DES Review CCD Focal Plane and Camera Roger Smith George Ricker

CCD Manufacturing Recommendations: CCD fabrication is progressing well. Packaging is mature. Yields are acceptable (26%) and now known to appropriate fidelity. However, there is risk of clustered failures not predictable from recent experience.

CCD Manufacturing Recommendations: We recommend CD-3a support for WBS 1.2 Mitigate risk by advancing the CCD fabrication work at DALSA/LBNL so that production is not limited by funding profile. a) procurement of 15 wafers be advanced from Oct 08 to May 08, resulting in a shift of $315K; b) procurement of 10 wafers be advanced from Oct 09 to Feb 09, resulting in a shift of $210K. retires risk on the CCDs 5 months additional schedule insurance save money through efficiency and yield increases Make all guider CCDs with same thickness so that they will be interchangeable; adjust heights of focus CCDs by machining support plate.

CCD Testing Previous recommendations have been followed. The testing program is in good shape.

Focal plane assembly Recommendations Evaluate a sample of glass from the C5 melt for potential Potassium-40 contamination by placing it in front of a DECam CCD and monitoring the change in CR event rate. Similarly evaluate a sample of the fiberglass used to make the VIB for potential radioactivity. If Beryllium is used (eg alloyed with copper) then test this for radioactivity too. Plan for profilometry of whole focal plane when cold though a flat window; calibrate against reference flat.

Dewar and Vacuum System Recommendations: Incorporate a small auxiliary valve for bleeding dry air into the vacuum during initial pump down to promote outgassing and transport of water vapor. Consider how to heat the dewar exterior during initial pump down to speed up outgassing. Seriously consider using a combination of accessible zeolite getter and an activated carbon getter (which may be inaccessible), in place of the Ion Pump. If the Ion Pump I retained, measure the optical light emitted inside the vacuum housing; trace anticipated flux from multiple scattering paths or if a convincing case can be made that the multi CCD test dewar is a worst case scenario, then test this.

Cooling System ( again ! ) Comments: The recirculating Nitrogen system was chosen due to space constraints created by the f8 secondary (top end flip), exacerbated by the size of the ion pump and gate valve. The f8 is no longer stored behind DECam. All three cooling options (recirculating N2, parallel JT coolers, on board tank) appear to be technically feasible, but vary greatly in cost. Recommendation: Seriously consider reverting to a conventional Nitrogen tank in order to reduce cost.