1. Astronomy 920 Commissioning of the Prime Focus Imaging Spectrograph
Meeting 4
CCD Detector; Signal/ Noise Model
Ken Nordsieck
Sept 30, 2005
Ast 920 Meeting 4

2. Grating Optics CCD Detectors Signal/ Noise Geometry Efficiency
Gain and Fullwell
Readout Noise and Speed
Signal/ Noise
Target and Sky Photon noise
Readout
Flatfield
Sept 30, 2005
Ast 920 Meeting 4

3. CCD Geometry
3 CCD's each 2048 x micron square pixels, 62 x 93mm
mosaic gap ~90 pixels
CCD focal plane scale 177 microns/ arcsec
F/2.2: Effective focal length = 11000x 2.2 = 24,200 mm FPS = ( arcsec/rad)/ 24200mm= 8.5 arcsec/mm
7.8 pixels/ arcsec
since seeing ~ arcsec, this is "oversampled", bin ~2 pixels
8 arcmin FOV
4096
2048
Dispersion
Sept 30, 2005
Ast 920 Meeting 4

4. Put UV CCD in the right place
Efficiency
The 3 CCD’s have somewhat different efficiencies
Put UV CCD in the right place
SALT 03
SALT 04
SALT 06
Sensitivity350 nm
400 nm
500 nm
650 nm
900 nm
1000 nm
(%)
(%)
(%)
Readout noise: (e-/pix)
Sept 30, 2005
Ast 920 Meeting 4

5. Binning, Gain and Fullwell
Pixel read out by shifting rows into serial shift register ("parallel shift"), then shift pixels in that register into amplifier
"Prebin" pixels in y-direction by doing several parallel shifts before reading out serial register
Prebin in x direction by doing several serial shifts before doing an amplifier sample
Maximum signal ("fullwell") set by "depth" of physical pixel (150,000 e-), serial register (3x pixel), or amplifier (4x pixel).
Each amplifier converts voltage to digital value (ADU)
ADU = e- / Gain
max ADU = 216
PFIS has two Gain setting, Bright and Faint
use Bright to collect more photons/ readout
PFIS has separate "image" and "storage" halves, which may be clocked separately, for high-speed "frame transfer"
Each PFIS CCD has two amplifiers, reading out 1024x4096 pixels
Sept 30, 2005
Ast 920 Meeting 4

6. Readout Noise and Speed
Associated with each bin readout is a readout noise. Depends on gain and readout speed: 1 2 3 4 5 6
6.34
FAST
FAINT
6.59
6.78
6.15
5.88
7.82
4.81
10.04
BRIGHT
10.20
10.68
9.82
9.18
10.81
9.57
2.51
SLOW
2.46
2.42
2.33
2.43
2.91
2.50
3.73
3.74
3.78
3.62
3.56
4.14
3.54
Use fast readout to minimize overhead time when need many readouts (bright objects, polarimetry, Fabry-Perot
Fast Readout: 3 microsec/ bin. 1x1 in 12.5 sec
Slow Readout: 10 microsec/bin. 1x1 in 43 sec
Sept 30, 2005
Ast 920 Meeting 4

7. Signal/ Noise Signal: Noise
S = Flux * Effective Area * Throughput * Exposure
units: detected photons
Throughput:
mirrors * lenses * slit thoughput * grating eff * CCD eff
Noise
N = sqrt (Star + Ssky + RO2)
Sept 30, 2005
Ast 920 Meeting 4