# Presentation: 004632

**Dark Energy Survey**
**Science Requirements**
**and**
**Technical Specifications**

## Science Goals

**IMAGING SURVEY**
**5000 Square Degrees**
**South Pole Survey**
**Equatorial Stripe**
**Connector area**
**1/2  L* elliptical galaxies ****@ z = 1**
**Photometric redshifts to 20 galaxies per sq. arcmin (10 galaxies/sq. armin for weak lensing)**
**TIME DOMAIN SURVEY**
**1900 Supernovae 0.25 < z < 0.75**
**Sampling 3 to 6 nights**

## Constraints and Conditions

**Operating Parameters**
**30% observing time for 5 years**
**CTIO Site seeing (90% of time FWHM < 1.1 arcsec)**
**Technology Constraints**
**4 meter primary mirror**
**LBNL CCDs**
**15 ****μ pixels**
**7.5 μ (sigma) diffusion**

## Science Requirements - Overview

**Science program is design based on existing performance of Blanco + P.F. Corrector + Mosaic II**
**==> DES should do no worse than existing telescope.**
**Operational constraints drive the requirements as much as science requirements**
**Requirements require balance of cost, performance, schedule, and risk**

## Wavelength Coverage

**Coverage is 400nm to 1000nm in 4 bandpasses (g, r, i, z)**
**UV: Good performance is not a requirement,  but we are asked to report on image quality and throughput (*****BIRP committee report*****)**
**Current design at 0.35 ****μ**
**0.5 arcsec FWHM**
**Glass transmission is 68%**

## Limiting Magnitude

**Bottom row gives final requirement**
**Note: The actual survey will do better in *****g***** and *****r*****.**

## Photometric Calibration

**Systematic error in each filter shall be no more than 2%.**
**Main impact is on ghosting - require 0.6% rms residual error due to ghosting**

## Point Spread Function

**Requirement: Instrument + telescope shall degrade PSF by no more than 0.55 arcsec**
**This is performance of current telescope + Mosaic II + PF corrector**
**Further breakdown later in PSF budget**
**Weak Lensing: Systematic errors in calibrating PSF shall contribute no more than 10% to statistical error on a scale of 2****o****.**
**Estimated statistical error is 0.1% shear (1 sigma)**
**Equivalent "whisker" for galaxy 1.2 arcsec in diameter is 0.06 arcsec**
***No requirement on static PSF***

## Current Performance

**Site Seeing**

**Actual image quality from SuperMACHO**
**campaign**

## CTIO Seeing

## =

**=**

*****

**Elliptical PSF**

**Circular PSF**

**"Whisker"**

**Circular PSF has contributions from intrinsic PSF & seeing**
**Whisker PSF has contributions from intrinsic PSF & tracking**
**      ****errors.**
**I will use the amplitude of a "whisker" from the intrinsic PSF**
**as my metric for  weak lensing calibration, since this mimics**
**the gravitational shear signal and is easy to calculate.**
**For convolution, FWHM's add in quadrature.**
**A 0.1% error in shear for 1.2" FWHM galaxies => 0.06"**
**FHWM error in the equivalent PSF "whisker".**

## PSF Budget

**Requirement:  0.55**

## PSF details

**Requirements on optical corrector:**
**Design: 0.27'' FWHM (r, i, z) - same performance as current PF corrector**
**No requirements on static contribution to whiskers for weak lensing - assume we will calibrate with multiple frames.**
**Manufacturing, assembly: Assume good engineering practices.**
**CCD Diffusion**
**This is the biggest single item in the error budget!!!**
**Performance of current CCDs is unknown.**
**HST ACS WFC uses same devices - sigma is 5 to 10 ****μ**

## Ghosting

**Ghosting**

**Prediction from design**

**Exit Pupil Ghosting**

**Mayall 4 m Mosaic I image**

## Ghosting Requirements

**Exit Pupil: Gradient across length of 1 CCD no more than 3%.**
**Assumes 15% CCD reflection, 1% lens surface reflection**
**Induced photometric error (if uncorrected) is 0.9% for a single frame.**
**Bright star: 6th mag star produces ghost no brighter than 25th mag/sq. arcsec**
**Main impact is spacing of window from focal plane.**

## Field of View

**Field diameter of 2.2 degrees**
**Needed to complete survey in 5 yrs**
**25% contingency (last year)**
**Minimum of 4 tilings (significant part of sky covered only 3 times)**
**Minimum exposure time of 100 sec.**
**Overhead is ~30 sec per exposure**
**Note: Statement of achieved limiting magnitude assumes 4 tilings, all contingency used for observing.**

## Transmission

**Original requirements presumed fused silica and thus no requirement was provided.**
**Science plan was constructed assuming ~75% throughput (mirrors + lenses)**
**Current Blanco is ~78%.**
**Glass transmission 97%**
**Coatings (SolGel + MgF2) 88% total.**
**Note: UV transmission (glass only) is 68%**

## ADC (or lack thereof)

**No requirement for ADC**
**Detriment to weak lensing (r, i, z)**
**In good seeing, image degradation due to diff. refraction is slightly smaller than degradation due to (secz)****0.6**** effects for g band; negligible for all other bands.**
**Anecdotal evidence is that current observers do NOT use existing ADC (e.g, weak lensing survey, SuperMACHO).**

## Impact of Diff. Refr. on Image Size

## Whisker length vs. differential refraction

** ****Filter zenith   may11    **
**         ****angle  Mean   diff   **
**r       0   0.15   0.00**
**r      30   0.21   0.20**
**r      45   0.32   0.29**
**i       0   0.23   0.00**
**i      30   0.23   0.11**
**i      45   0.24   0.17**
**z       0   0.23   0.00**
**z      30   0.23   0.06**
**z      45   0.23   0.09**

- Survey operates
- to zenith = 45 deg

## Trade Studies

**Figure of merit:  Exposure time per sq. deg. of sky to reach a target S/N for sky-limited objects.  Small is better.**
**t = FWHM****2**** / (A*T)**
**     ****FWHM = Image size**
**     ****A = Field size**
**     ****T = Transmission (throughput)**
**For current design, increasing area wins over degradation of image size always.**