Dark Energy Camera Observing Strategy

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Dark Energy Camera Observing
Strategy

• James Annis
• Experimental Astrophysics Group
• Fermilab

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Footprint

• Extinction Map
• Centered SGP
• Also Alt-Az at midnight, Halloween, CTIO
• Red circles
• Gal latitude/ Elevation
• Offset red circles
• Equatorial coords
• SPT visibility region green
• Strawman survey area blue
• 5000 sq-degrees total
• 3100 sq-deg South
• 1600 sq-deg overhead
• 200 sq-deg SDSS stripe 82

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DEC and SDSS Footprints
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Visbility

• RA 22 hrs to 6 hrs
• 22 hrs overhead at midnight Sept 15
• 6 hrs overhead at midnight Jan 15
• Optimal
• October, November, December, January

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Weather

• December
• 6.6 hrs/night photometric
• 0.7 hrs/night cirrus
• 1.00 fraction time at airmass lt 1.5
• 0.65 median site seeing
• January
• 6.7 hrs/night photometric
• 1.0 hrs/night cirrus
• 0.66 fraction time at airmass lt 1.5
• 0.60 median site seeing

• October
• 5.5 hrs/night photometric
• 1.7 hrs/night cirrus
• 0.66 fraction time at airmass lt 1.5
• 0.85 median site seeing
• November
• 6.0 hrs/night photometric
• 1.2 hrs/night cirrus
• 1.00 fraction time at airmass lt 1.5
• 0.65 median site seeing

Totals 630 hours/year of photometric time
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Work in Gray/Bright Time?

• Doug Tucker SDSS PT
• z band
• No effect
• i band
• lt 0.2 mag
• for gt 60 degree separation

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Size of Camera Field

• Footprint 5000 sq-degrees
• Available time 3000 hours
• Exposure time 1.6 hrs/field
• 5000 sq-degrees in 3000 hours
• 1.67 sq-degree/hour
• Field of View
• 1.67 sq-deg 1.6 hrs/field

2.6 sq deg/field
Exposure time split over 5 separate images
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Circles Tessellate

• The geometric meaning of the word tessellate is
  "to cover the plane with a pattern in such a way
  as to leave no region uncovered."

The overlap doesnt count towards the five.
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3 Regular Polygons Tile

• To tile is to tessellate is without overlaps

• Equilateral Triangle
• Square
• Hexagon

• Only 3 regular polygons can tile the plane.
• Others violate the rule that the angles of a
  vertex must add to 360.

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Hexagons Tile Optimally

• Theorem (Honeycomb conjecture) Any partition of
  the plane into regions of equal area has
  perimeter at least that of the regular hexagonal
  honeycomb tiling. (Hales
  1999)

Hexagons maximize the unique area
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The Relevant Area is the Hexagon
LBL devices
E2V devices
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Sky Map

• 2500 hexagons
• 50x50
• Image to the right is 13x13

Use the hexs to count the 5 exposures
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Large Survey Photometry I

• Unique Properties
• Single stable instrument
• Huge homogeneous photometric data set
• System defined by 108 magnitudes of the survey

• Survey Systems
• System defined by detailed response of survey
  camera
• Zeropoints set by observations of standard stars

• Goals
• Change brightness of object and move it 10s of
  degrees
• Effect is only- that of -2.5 log(D flux)
• Given a calibrated spectrum and detailed response
  curves, one can accurately predict measured
  magnitudes

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Large Survey Photometry II

• Equations are of form
• i -2.5 log(Counts) zp ki(t) X
• Where
• zp is zeropoint
• X is airmass
• ki(t) is the extinction coefficent
• No cross terms
• Neglect of 2nd order color/airmass term limits
  precision to 1

• For a given night,
• 1 filter
• Maximizes numbers of standards/filter/night
• Keep airmass the same.
• kg 0.15
• kr 0.11
• ki 0.10
• kz 0.05
• If airmass changes
• from 1.2 to 1.4, then change in g is 0.02

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Large Survey Strategy I

• Tile the plane
• Then, tile the plane with hex offset half hex
  over and up
• This gives 30 overlap with three hexagons
• Repeat

• 2 hour stripes
• Roughly same airmass
• Same filter
• 40 hexagons
• Maximizes photometric uniformity of 40 hex stripe
• Then go observe a standard

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Large Survey Strategy II

• Dither pattern is thus a large scale Y shape
• ¼ -gt ½ hex scale
• But dither is tied intimately to photometric
  calibration scheme.

• Tie the hex stripes together using these offset
  tilings.
• Maximize rigidity by using one hex tiling taking
  stripes vertical, across all hex stripes. This
  breaks the constant airmass, but eliminates large
  scale non-uniformities.
• Then zeropoint the rigid map.

Akin to the ground based CMB mapping strategies
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Summary
Calibration / Dithering
Footprint

• Use 1 filter/night, taking long connected swaths
• Aim to make photometrically rigid map by
  maximially interconnecting observations.
• Then zeropoint the rigid map

• Southern Galactic Cap
• Use Oct, Nov, Dec, Jan
• Observe i,z during moony times
• Hexagons form the best map to count unique
  observations
• Need 2.1o diameter corrector for a 2.6 degree2
  hexagon camera

Tiling
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The South Galactic Cap
SGP
Midnight, Nov 16
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Area on Sphere

• Area (sq-degrees)
• (sin dec1 sin dec2) D RA 41,000
• ------------------------
• 2360

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