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CCD detectors

The AAT has several CCD (charge-coupled device) detectors available, and the FLAIR spectrograph at the Schmidt telescope also has its own CCD system (described in AAO UM 32: FLAIR CCD users' guide). CCD detectors can be used with all the spectrographs described in chapter 5, as well as for direct imaging as described in chapter 7.

At present there are 6 CCDs in use at the AAT; one thinned TEKTRONIX device, one thinned RCA device, two unthinned GEC devices, one of which (GEC19) has been coated to improve its blue response, and two unthinned Thomson CSF devices, one of which has a Metachrome coating. Table 6.1 lists the basic physical properties of these devices, while Table 6.2 gives a brief summary of their main applications.  Figure 6.1 and Table 6.7 compare the quantum efficiencies of the AAO's CCDs.

Several other large-format CCDs are also planned for commissioning in the near future (for example 1024x1024 CCDs will be permanantly mounted in the two 2dF spectrographs), so it is advisable to consult the AAO Newsletter or the responsible  instrument scientist at Epping for up-to-date information on what is available.


Table 6.1: Physical properties of AAT CCDs
Chip Format Pixel
Size(um)
Fringing? Bias Stability Cosmic Ray Rate
(hits / frame / hour)
TEKTRONIX (TEK) 1024 x 1024     24 1% at 7300Å
20% at 9000Å
stable 1300
RED THOMSON (THX) 1024 x 1024     19 none variable 360
BLUE THOMSON (THX) 1024 x 1024     19 none variable 360
RCA 512 x 320     30 at > 5000Å excellent 630
GEC16 576 x 385     22 none excellent 100
GEC19 576 x 385     22 slight excellent 100


Table 6.2: CCD Applications
Chip Advantages Main Applications
TEK High QE, good UV and blue response
Low read-noise, Large Format
Direct imaging at f/3.3,  RGO Spectrograph,
UCLES, Taurus II,  ie. almost everything
RED TH Large Format, low read noise. f/1.1 (VRI) imaging
BLUE TH Large Format, low read noise. f/8 imaging on the auxiliary port when TEK
is used for RGO.
RCA High QE, good blue response. Direct imaging at f/3.3 (essentially superseded
by TEK chip)
GEC16 Low read noise, no fringing Permanently mounted in FORS
GEC19 Low read noise, good UV response U band imaging at f/3.3 (essentially superseded
by TEK chip)

 
Figure 6.1: Quantum efficiency of detectors currently available at the AAT.
Click on the image or here for a larger version. This data is available in
tabular or file form in Table 6.1.

Data acquisition

Control of all CCD exposures is handled by the VAX computer running the OBSERVER acquisition software. Four readout speeds are usually available, SLOW, NORMAL, FAST and NONASTRO, but some extra ones (SLOW_BESTCTE, NORMAL_BESTCTE) are available with the Thomson chip to give improved charge transfer. The fastest speed, NONASTRO, has very high gain and readout noise, and is normally used only for tests. It is, however, safe to use NONASTRO for direct imaging with the Thomson CCD and LBL f/1  focal reducer (see § 7.3) and most observers choose to do so.

It is important to note that the readout noise, gain, and bias level will change if the readout speed is altered. In general, slower readout speeds give lower readout noise and are used for spectroscopy where the count rate is low. Faster readouts save time at the expense of increased noise, and are useful in direct imaging when exposures quickly become sky limited. On-chip binning is available for all the chips and windows can be set to read out only part of the chip, again saving in readout time. A number of special readout modes, such as time-series readout, are also available.

AAO UM 33: The OBSERVER software system describes the data acquisition commands in detail, while AAO UM 17: A user's guide to CCD detectors at the AAT gives more information about the use of CCDs for imaging and spectroscopy. Most of the devices are slightly non-linear, and AAO UM 17 gives details of the (small) correction which should be applied before data reduction.

A few of the most important features of each chip are discussed below. Note that the readout times given are approximate, and do not include the time needed to transfer the data from external memory or to the VAX disk. Thus the cycle time between one observation and the next is somewhat longer than the readout time.

The TEKTRONIX CCD

Since its commissioning in July 1992, the 1024x1024 TEK #2 chip has become by far the most requested CCD detector on the AAT. It is used for both spectroscopy and imaging. Only in a number fo  fairly specific applications are the other CCDs available to be preferred over it. Table 6.3 summarises the readout performance of the array.


Table 6.3: Readout parameters of the TEK CCD (for a 1050x1024 window).
 Readout Speed (TEK)
XTRASLOW SLOW NORMAL FAST NONASTRO
Readout time (s) 394 120 75 52 33
Readout noise (e-) 2.3 3.6 4.8 7.2 11
Gain (e-/ADU) 0.34 1.36 2.74 5.5 11
Saturation (ADU) 65535 65535 65535 65535 35000

The QE of the TEK chip is a strong function of its operating temperature. Therefore it is offered to observers at two operating temperatures &endash; 170K (or "cold") and 200K (or "warm"). Running the CCD at 200K, rather than the more usual 170K, increases the QE by 20% in the UV, about 16% at B, 10% in V and R, and from 10% to more than 100% at wavelengths increasing through the I band. However, this comes at the expense of increased dark current of about 0.1e-/pix/sec.

The 170K mode is therefore necessary for spectroscopic observers who nead to minimise noise, while the 200K mode is suitbale for direct imaging where even very short observations are sky noise limited.

The TEK chip also has an extra readout speed XTRASLOW, which gives a read noise of 2.3e-. By using smaller windoes and/or binning the readout times can be made not excessively long. XTRASLOW requires the preamplifier offset to be accurately set &endash; observers should check with electronics staff that this adjustment is satisfactory.

Binning along the row does not increase the readout noise (as does the Thomson), though binning by large factors in X and/or Y may show up an excess readout-induced dark current. This effect has been adequately suppressed for typical binning factors (eg. 3-5). Users of larger binning factors should take bias frames and ensure the overscan region truly represents the bias level in the image area. This is more critical when using XTRASLOW.

The TEK CCD is thinned and shows fringes in the red. These are about 1% peal-to-peak at 7300Å, rising to more than 20% above 9000Å. Prime focus imaging through an I badn filters shows fringes at about the 3% level.

Readouts in the XTRASLOW, SLOW, NORMAL and FAST modes are all ADC limited and (except for FAST) at 65K the CCD is noweher near saturation, which occurs at about 450000e-/pixel.

At 200K there is some evidence to indicate that the dark current is a rising function of integration time. About 50 rows at the top and bottom of a dark frame  are affected by a brightening of the dark current which rises to a level 60% above the average of the rfirst and last rows of the dark frame.

At 170K the dark current is about 0.55e-/pix/2000s, providing the CCD is not recovering from power-on, or residuals from saturation or high light levl illumination.

The cosmic ray event rate is about 730 per 2000s. This is double the rate per area of chip for the Thomson devices.

Residual images from gross overexposure are very weak. At 200K, an illumination of 10 times saturation produces a residual image of only about 4e-/pix in a subsequent 100s dark frame, and had disappeared by a second 100s dark frame. At 170K the same test produces only 2e-/pix in a subsequent 500s dark frame, and had reduced to 1e-/pix in a second 500s dark frame. Smearing above the saturated image areas due to shifting the saturated image towards the readout amplifier produced weak residuals of less than 0.3e-/pix in the first frame.

The Tektronix CCD recovers far faster than the Thomson when powered off and then on again while cold. At 200K dark current s after 5 minutes have settled to within 30% of their final values. At 170K, dark current falls to 10e-/pix/2000s after 5-10 minutes, and to less than 2.5e-/pix/2000s after an hour.

The time needed to change between 170K and 200K operation depends on the temperature of the dewar. Warming from 170K to 200K may take over an hour in cold ambient temperatures, and coolling from 200K to 170K takes up to 75 minutes in warm ambient temperaures. Whenever the temperature is changed, the pre-amplifier offset must be readjusted by a technician.

The Thomson (THX) CCDs

Table 6.4 gives the readout parameters of the uncoated Thomson (THX) CCDs. The coated chip has similar properties. These chips are linear for all practical purposes, unless working at the fastest speeds and very high count levels. The cosmic ray event rate is ~ 200 per frame per 2000 seconds.


Table 6.4: Readout parameters of the Thomson (THX) CCD
 Readout Speed
SLOW SLOW
_BESTCTE
NORMAL NORMAL
_BESTCTE
FAST FAST
_GOODCTE
NONASTRO
No
Binning
No
Binning
Bin by 2
along row
Bin by 4
along row
No
Binning
No
Binning
No
Binning
No
Binning
No
Binning
Readout time (s) 100 160 160 160 60 110 38 70 28
Readout noise (e-) 2.8 3.1 3.4 3.7 3.4 5 10
Gain (e-/ADU) 0.63 0.63 0.63 0.63 1.26 1.26 2.5 2.5 5
Saturation (ADU) 65535 65535 65535 65535 65535 65535 65535 65535 ~30000

Readouts at SLOW, NORMAL and FAST rates are all ADC limited and even at the 65K ADU level the CCD is nowhere near saturation. CCD saturation occurs somewhere about 400,000e-/pixel. Readouts using NONASTRO are limited to a maximum of about 30K ADU because of the saturation of the electronics amplifiers ahead of the DCS stage and NOT the CCD.

During commissioning of the Thomson chip, the fast CCD clock rates recommended by the manufacturer were found to give poor horizontal charge transfer efficiency (HCTE). Major improvements were possible by slowing the clock rate, but the readout noise suffers and readout times are increased by about 60 seconds. For spectroscopy, the SLOW_BESTCTE or NORMAL_BESTCTE readout speeds should be used if the background level is less than a few hundred electrons per pixel.

Unlike other CCDs in use at AAO, the Thomson CCDs are seriously affected by overexposure to light. While this causes no physical damage to the chip, recovery from saturation is slow, and high dark current and after images may persist for several hours if the chip is grossly saturated. Observers must be extremely careful not to everexpose the detector while, for example, measuring bright position reference stars or taking flat fields.

The RCA CCD

Table 6.5 lists the readout characteristics of the chip. The high readout noise means that the RCA CCD is rarely used for spectroscopy except at low-dispersion in the blue with LDSS. The strong fringes present at wavelengths greater than 5000Å can be removed from photometric data using a combination of dome and dark sky flat fields, but attempts to use the RCA for spectroscopy at red wavelengths have been unsuccessful because of fringing.

The cosmic ray rate is ~ 350 events per frame per 2000s elapsed time.


Table 6.5: Readout parameters of the RCA CCD
 Readout Speed (RCA)
SLOW NORMAL FAST NONASTRO
Readout time (s) 42 35 14 7
Readout noise (e-) 45 37 39 89
Gain (e-/ADU) 2.3 4.7 9.4 18.8
Saturation (ADU) 65535 65535 53191 26595

The GEC CCDs

Table 6.6 lists the readout characteristics, which apply to both GEC16 (the uncoated chip, mounted permanently in FORS) and GEC19 (the coated chip).


Table 6.6: Readout parameters of the GEC CCD
 Readout Speed (GEC)
SLOW NORMAL FAST NONASTRO
Readout time (s) 55 45 18 9
Readout noise (e-) 5.2 6.6 8.2 24
Gain (e-/ADU) 2.0 4.1 8.2 38.9
Saturation (ADU) 65535 65535 40200 9000

The cosmic ray rate is ~ 55 events per frame per 2000s elapsed time.

GEC16 in FORS has no detectable fringing, whereas GEC19 has slight fringing (at the 1-2% level) due to the dye coating. The fringes are most pronounced at wavelengths above 5500Å, and can be removed using twilight sky or continuum lamp flat fields.

The coating process produces some randomly located pixels with enhanced gain, giving intensities between 1% and 25% above the background level. Variations in the thickness of the coating also produce small changes in sensitivity over large patches of the chip. These gain variations are only seen at blue wavelengths ( 4500Å), and can be accurately removed with flat field exposures taken at the observing wavelengths.



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This Page Last updated: Feb 21, 1996, by Chris Tinney.