ACS Drizzling Overview J. Mack; DA Training 10/5/07 Distortion Dither Strategies MultiDrizzle ‘Fine-tuning’ Data Quality Photometry

Optical Telescope Assembly ACS optics designed with minimum components Result is high throughput, but focal surfaces far from normal to principal rays. Off axis position also contributes to total distortion WFC3 images will also have significant distortion Primarily due to tilt of focal plane w/r/t optical axis (required to achieve uniform focus across detector), leading to an elongation of the field of view. WFC3/ACS - 2 chips; ‘fixed’ WFPC2 - 4 chips; ‘free’ (chip-to-chip shifts)

ACS Distortion 1.Axes are not perpendicular (85 degrees, rhombus) 2.Pixel area and scale varies over detector FOV

Reasons to Dither Remove detector artifacts Improve pixel sampling Create larger mosaic

ACS Dither Patterns TypeShiftPurpose BoxOptimal half-pixel sampling Line2x2Reject of cosmic rays, hot pixels Line5x60Shift over WFC interchip gap Line75x8Shift over HRC occulting finger Line10x10Shift over SBC bad annode and repeller wire

MultiDrizzle Reprocessing Beware of using IRAF/PyRAF tasks as a ‘black box’ Always test critical input parameters Pipeline values (see MDRIZTAB) not always optimal Key reprocessing steps: Fine-tune image alignment (delta shift+rotation) Improve sky subtraction Tweak cosmic-ray rejection (after shift refinement) Find optimal scale/pixfrac values

Shift adjustments f606w_01_flt.fits f606w_03_flt.fits f606w_05_flt.fits f606w_07_flt.fits f606w_09_flt.fits f606w_11_flt.fits f606w_13_flt.fits f606w_15_flt.fits f435w_01_flt.fits f435w_03_flt.fits f435w_07_flt.fits f435w_17_flt.fits f435w_09_flt.fits Filename Pos-Targ dx dy drot (arcsec) Dither Box (small step) Dither Line (large step) Visit 1 Visit 2

ACS Data Quality Data Handbook Table 3.4: Flags for the DQ array Flag ValueDefinition 0good pixel 1Reed-Solomon decoding error 2data replaced by fill value 4bad detector pixel or beyond aperture 8masked by aperture feature 16hot pixel (dark current >0.08 e/s) 32CTE tail of hot pixel 64warm pixel (dark current 0.02 to 0.08 e/s) 128bias structure (mostly bad columns) 256saturation (full well or a-to-d) 512bad pixel in reference file 1024weak trap 2048a-to-d saturation 4096cosmic ray rejected by MultiDrizzle 8192cosmic ray rejected by ACSREJ (CR-SPLIT)

Example ACS/WFC file extensions: sci err dq image_flt.fits [1] [2] [3]Chip 2 image_flt.fits [4] [5] [6]Chip 1

The ‘Bits’ Parameter Data quality flags set during CALACS can be used as bit masks when drizzling. The user may specify which bit values should be considered 'good' and included in image combination. The bits parameter indicates which flagged pixels to keep. Example: Full well saturation DQ flag = 256. [Tests shows that counts still accumulate linearly and that photometry of saturated stars is robust when using a gain value that samples the full well depth.]

Cosmic Ray Rejection For optimal flagging of cosmic rays, sky subtraction must be performed when the sky is more than a few electrons. Statistical checks assume the sky is removed. Flagging criteria: |data_image - blotted_median_image| > scale * deriv_image + SNR * noise where scale and SNR are free parameters in MultiDrizz

Effect of Drizzling on PSF ImageFWHM (pixels) Scale (“/pix) FWHM (arcsec) single_flt.fits single_sci.fits final_drz.fits (32 images) final_drz_sc07px08.fits WFC 4-pt Dither Box Pattern

The Scale & Pixfrac Parameters The drizzle pixfrac and scale values depend on: the number of input images and the size of the dithers. Choosing the drizzle.scale The PSF FWHM in the final image should be around 2.5 pixels. Choosing the drizzle.pixfrac The pixfrac parameter should be slightly larger than the scale. This allows some of the drop to spill over to adjacent pixels. Image statistics in the center of the final drizzle weight image should have an r.m.s. less than 20% to 30% of the median

Photometry using DRZ products DAOPHOT Reference Guide (Lindsey Davis) “The sky background should NOT be subtracted from imaging prior to photometry. DAOPHOT fitting routines use an optimal weighting scheme which depends on the readnoise, gain, and true counts in each pixel. If the mean sky has been subtracted, the computed weights will be incorrect.” ‘For the same reason, users should not correct their images for exposure time.’ Photometry UNITS = counts(electrons or DN) Must also specify the gain header keyword