wfc3/ir lessons for wfirst jay anderson stsci. wfc3/ir lessons for wfirst 1)absolute astrometry...
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WFC3/IR LESSONS FOR WFIRST
Jay AndersonSTScI
WFC3/IR Lessons for WFIRST
1) Absolute astrometry2) PSF modeling and variation
(space/time)3) Bulge-type fields
WFC3/IR Lessons for WFIRST
1) Absolute astrometry2) PSF modeling and variation
(space/time)3) Bulge-type fields
Absolute Astrometry:What Can WFC3/IR Do?
• hst2galign software written for FrontierFields• Excellent data set of UDF in F160W
– 50 well-dithered obsns at each of 2009, 2010, 2012– Same orientation
• Procedure– Found a few bright objects to align all images to first image– Made a x4 supersampled stack (iterative)– Extract high-res template for each galaxy: a “GSF”
• Simply gives distribution of light (convolved with PSF)• Pick an arbitrary point as the “handle”• Find that handle in each exposure• Fit to 5×5 pixels in each exposure• Properly account for distortion
– Result: set of consistently measured positions for each object in each exposure
– Next: assess quality of transformations
14×11
Transformations• Distortion: xraw xcorr
• General linear transformation:– Local frame (x,y) to – Master frame (u,v)– Really only 6 parameters
( A BC D( ))u-uo
v-v0
= ( )x-xo
y-y0
(x,y)
(u,v)
The High Achievers• The “best” objects are stars• Defined by consistency
residual– xobs − xtrans(Umast,Vmast)
– yobs − ytrans(Umast,Vmast)
> 100 objects!
Transformation precision ~ 0.05/√100
Proper Motions
• Each point is an observation from one exposure
• Scatter within epoch Measurement error:
• Inter-epoch trend® Proper motion!
• Galaxies used to define transformations
2009 2010 2012
Two neighboring stars…
• Open green circle is fixed in reference frame• A binary orbit?• Can see motion with your eyes!
WFC3/IR Lessons for WFIRST
1) Absolute astrometry2) PSF modeling and variation
(space/time)3) Bulge-type fields
WFC3/IRF110W PSF(harshstretch)
WFC3/IRF110W PSF(mediumstretch)
WFC3/IRF110W PSF(linearstretch)
40% oflight incentral
pixel
WFC3/IRF110W PSF(linear spatialvariation)
WFC3/IRF110W PSF(mediumstretch)
WFC3/IRF110W PSF(mediumstretch;spatial grad)
WFC3/IRF110W PSF(x profile)
TIME DEPENDENCE OF F110W PSF(0.5% here, but
~10% for ACS/UVIS)
WFC3/IR Lessons for WFIRST
1) Absolute astrometry2) PSF modeling and variation
(space/time)3) Bulge-type fields
Tom Brown’s Treasury Program
Sweeps
SAHU’S BULGE MICROLENSING PROGRAMTO FIND BHs, NSs IN DISK
Fast BH, NS, WD or slow BD?
Schematic of event
Duration of event mass Astrometric offset mass
Astrometry
Photometry
OBSERVING STRATEGY• NUMBER OF TARGETS– Each ACS field has ~300,000 stars
• 50% have S/N > 100– Each WFC3/UVIS field has 200,000 stars– Total of > 2,500,000 stars
• OBSERVING CADENCE– Optimized for long-duration events– One visit every 2 weeks over two 4-month windows
• 64 visits per year
• EXPECTATIONS: (54 / 120 events “astrometric”)– 18 events due to BHs– 14 due to NSs– 22 due to MS stars
SWEEPS 2003/4F814W STACK
SWEEPS 2012/4 F814W STACK
SWEEPS 2012/4 STACK+REG
SWEEPS 2012/4 F814W STACK
SWEEPS 2012/4 F814W SUB
SWEEPS 2012/4 F814W STACK
BROWN 2010 F110W STACK
BROWN 2010 F110W INDIV
SWEEPS 2012/4 STACK+REG
SWEEPS 2012/4 STACK+REG
300,000 stars in one ACS Field300,000 / (202”)2
7 stars per square arcsecACS typ sep = 10 pixels
WFC3/IR typ sep = 3.5 pixels
Faint stars will bebrighter in IR…
WDs(Calamida et al 2014)
Calamida et al (2015), soon to be submitted
0.15 Msun
Calamida et al (2015), soon to be submitted
Challenges• For Sahu’s ACS/UVIS program
– use general software routine for finding and phot-ing• Finding done on combined
products• Measuring done on individual
images, simultaneously– ok, since most stars separated:
“semi-crowded”• neighbors are perturbations• PSFs can be extracted easily
– PMs not major issue, can be measured after the fact
– easy to triage complicated cases
• For WFIRST– will need specialized software
• ground does crowded-field well, but not with undersampling
• undersampling means we must operate on original images
– need to find all stars• even neighbors within 1 pixel• PSF creation will be complicated
– crowding, color, spatial variations, – must do with neighbors in mind
• 10x more stars affect 5x5 fitting aperture (>1 per)
– PMs need to be folded in– harder to triage complicated cases (too many)