Dark Energy Camera Observing

Strategy

James AnnisExperimental Astrophysics Group

Fermilab

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

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

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

DEC and SDSS Footprints

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

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

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Weather October

5.5 hrs/night photometric 1.7 hrs/night cirrus 0.66 fraction time at

airmass < 1.5 0.85” median site seeing

November 6.0 hrs/night photometric 1.2 hrs/night cirrus 1.00 fraction time at

airmass < 1.5 0.65” median site seeing

December 6.6 hrs/night photometric 0.7 hrs/night cirrus 1.00 fraction time at

airmass < 1.5 0.65” median site seeing

January 6.7 hrs/night photometric 1.0 hrs/night cirrus 0.66 fraction time at

airmass < 1.5 0.60” median site seeing

Totals 630 hours/year of photometric time

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Work in Gray/Bright Time?

Doug Tucker SDSS PT

z band No effect

i band< 0.2 magfor > 60

degree separation

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

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/fieldExposure time split over ~5 separate images

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Circles TessellateThe 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 doesn’t count towards the five.

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Equilateral Triangle

Square

Hexagon

3 Regular Polygons TileTo tile is to tessellate is without

overlaps Only 3 regular polygons can

tile the plane.

Others violate the rule that the angles of a vertex must add to 360.

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

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

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

The Relevant Area is the Hexagon

E2V devices

LBL devices

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Sky Map

2500 hexagons 50x50 Image to the right

is 13x13

Use the hexs to count the 5 exposures

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

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( flux) Given a calibrated spectrum and detailed response

curves, one can accurately predict measured magnitudes

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Large Survey Photometry II

Equations are of form:i = -2.5 log(Counts) – zp – ki(t) X

Where zp is zeropointX 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 changesfrom 1.2 to 1.4, then

change in g is 0.02%

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Large Survey Strategy I

2 hour stripes Roughly same airmass Same filter ~ 40 hexagons

Maximizes photometric uniformity of 40 hex stripe

Then go observe a standard

Tile the plane Then, tile the plane with

hex offset half hex over and up

This gives 30% overlap with three hexagons

Repeat

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Large Survey Strategy II

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.

Dither pattern is thus a large scale Y shape

¼ -> ½ hex scale

But dither is tied intimately to photometric calibration scheme.

Akin to the ground based CMB mapping strategies

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Summary

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

Use 1 filter/night, taking long connected swaths

Aim to make photometrically rigid map by maximially interconnecting observations.

Then zeropoint the rigid map

Footprint

Tiling

Calibration / Dithering

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis

Dec 5+6 2003 Fermilab Dark Energy Camera Workshop Jim Annis