Transcript "Lessons from WFC3/IR about Crowded Bulge Photometry"

WFC3/IR LESSONS FOR WFIRST
Jay Anderson
STScI
WFC3/IR Lessons for WFIRST
1) Absolute astrometry
2) PSF modeling and variation
(space/time)
3) Bulge-type fields
WFC3/IR Lessons for WFIRST
1) Absolute astrometry
2) PSF modeling and variation
(space/time)
3) Bulge-type fields
Absolute Astrometry:
What Can WFC3/IR Do?
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hst2galign software written for FrontierFields
Excellent data set of UDF in F160W
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50 well-dithered obsns at each of 2009, 2010, 2012
Same orientation
Procedure
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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
(u,v)
• Distortion: xraw  xcorr
• General linear transformation:
– Local frame (x,y) to
– Master frame (u,v)
– Really only 6 parameters
(x,y)
( ) ( )( )
u-uo
v-v0
=
A B
C D
x-xo
y-y0
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
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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…
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Open green circle is fixed in reference frame
A binary orbit?
Can see motion with your eyes!
WFC3/IR Lessons for WFIRST
1) Absolute astrometry
2) PSF modeling and variation
(space/time)
3) Bulge-type fields
WFC3/IR
F110W PSF
(harsh
stretch)
WFC3/IR
F110W PSF
(medium
stretch)
WFC3/IR
F110W PSF
(linear
stretch)
40% of
light in
central
pixel
WFC3/IR
F110W PSF
(linear
spatial
variation)
WFC3/IR
F110W PSF
(medium
stretch)
WFC3/IR
F110W PSF
(medium
stretch;
spatial grad)
WFC3/IR
F110W PSF
(x profile)
TIME DEPENDENCE
OF F110W PSF
(0.5% here, but
~10% for
ACS/UVIS)
WFC3/IR Lessons for WFIRST
1) Absolute astrometry
2) PSF modeling and variation
(space/time)
3) Bulge-type fields
Tom Brown’s Treasury Program
SAHU’S BULGE MICROLENSING PROGRAM
TO FIND BHs, NSs IN DISK
Sweeps
Astrometry
Photometry
Schematic of event
Duration of event  mass  
Astrometric offset  mass
Fast BH, NS, WD or slow BD?
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 2003/4
F814W 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 Field
300,000 / (202”)2
7 stars per square arcsec
ACS typ sep = 10 pixels
WFC3/IR typ sep = 3.5 pixels
Faint stars will be
brighter in IR…
WDs
(Calamida et al 2014)
0.15 Msun
Calamida et al (2015), soon to be submitted
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)