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CCD Performance with the New AAO-2 Controllers

Introduction

This page provides a brief summary of AAO CCD performance data for CCDs controlled using the new AAO-2 Optical Dectector Controllers (ODCs). It was originally compiled by Scott Croom, now at the University of Sydney.

General Notes on Performance

Here we discuss the various parameters used to quantify the performance of AAO CCDs.

SPEED: There are 6 readout speeds generally available: XTRASLOW, SLOW, NORMAL, FAST, NONASTRO and ULTRAFAST (from slowest with the best read noise performance, to fastest with the worst performance). These can be selected from the instrument control software. For some detectors only a subset of readout speeds are available. Only XTRASLOW, SLOW, NORMAL and FAST are generally suitable for scientific use. NONASTRO and ULTRAFAST should NOT be used for taking astronomical data.
READOUT TIME: The readout time (in seconds) is usually the total effective readout time derived from a sequence of 20 bias frames [i.e. (UTSTART_20 - UTSTART_1)/19] with the instrument on the telescope, and taking into account all overheads. In some cases we also list readout times derived in the lab (which are generally a little shorter, as they do not include all overheads).
GAIN: Gain (actually inverse gain) measurements are made in the lab (either in Epping or at the AAT off the telescope) using a stable light source.
READOUT NOISE: Readout noise measurements are generally performed on the telescope.  In some cases we also include values measured in the lab.
SATURATION: The point at which saturation occurs is measured during lab tests.  In most cases this is set by the analogue-to-digital conversion (ADC) which limits the range to 65536 ADU (16 bit).
NON-LINEARITY: Although CCD detectors are generally considered "linear" devices, they can often have non-linearities at levels from a fraction of a percent to a few percent (particular at high illumination levels). The non-linearity can be approximately parametrised by Alpha, where

Nm = Nt  *  [ 1 + Alpha * Nt ]
,

Nm
 are the measured (i.e. non-linear) counts in ADU above the bias level, and Nt are the `true' (i.e. expected linear) counts in ADU above the bias level.  Alpha can be estimated by taking a series of exposures at different exposure times with a constant illumination source. Then a straight line is fitted to a plot of
Nm /t    vs    t,

(where t is the exposure time at constant illumination). From the slope m and y-intercept c of this fit, you find

Alpha = m / (c*c)

The measurements of Alpha have been done in the lab as part of the gain measurements.  On the telescope linearity measurements can be made taking exposures of the dome flat lamps (which are not constant), regularly calibrated (every ~5 minutes) for slow changes in the lamp's flux over periods of about an hour.

The above only gives an approximate estimate of non-linearity, as it uses only a single parameter, Alpha, so that the non-linearity is assumed to be essentially 'linear'.  Proper correction for non-linearity can require the fitting of higher order functions (quadratics, cubics or higher) in order to properly trace the non-linear response of the detectors.

CCD Performance Data


Tek 5 (2dF):

SPEED
READOUT
TIME (s)
GAIN
(e-/ADU)
READOUT
NOISE (e-)
SATURATION
(e-)
NON-LINEARITY
(ALPHA x10-6)
XTRASLOW 402
0.36
(2.03)
24,000 (ADC)
-0.02
SLOW
122
1.42
3.1 (3.09)
93,000 (ADC)
-0.06
NORMAL
80
2.83
4.5 (4.93)
185,000 (ADC)
-0.03
FAST
59
5.6
6.9 (8.2)
367,000 (ADC)
-0.08
NONASTRO
41
11.0
(12.0)
385,000
-0.10
ULTRAFAST
10





Notes:
SPEED: Only SLOW, NORMAL and FAST are used in normal scientific operation.
READOUT NOISE: Readout noise measurements made in the lab are listed in parentheses.


Tek 6 (2dF):

SPEED
READ OUT
TIME (s)
GAIN
(e-/ADU)
READ OUT
NOISE (e-)
SATURATION
(e-)
NON-LINEARITY
(ALPHA x10-6)
XTRASLOW
402
0.34
(2.65)
22,000 (ADC)
0.03
SLOW
122
1.41
3.3 (3.30)
92,000 (ADC)
-0.03
NORMAL
80
2.82
5.1 (4.76)
185,000 (ADC)
-0.04
FAST
59
5.7
8.5 (7.1)
374,000 (ADC)
-0.06
NONASTRO
41
11.0
(12.0)
385,000
-0.13
ULTRAFAST
10





Notes:

SPEED: Only SLOW, NORMAL and FAST are used in normal scientific operation.
READOUT NOISE: Readout noise measurements made in the lab are listed in parentheses.

EEV2 (UCLES):

SPEED
READ OUT
TIME (s)
GAIN
(e-/ADU)
READ OUT
NOISE (e-)
SATURATION
(e-)
NON-LINEARITY
(ALPHA x10-6)
SLOW
445 (403)
0.37
2.45 (2.3)
24,000 (ADC)
0.14
NORMAL
178 (158)
1.30
3.15 (3.2)
85,000 (ADC)
0.60
FAST
132 (132)
2.80
5.27 (5.35)
183,000 (ADC)
0.78
NONASTRO
89 (74)
6.67
9.74 (9.8)
437,000 (ADC)
1.92

Notes:
SPEED: No XTRASLOW mode available.
READOUT TIME: Lab measurements are given in parentheses.
READOUT NOISE: Lab measurements are given in parentheses.

MITLL3 (UCLES):

SPEED
READ OUT
TIME (s)
GAIN
(e-/ADU)
READ OUT
NOISE (e-)
SATURATION
(e-)
NON-LINEARITY
(ALPHA x10-6)
XTRASLOW
(936)
0.074
(1.16)
5,000 (ADC)
-0.23
SLOW
300 (296)
0.37
1.84 (1.7)
24,000 (ADC)
0.23
NORMAL
163 (154)
1.1
2.38 (2.4)
72,000 (ADC)
0.36
FAST
127 (120)
2.2
3.47 (3.3)
141,000 (ADC)
0.19
NONASTRO
85 (75)
4.5
6.55 (6.3)
178,000
0.02

Notes:
READOUT TIME:
Lab measurements are given in parentheses.
READOUT NOISE:
Lab measurements are given in parentheses.