# Presentation: 492511

## Dark Energy Survey:Optical Design and Issues

**Dark Energy Survey:****Optical Design and Issues**

**2.2 Deg. Field of View Corrector**
**Requirements**
**Performance**
**Issues**

- Steve Kent, Fermilab, for the DES Collaboration
- Dark Energy Survey
- BIRP, Aug 12,  2004

## 2.2 Deg. Field of View Corrector

**14 requirements total**
**0.39 to 1.1 ****μ (SDSS filter bandpasses)**
**Scale 17.7 arcsec/mm**
**Field size 450 mm diameter (2.2 degrees)**
**D80 < 0.64 arcsec everywhere (FWHM < .4 arcsec)**
**No ADC (Atmospheric Dispersion Corrector)**
**Minimize ghosting**
**Space for filter, shutter**
**Design choices should minimize procurement, fabrication schedule.**

## Gladders may11 design

**Features:**
**Flat focal plane**
**Five lenses + Filter**
**(including dewar window)**
**All fused silica**
**One aspheric surface**
**Largest diameter 1.1 meters**
**Flexibility spacing elements**
**Low distortion (<1%)**
**Good ghosting properties**
**star halos**
**exit pupil image**

**Filter**

**Shutter**

**C1**

**C2**

**C3**

**C4**

**C5**

**Filter**

## Obligatory Spot Diagrams

**g filter**

**r filter**

**i filter**

**z filter**

**2"**

**0 mm**

**158 mm**

**225 mm**

**Field**
**Radius**

## Image Quality

**Requirement: < 0.64  (FWHM = .40")**
**Achieved:****	****	****< 0.59  (FWHM = .37")**
**Global rms:  0.34  (FWHM = .22")**

**	****	****	****D80 vs. filter, field position**
**Field**
**Radius      g          r          i       z**
**(mm)**
**  ****0       0.32"      0.11       0.17    0.31**
** ****45       0.35       0.19       0.21    0.34**
** ****90       0.39       0.14       0.22    0.33**
**135       0.40       0.21       0.25    0.33**
**180       0.50       0.32       0.36    0.42**
**226       0.59       0.37       0.41    0.47**

## Impact of no ADC

**g**

- r

- i

- z

**LMC, SMC limits**

**0.4 arcsec**
**FWHM**

**Survey limit**

## Ghosting

**Uniform Illumination:**
**Central intensity of**
**ghost is 7% of**
**direct recorded**
**light.**

## Design Issues

**Fused Silica**
**Expensive**
**Excellent transmission**
**Low CTE (factor 10x less than BK7) ==> shorter polishing, testing times**
**Largest lens requires blank to be slumped; 5 month procurement time.**
**dn/dT is 3x BK7**

## More Issues

**Lateral, longitudinal chromatic optimized only within SDSS bandpasses.**
**Coatings**
**Options are being evaluated**
**Assume MgF2 for inside dewar, on front surface of C1**
**Assume solGel for all other surfaces**

## Tolerance Analysis

**Preliminary analysis only**
**Fabrication errors in radii, thickness, refractive index are compensated by repositioning of lenses**
**Static errors**
**inhomogeneity, thickness, radii, centering & positioning, wedge & tilt**
**polishing errors**
**primary mirror alignment**
**Combined: 0.33" D80**

## Thermal Analysis

**Assume steel cell.**
**Include dn/dT of FS, CTE of FS**
**30 deg C range**
**D80 changes by .1"**

## Summary

**We have a corrector design that will work and can be fabricated by existing vendors.**
**Next steps**
**Firm up requirements**
**Pixel scale****	****	****	****	****PSF uniformity**
**Distortion****	****	****	****	****Ghosting amplitude**
**Hire optical engineer to**
**review design, identify areas where cost and/or schedule savings can be achieved.**
**Improve analysis of tolerancing and image size**