Dark Energy Camera Observing Strategy

1. Dark Energy Camera Observing Strategy James Annis Experimental Astrophysics Group Fermilab

2. 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 Dark Energy Camera Workshop Jim Annis

3. DEC and SDSS Footprints Dark Energy Camera Workshop Jim Annis

4. 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 Dark Energy Camera Workshop Jim Annis

5. 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 Weather Totals 630 hours/year of photometric time Dark Energy Camera Workshop Jim Annis

6. Work in Gray/Bright Time? • Doug Tucker SDSS PT • z band No effect • i band < 0.2 mag for > 60 degree separation Dark Energy Camera Workshop Jim Annis

7. 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 Dark Energy Camera Workshop Jim Annis

8. 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 doesn’t count towards the five. Dark Energy Camera Workshop Jim Annis

9. 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. Dark Energy Camera Workshop Jim Annis

10. 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 Dark Energy Camera Workshop Jim Annis

11. The Relevant Area is the Hexagon LBL devices E2V devices Dark Energy Camera Workshop Jim Annis

12. Sky Map • 2500 hexagons • 50x50 • Image to the right is 13x13 Use the hexs to count the 5 exposures Dark Energy Camera Workshop Jim Annis

13. 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 Large Survey Photometry I • 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 Dark Energy Camera Workshop Jim Annis

14. 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% Large Survey Photometry II • 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% Dark Energy Camera Workshop Jim Annis

15. 2 hour stripes Roughly same airmass Same filter ~ 40 hexagons Maximizes photometric uniformity of 40 hex stripe Then go observe a standard 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 Dark Energy Camera Workshop Jim Annis

16. 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. Large Survey Strategy II • 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 Dark Energy Camera Workshop Jim Annis

17. Summary Calibration / Dithering Footprint • 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 Tiling Dark Energy Camera Workshop Jim Annis

18. Dark Energy Camera Workshop Jim Annis

19. Dark Energy Camera Workshop Jim Annis

20. Dark Energy Camera Workshop Jim Annis

21. Dark Energy Camera Workshop Jim Annis

22. Dark Energy Camera Workshop Jim Annis