Progress Report on PSFs and Pixels

Jay Anderson, Elena Sabbi, Kailash Sahu, and Matthew Bourque

TIPS Feb 19, 2015

Lots of Inter-related Issues• Why do we need a good PSF? (Or do we?)• How do we get a good PSF? • How do we use a good PSF?

• Issues involve:– Distortion solutions– Image alignment and reference frames– High-precision transformations– PSF modeling (spatial and time variability)– Forward modeling into data domain– Deconvolution– Background matching– Pixel-area corrections

ChickenandEgg

Problems

Why Do We Need a Good PSF?• Not everyone does…• High-precision science– PSF-fitting of bright stars

• Photometry to 1%• Astrometry to 0.01 pixel

– PSF-fitting of faint stars• Simultaneous photometry to optimize S/N

– Fitting barely-resolved galaxies• Weak-lensing analysis, profiles, point-source removal,

deconvolution

How Do We Get a Good PSF?• Deal with undersampled nature of detectors

– WFC3/IR: up to 40% of light in central pixel – ACS/WFC: up to 23% of light in central pixel– WFC3/UVIS: up to 18% of light in central pixel

• Deal with spatial variations– Every ~500 pixels the PSF changes appreciably– Geometric optics, Charge diffusion variation– Tiny Tim models not very good

• Deal with time variations– Focus changes… model?– Often not enough stars in an image to extract– “Library” PSFs actually quite useful

• No standard format for PSF models, no obvious tools– “effective” PSF is one approach– Purely empirical -vs- model-based– Will suggest a “standard” format

How to Use a Good PSF?• Push through astrodrizzle– Must plant stars in multiple FLTs consistently– “Fuzzy” constraints

• uneven sampling, correlated noise

• Fit to the individual FLT/FLC pixels– The only “hard”, independent constraints

• Can properly treat the errors

– Easier for bright isolated point-sources• “one-pass”

– Harder for faint or resolved objects• Must simultaneously interrelate the FLT pixels with scene• Must properly convolve model of scene to match pixels

The Impasse• Many tasks at once

– Provide PSFs– Provide access to pixel mappings– Provide tools to use

• Institute is in a unique position– Users can’t be expected to solve all these probs

• Requires astronomer-decades…

– Legacy value for HST (higher-level data products)– Practice for JWST (on hook)

• This talk– Progress report– Stimulate ideas, pique interest

This Talk• Intro• Mapping– Frontier Fields: hst2galign routine– Enables inter-image mapping

• PSFs– PSF study of F606W WFC3/UVIS archive– Suggested “standard” PSF format

• Tools– “bundles”: convenient structure for modeling

Frontier Fieldshst2galign

Frontier Fieldshst2galign

“CENTROID” POSITIONS

“TEMPLATE-FIT” POSITIONS

Other Filters/Detectors

hst2galignsoftware takes *_flc.fits list and flagsit outputs “mat” files of point-association

lists allows mapping of FLT to REF frame

uses distortion reference images

This Talk• Intro• Mapping– Frontier Fields: hst2galign routine– Enables inter-image mapping

• PSFs– PSF study of F606W WFC3/UVIS archive– Suggested “standard” PSF format

• Tools– “bundles”: convenient structure for modeling

PSFs: Spatial variation F606W WFC3/UVIS

• Spatial variations

Star Images from Entire UVIS Archive5,013,607 star images extracted

raster of 21×21 pixels extracted for eachrecord (i,j) location and sky and fitted position/flux

Star Images from Entire UVIS Archive5,013,607 star images extracted

raster of 21×21 pixels extracted for eachrecorded (i,j) location and sky and fitted position/flux

S/N Distribution

PSF Moments

“A” excess

“B”

exce

ss

PSF Moments

“A” excess

“B”

exce

ss

“A” corner

How the upper-left PSF changes with

Focus

How the upper-left PSF changes with

Focus

Solving for the Focus in an exposure

To do: * extend analysis to entire chip* determine how many stars are needed* correlate with focus model

Encouraging!

Suggested “Standard”PSF format

• A 3-D fits image cube– NX×NY = NTOT PSFs

• Specify i,j fiducial locations

– Each PSF is 101×101 pixels• Super-sampled ×4• Covers 25×25 pixels (out to r=12.5)• Normalized to have flux of 1.0

within 10 pix

– Already available for many filters/detectors

• Tools:– get_psf[iCEN,jCEN,psfloc()]

• Returns local PSF at detector location

– rpsf_phot[Δx, Δy,psfloc()]• Returns fraction of light in particular pixel

NPSFs

%

101101

Example ofthe fitsheader fora STD PSF

This Talk• Intro• Mapping– Frontier Fields: hst2galign routine– Enables inter-image mapping

• PSFs– PSF study of F606W WFC3/UVIS archive– Suggested “standard” PSF format

• Tools– “bundles”: convenient structure for modeling

A High-level Product: Pixel Bundles• Putting it all together– Uses the alignment from hst2galign– Identify an object of interest and size (ie, 21×21 pix)

• Bundle-extraction routine: hst2bundle

bundle_21x21_00630_00979.fits

The Bundle Itself• A set of NRAST × NRAST × NIMs fits images– in 8 extensions

1-3 FROM FLT4-5 DIRECT MAPPING EACH PIXEL TO REF-FRAME6-7 REVERSE MAPPING FROM REF-FRAME TO LOCAL PIXEL 8 PSF (actually 101 × 101 × NIMs)

The Pixels in theBundle

A Bundle is Like an “object”

• Can imagine python “methods” to analyze• Bundle2process example routine– DUMP (output P,E,DQ,I,J,U,V,PSF…)– PIXCOMP (to rem artifacts, match sky)– XYZFIT_IND – XYZFIT_SIM– STACKING

• Point-source fitting• Deconv fwd-model

• Documentation…

This Talk• Intro• Mapping– Frontier Fields: hst2galign routine– Enables inter-image mapping

• PSFs– PSF study of F606W WFC3/UVIS archive– Suggested “standard” PSF format

• Tools– “bundles”: convenient structure for modeling

• Discussion…