Prospects for the Dark Energy Survey
Including an Overview of the Dark Energy
Camera’s Central Nervous System
Kyler Kuehn
Center for Cosmology and AstroParticle Physics,
The Ohio State University
CCAPP Symposium
Columbus, OH
14 October, 2009
Life, the Universe, and Everything
• Dark energy can be described in terms of an equation of state: w(z) = p/;
we parameterize w(z) as follows: w(z) =w0+wa(1–a), where a=(1+z)-1
Dark Energy Survey Science Goals:
constraints on w0 and wa
w(z) =w0+wa(1–a)
• DES will measure the expansion history of the Universe through the dependence
on redshift of the luminosity distance, angular diameter distance, and volume
element and the growth rate of structure.
• Three observing techniques will measure the (3-D) spatial distribution of 300M
galaxies in four bands to ~24th magnitude. The fourth, a search for SNe Ia, will
3
yield independent constraints on Dark Energy, along with an independent measure
of systematic errors.
Additional DES Science
Beyond Supernovae, Galaxy Cluster/BAO studies, Large Scale
Structure, and Weak Lensing, DES will contribute to progress in
(among other topics)…
• Photometric redshift estimation methods
• N-body LSS Simulations
• Strong Lensing
• Non-Type Ia SNe
• Other transients (e.g. GRBs), through detection and follow-up, as
well as pre-discovery images; transient host properties; multiwavelength and multi-messenger campaigns
• Galaxy Properties and Evolution: QSOs, luminosity functions, stellar
populations, DM Halo correlations, interacting galaxies
• Milky Way/Local Group “Archaeology”: Galactic (sub-)structure,
stellar metallicities (e.g. 47 Tuc), variable stars, proper motions
Focal Plane and Survey Characteristics
Focal plane with 62 2k x 4k Image CCDs:
~520 Mpixels  ~1GB images (+ header)
8 2k x 2k Focus/ Alignment CCDs
4 2k x 2k Guide CCDs
DES incorporates two multiband surveys:
- Main Survey: “Tiled” pointings of 3 deg2
each, covering an area of 5000 deg2
twice per year in each of four filters
- SNe Ia Survey: 9-15 deg2 within Main
Survey field
- Filters: g, r, i, z, Y, from DES augmented
with J, H & K from VISTA Survey
- Surveys will take place during 2011-2016
(525 nights). Additional nights allow
significant opportunities for Community Use
of DECam.
DES Instrument: DECam
CCD
Readout
Mechanical Interface of
DECam to the Blanco
Filters
Shutter
Hexapod
Optical
Lenses
6
Dark Energy Camera CCDs
LBNL CCDs:
• 31.5 mm x 63 mm
• 250 μm thick, fully depleted
• 2 Readouts per chip
• 2k x 4k pixels @ 15 μm (0.28”)/pix
• Extremely red sensitive:
QE > 50% at 1000 nm
Fabrication and testing
at Fermilab have yielded
nearly a full focal plane
of science-grade
SISPI Overview
Officially title: “Survey Image
System-Process Integration”
(SISPI)
Software Infrastructure also
known as
“Mountaintop Software”
or the
“DECam Central Nervous
System”
Testing non-DES +
simulated images,
along with related
telemetry data.
Data
Transport
System
Infrastructure/Applications
Schematic
Feedback to Observer
• Real Time Display
– Full (compressed) focal plane image of every exposure
• Instrument Heath
– SISPI process that accumulates information from ICS,
FCS, TCS and prepares time-history plots, alarms…
• Image Health
– Within Image Builder; Very fast “SExtractor-Lite” check of CCDs:
Sky BG (“Is this thing on?”), Bright Star PSF & Shape Properties
• Quick Look
– Interactive observer workstation
– Display a copy (using DS9 or similar package) of every image
• Quick Reduce
– More sophisticated image processing on complete images
– Sampling datastream every N (~5) images
Real-Time Image Display
Original Image
Compressed Image
1GB  4MB (~1/256 compression) in < 1 sec.
Instrument Health
Instrument Health displays current parameter value and
time history plot. Any parameters generated by site,
telescope, or camera telemetry can be displayed.
Image Health
imagehealth.c (incorporating cfitsio library):
• Sets chip stepsize, defines object by threshold above image mean and
number of pixels to search around bright pixel
• Opens image (currently from disk), checks image header for keyword
values (e.g. saturation, axis size), defines number of chips in MEF file
• Reads pixel values to data structure
• Finds mean of (non-saturated) pixel values
• Gets bright pixels (pixel value ≈ saturation/2)  defined as object
• Finds all pixels in object—if saturated, pick new object
• Calculates image/object statistics: Flux, FWHM, Object BG, Sky BG,
Major [Minor] Axis, Ellipticity, Orientation Angle
• Iterate over all chips
• Output provided for Image-by-Image and Time History Displays
IH Execution with MEF FITS Images
2500
Sample Pixel Positions of IH Objects
• User-defined parameters include:
– “Full Counting” or “Pixel Skipping”
mode (latter mode is faster, with
little loss of accuracy)
– Object Searchbin Size
– Object Intensity Threshold
– Pixel Saturation Value (option to
read value from FITS Header)
– Object Size Requirements (for
cosmic ray rejection)
– Limit to Number of Potential Objects
2000
1500
1000
• With DES images consisting of
124 amplifiers (2 per chip), ~100
objects found by IH, with some
filter/threshold dependency
• Object search starts at position
0,0, but no start bias evident in
output object positions.
500
0
0
500
1000
1500
2000
2500
3000
3500
4000
450
ImageHealth Output Testing
• Compare ImageHealth to SExtractor ( = “truth”)
Sample results for Mean(IH–SE), full counting (pixel skipping = 8):
– ΔPosition: 1.81 (1.72) pix
– ΔBG: 0.35 (0.24)%
– ΔFlux: 0.53 (0.68)%
– ΔFWHM: 0.22 (0.24) pix
– ΔEllipticity: 0.03 (0.04)
– ΔAngle: 8.6° (7.4°), or 5.0° (3.7°) excluding 5 largest outliers
– Low Quality Images (large FWHM) in Blanco data identified routinely
• ImageHealth is not instrument-specific!
– DES grizy images: initial tests complete, more detailed tests underway
– Y4KCam science, engineering images (courtesy of R. Pogge)
– Blanco (Coma Cluster) images
• “SExtractor-Lite”: streamlined processing with more focused output
– ~600 lines of code (cf. 1700 for SExtractor)
– Distinct file I/O and object analysis (not the case for SExtractor)
– Executes in 30 seconds in full counting mode or as little as 6 seconds in
pixel skipping mode (cf. 70 seconds for SExtractor)
DECam Infrastructure:
Under Construction
•
•
•
DES expects first light in
2011/2012. Its five year mission:
to explore 5000 deg2 of the
Southern Sky, to seek out faint
galaxies and new supernovae, to
provide constraints on the Dark
Energy Equation of State that no
one has provided before.
SISPI (the “Central Nervous
System” of the Dark Energy
Camera), is in the “prototype”
stage, with working components
being tested at Fermilab and on
the sky at CTIO.
ImageHealth toolkit will be
extremely useful for DES
collaboration, as well as any other
astronomers needing real-time
analysis of large (GB) images.
Committee on the Internationl Freedom of Scientists
http://www.aps.org/about/governance/committees/cifs/index.cfm
•
“To achieve its full potential and to benefit all mankind, science requires that governments
respect basic human rights, allow open communication, and avoid interference with the
rights of scientists as they carry out their professional
work.”
-- from the
APS Statements on Human Rights
•
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scientists throughout the world. It shall apprise the [Society] of problems encountered by
scientists in the pursuit of their scientific interests or in effecting satisfactory communication
with other scientists and may recommend appropriate courses of action designed to
alleviate such
problems.”
-- from the APS CIFS Statement of Purpose
•
Cases currently being reviewed from China, Cuba, Gaza Strip/Israel, Iran, Iraq, Mexico,
Saudi Arabia, Turkey, Russia, and the United States, involving allegations of everything
from scientists being unfairly tried and incarcerated to students prevented from traveling
internationally to participate in educational opportunities.
•
CIFS also participates in the awarding of the APS Andrei Sakharov Prize,
“to recognize outstanding leadership and/or achievements of scientists in
upholding human rights.”