# Presentation: 313797

## Dark Energy Camera                Observing Strategy

**Dark Energy Camera                Observing Strategy**

**James Annis**
**Experimental Astrophysics Group**
**Fermilab**

## 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 and SDSS Footprints

## 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

## 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

## Work in Gray/Bright Time?

- Doug Tucker  SDSS PT
- z band
- No effect
- i band
- < 0.2 mag
- for > 60 degree separation

## 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

## 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.

## 3 Regular Polygons Tile

- Square
- Hexagon

- To 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.

## 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

## The Relevant Area is the Hexagon

- E2V devices

- LBL devices

## Sky Map

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

- Use the hexs to count the 5 exposures

## 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

## 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%

## 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

## 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
- 1⁄4 -> 1⁄2  hex scale
- But dither is tied intimately to photometric calibration scheme.

- Akin to the ground based CMB mapping strategies

## 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

## The South Galactic Cap

- Midnight, Nov 16

- SGP

## Area on Sphere

- Area (sq-degrees) =
-       (sin dec1 – sin dec2) *  RA * 41,000
-        ------------------------
-             2*360