Optics: design, procurement, assembly and testing plans

1. Optics: design, procurement, assembly and testing plans Dr Peter Doel, University College London

2. Contents WBS 1.4 • Optical design 1.41- 1.4.2 • Optics procurement and manufacture 1.4.3 -1.4.5, 1.4.9 • Lens cell design and lens mounting 1.46 -1.47 • Lens alignment in camera 1.48

3. Optics Roles • University College London • Optics procurement and testing • Lens cell design/construction and mounting • Optical alignment • University of Michigan • Optical Design/Stray light analysis • Filter procurement • Filter mechanism design/build • Shutter design /build • Fermilab • Barrel design/build • Chicago • Active alignment sensing

4. Optical Specification • Field of view • 2.2 degree diameter • Pixel scale • 57 µm/pixel (f/2.9) • Image quality: • Resolution Element: < 2.25 pix (0.59”, 34µm) • Goal (as-built) fwhm: ~ 0.33” (18 µm) • Goal (design) fwhm: ~ 0.27” (15 µm) RMS spot size Rrms= 8.7µm (area weighted) • Wavelengths: • g, r, i, z (0.4 – 1.0 µm) • with a secondary goal of good performance down to 0.32 µm • System throughput • Greater than 60% over the focal plane • Pupil ghost: • intensity gradient < 3% over 61mm

5. Status of Optical Design • Preliminary design development (Nov 2005-Feb 2006) • Number of elements • Glass choice • Overall size and volume (diameter of first element) • Number of aspheric lens surfaces • Preliminary Design Review (Feb 2006) • Final design development (Feb-Oct 2006) • Feedback from PDR included and optimisation of design • Preliminary sensitivity analysis completed • Critical Design Review (Feb 2007) • UK provisional location • Final review before placing orders

6. Optical Design • 5 lens design • All fused silica • Two aspheric surfaces • C1 lens diameter 0.98m • Minimum thickness ratio 1:10 • Image scale 1" = 57μm • 0.26"/pixel • Sol-Gel/MgF2 coatings • Rrms= 8.7 um Design due to Rebecca Bernstein

7. Lens Parameters * Aspheric surface

8. Blank Procurement WBS 1.4.3 :Status • Preliminary blank specification (Nov 2005-Feb 2006) • Choice of blank material (fused silica, BSL7-Y,etc) • Discussion with manufacturers (Corning, Ohara, Schott, Heraeus) • Availability • Manufacturing issues (slumping, homogeneity) • Schedule • Cost • Fused Silica chosen (Corning, Heraeus) • Good performance (especially in blue) • Available in sizes and homogeneity required • Production schedule ok • Tender will be issued in December 2006

9. Corning Fused Silica HPFS Grade C is quoted as have a homogeneity of better than 2ppm. B is better that 1.5ppm and A better than 1ppm. The numeric refers to the inclusions: 1 has a total inclusion cross section per 100cm3 of glass of less than or equal to 0.1mm2 and 0 less or equal to 0.03mm2

10. Optics Polishing WBS 1.4.5 :Status • Input to preliminary optical design (Nov 2005-Feb 2006) • Discussion with manufacturers (SAGEM, SESO, Tinsley) • Manufacturing issues • Lens size • Asphere on convex or concave surface • Size/gradient of asphere • Slope 1mm departure over 50mm was acceptable • All companies happy to quote on the proposed (or similar) design • Testing issues • All optics tested in transmission to allow compensation for inhomogeneity • Schedule • Cost • Tender for polishing will be issued in February 2007 • This will state optical specification and requirements on testing results during manufacture

11. DES Filters WBS 1.4.9 :Status U. Michigan will • handle procurement and testing of the filters • match SDSS – g,r,i,z and introduce a well defined cut-off at high wavelength • design and fabricate or procure a combined filter changer and shutter

12. Filters bids as of June 2006 (Bruce Bigelow)

13. Optics Assembly and Test • Lens mounting C1-C4 (WBS1.4.6) UCL • Design of lens cell and bonding technique (FEA, testing) • Handling of lenses • Alignment procedure definition • Lens mounting C5 (WBS 1.4.7) FermiLab • Bonded straight to detector vessel • Design of lens cell and bonding technique (FEA, testing) • Alignment procedure definition • Alignment of lenses in barrel (WBS 1.4.8) UCL • Handling of lens and cells • Alignment procedure definition

14. Production Procedure • Blanks produced and ground to shape (US?) • Blanks shipped to optical polishers (France?US?) • Polished lenses shipped for coating (C1 and C5 could be coated by polishing firms) (US) • Coated lenses shipped to UCL • Lenses mounted in lens cells at UCL • Lens and cells mounted in camera barrel at UCL • Shipped to CTIO

15. Alignment Tolerances

16. Lens Mounting • Baseline athermal elastomeric (RTV rubber) bonding technique • Looking at two cell options • Invar lens cell + flexures + thin RTV layer (see figure) • Steel cell + thick RTV layer LBT lens mounting (from Diolaiti et al. SPIE 4841)

17. Lens to Cell Alignment • Lens to cell • Lens to cell alignment performed using rotary table and digital dial gauges. RTV inserted into gap D.G.I. Cell Adjustment Screws Cell Translation Stage Lens Rotary Table

18. Lens/cell to Barrel Lens-cell alignment in barrel • Mechanical alignment • Passive: using machined fiducials • Active: using rotary table and digital dial gauges • Optical Alignment (see figures) • Using rotary table and laser beam

19. Optics Cost/Time Allocated

20. Milestones