# 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:- Near the start and end of observations on a single photometric night
- At some time on at least one other photometric night
- At some time on at least one non-photometric (but 'useful') night

- 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.