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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…