DES – Optical corrector

Optical corrector for Dark Energy Survey

optical_thales00THALES SESO manufactured one of largest lens in the world the Dark Energy Survey (DES) project with a diameter of 980mm. This project included the grinding, polishing, control and coat and AR coatings of 5 large lenses (some evenaspheres).

DES is a major ground-based Astronomy project designed to probe the origin of the accelerating universe. To do that, an extremely sensitive high resolution/high field of view camera (DECAM) is mounted at the focal plane of the existing Φ 4m Blanco Telescope (location Cerro Tololo Observatory in Chile).

The optical corrector system of the DECam is a Wynn-style five lens solution, with two aspheres design (C2 and C4). It provides a 2.2 degrees field of view image at 0.27″/pixel while contributing less than 0.3″ FWHM to the image quality. The lenses are all made out of Silica. The blanks were grinded/polished and coated by Thales SESO in France. The biggest of these lenses (i.e. the entrance lens named C1 element) is almost 1m in diameter and weights 380 pounds (172kg)!

View of the DECAM
Courtesy of UCL-UK and Fermilab-USA
Inspection of CV side of C1

Key- requirements of these lenses:

Element Type Size Optical Quality per surface
C1 CX spherical /CV spherical Φ 980mm
C2 CX spherical / CV aspherical Φ 690mm <60/250nm RMS (spherical/aspherical sides)
C3 CX spherical /CV spherical Φ 650mm
C4 CX spherical /CV spherical Φ 600mm <60/250nm RMS (spherical/aspherical sides)
C5 CX spherical /CV spherical Φ 550mm

Other optical specifications were almost the following ones:

• Radii tolerances within about +/-0.5mm
• Thickness tolerances within about +/-0.2mm
• Tilts/wedges < 0.05mm
• S/D quality better than 60/40 (relaxed up to 80/50 for the largest C1 element)
• Microroughness better than 2nm RMS