.

Procedure

The time variability will be investigated under three circumstances:
  • Consistency within a single photometric night
  • Consistency between photometric nights
  • Consistency between photometric and nonphotometric nights

The minimum number of frames required to construct a useful fringe frame will also be measured.

The basic dataset is 10 dithered observations in the Y-band (z-band) with the survey exposure times. They should be obtained:
  1. Near the start and end of observations on a single photometric night
  2. At some time on at least one other photometric night
  3. At some time on at least one non-photometric (but 'useful') night
The analysis steps are:
  • Process each set of images to create a fringe frame (bias subtract, overscan, corrected flat field, mask bright objects)
  • Scale and subtract the fringe constructed from each dataset
  • Scale and subtract each fringe frame from the other datasets
  • Use the standard DESDM tool (or SExtractor by itself) to fit and subtract the large-scale background
  • Measure amplitude of residual fringing.
  • Repeat measurements with smaller numbers of files to determine minimum number of exposures required to construct the fringe frame

Analysis

Analysis relevant to this requirement is available on this wiki page: Fringing

Verdict

The fractional contribution of the fringing on scales of <=20" should be less than 10% of the shot noise. (Think about further)

Consequences

If the fringing correction is time variable, enforce a minimum number of Y-band (z-band) exposures in OBSTAC.