The Des Focal Plane

Our survey goals require that we have high Quantum Efficiency (QE) at the near infrared wavelength of ~1000 nm. Standard astronomical CCDs typically have a QE at this wavelength of 5-10% because the charge collection region is 10-20 ^m thick and the total device thickness is often less than 50 ^m. The absorption length in silicon is 205 ^m at a wavelength of 1000 nm, and thus thick sensors are required for a better QE at that wavelength. Photons at near infrared wavelengths will simply pass through 50 ^m of silicon. LBNL has developed thick, fully depleted, back illuminated CCDs 200-300 ^m thick [4,5]. For DES, we will use 250 ^m LBNL CCDs. The QE of LBNL CCDs is shown in Fig. 2 compared with a standard thinned astronomical CCD and a deep-depleted CCD. Table I includes a list of the requirements for the DES CCDs compared with the performance of the thick LBNL devices. Figure 3 shows our reference design for the focal plane layout with the 2Kx4K 4-side buttable LBNL CCDs. It contains 62 CCDs and a total tile active area of 3.0 deg2.

Sprinkler Room Design

Figure 3. A simulated image of the 62 CCDs in the DECam focal plane. Each CCD is a 2K*4K device and the total focal plane has a diameter of 50 cm.

Table I. CCD requirements for DECam compared with the performance for LBNL devices.

LBNL CCD

DECam requirements

Pixel array

2048 x 4096 pixels

2048 x 4096 pixels

Pixel size

15 |am x 15 |am

15 |am x 15 |am (nominal)

<QE (400-700 nm)>

~70%

>60%

<QE (700-900 nm)>

~90%

>80%

<QE (900-1000 nm)>

~60%

>50% at 1000 nm

Full well capacity

170,000 e-

>130,000 e-

Dark current

2 e-/hr/pixel at -150oC

<~25 e-/hr/pixel

Persistence

Erase mechanism

Erase mechanism

Read noise

7 e- at 250 kpixel/s

<10 e-

ChargeTransfer Inefficiency

<10-6

<10-5

Charge diffusion

8 |am

<10 |am

Linearity

Better than 1%

1%

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