# IQ-R4¶

## Requirements:¶

- TO.2: The delivered PSF from the telescope, instrument, and other factors exclusive of the site seeing will be no greater than 0.55 arcsec FWHM (DES-doc-806-v25).

- R-18: The PSF FWHM in each of the r, i, and z bands should, over all exposures and over the survey area, have a median of less than 0.9”. Moreover, for 95% of the survey area, there should be at least one exposure in each of these bands for which the mean PSF FWHM is 0.9” or smaller (DES-doc-20-v32).

## Prerequisites:¶

10 Regular exposures in each of the r, i, z band. The images needs to be processed (flat fielding, bias subtraction) and a list of bright stars (positions in CCD coordinates) are needed.

## Procedures:¶

- The PSF FWHM from DECam are compared to the CTIO DIMM seeing measurements according to the model FWHM^2 = DIMM^2 + \sigma^2. The best fit value is \sigma <= 0.55" across the full array. Therefore, we can measure the contribution of the PSF from the DECam by subtracting the DIMM seeing from the measured PSF by quadrature.

- Step 1: Take 10 regular survey exposures in each of the r,i and z band. The exposures are randomly chosen from the whole survey area.
- step 2: Extract the objects on the images and get a list of bright stars (magnitude range?). Here, we need the data reduction pipeline to reduce the data.
- Step 3: Measure the FWHM around bright stars after Flat-fielding and bias-subtraction
- Step 4: Get the DIMM FWHM from http://www.ctio.noao.edu/environ/environ.html and calculate the FWHM due to the DECam. The maximum of the sqrt(WFHM^2 - DIMM^2) should be less than 0.55". This is a check of TO.2.
- Step 5: Get the median of the FWHM and check whether it is less than 0.9" in r, i and z band.

## Code availability:¶

In terms of the Gaussian Fit, the codes are available to measure the FWHM.

## Caveats:¶

Here, the FWHM can be measured using different methods. We are going to measure all of them.