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The formats and major applications of the detectors most commonly used for imaging are summarised in Table 2.1. Note that all the CCDs can be used at a range of readout speeds - with the trade-off that shorter read-times mean larger read-noises. For most imaging applications, even very short observations are sky-limited (rather than read-noise-limited) even at the fastest available speeds. Only the most commonly used applications are listed below. For more details on speeds, read-noise, etc, see the sections on the individual CCDs in §2.1
Table 2.1: Major properties of AAO CCDs used for imaging
|1024 x |
|Most common use: Direct imaging at
Most common speed: FAST for broadband, NORMAL for narrowband.
|RED THOMSON |
|1024 x |
|Most common use: LBL f/1 wide field
imaging. 0.98"/pix, 16.73' FOV|
Most common speed: NONASTRO
|Most common use: f/8 imaging on Cass.
port when |
TEK used on RGO. 0.129"/pix, 2.20' FOV
Most common speed: FAST_GOODCTE
Table 2.2 summarises the quantum efficiencies (QE) of the major imaging detectors. All were meaured in their dewars, so the responses include losses due to the dewar windows. Note that the TEK CCD can be run at two temperatures. Running it at 200K ("warm") produces enhanced QE, but degraded dark-current performance (0.1e-/pix/s). For broadband observations this increased dark-current is swamped by the sky brightness, so running warm is the default mode. For narrow-band imaging 170K operation ("cold") is probably to be preferred.
2.2: Comparison of DQE for imaging
|Differential Quantum Efficiency|
Figure 2.1 - Graphical comparison of the QEs of imaging CCDs. The superior performance of the TEK chip is obvious. A larger version of this figure is available here.
By far the most popular imaging set-up is the TEK 1024 x 1024 Thinned CCD used at the f/3.3 Prime Focus. For f/3.3 imaging, neither of the Thomson CCDs offers any advantages over the TEK CCD - they both have lower quantum efficiency, and a smaller field of view (the seeing at the AAT is almost never good enough to gain any advantage from the Thomson chip's smaller pixels).
The Red Thomson CCD is used in the LBL f/1 Imaging System (because the f/1 optics are optimised for the red, their is no advantage to using the Blue Thomson at f/1).
The Blue Thomson CCD is occasionally used for imaging at the f/7.9 Cassegrain Auxiliary focus, when the TEK CCD is mounted on the RGO spectrograph, allowing the acquisition of very faint targets, or near simultaneous imaging and spectroscopy.
In July 1989, the AAO CCD controllers were fitted with a fast cleanout facility. Before this feature was added, the cleanout phase that takes place just after a CCD readout was achieved by the repetition (1000 times for small CCDs, 2000 times for the Thomson and Tektronix devices) of the sequence one vertical shift followed by a shift out of the full row (plus a few more pixels) of the readout register. The cleanout of a Thomson CCD in this mode would take about 120 seconds in '_BESTCTE' mode or 18 seconds otherwise. For the Tektronix CCD it would take about 30 seconds.
Fast cleanout is a repetition (1000 or 2000 as before) of the sequence one vertical shift followed by only one horizontal shift of the readout register. This cleans out the Thomson CCD in just over 0.1 sec in the `_BESTCTE' mode of operation. The Tektronix is cleaned out in just over 1 second. Fast cleanout is also used to reject the unwanted CCD rows ahead of, and following, an astronomer window. This avoids the otherwise long delays that would result if these unwanted rows were fully shifted out in the customary fashion. In fact, the minimum `_BESTCTE' readout time for a very small window of the Thomson CCD would be 72 seconds (17 seconds for the Tektronix), if the fast cleanout mode was not available.
HOWEVER, it should be recognised that the fast cleanout process in effect bins all of the unwanted rows into the readout register and if these rows have a high intensity background then there is a risk that the readout register will saturate (note that the readout register has a much higher charge handling capacity than an image area pixel) and saturating charge will spread down the columns from the readout register. The extent of downward spread from the unwanted charge existing above the window will generally not reach the astronomer window . Also, the readout register is thoroughly cleaned out by extra rows added above and read out just prior to the astronomer window.
But if there are a large number of unwanted rows below the astronomer window to the last row of the CCD, and these contain extended areas of high intensity (say a bright flat field image), the resulting saturation when these rows are binned in the readout register may spread down from the readout register into the astronomer window area (but not into his data as that has already been read out). The result will be the generation of higher dark currents emanating from the residuals of this saturated charge spread.
NOTE that this is a problem only when windowing, and then only when windowing in the vertical direction (i.e. full ROWS are being discarded), and then only if extended intense images are attempted such as high level flat fields with, say, 20,000e-/pix. Moreover, this is only a problem when using a CCD which suffers seriously from residual-image-generated dark currents. The resultant dark current effect on the next image are too difficult to quantify as they depend on the window size, position, intensity of previous bright images or flat fields, time since these images, etc.
Unfortunately, taking test exposures with small windows of flat fields with the Thomson CCDs satisfies all these criteria. It is, therefore, important to ensure that when testing dome- or sky-flat exposures with the Thomson CCDs and small windows, that the windows read an entire column - ie cover the entire vertical direction of the CCD. A sample suitable suitable window (11 x 1024 pixels) called THOMSON_F1_TEST can be found in DISK$USER:[OBSERVER.WINDOWS].
The Tektronix CCD does not suffer from residual-image-generated dark current at any significant level, so no special precautions need to be taken with it.
The bias level is a purely electronic offset. In itself it is noiseless, and no photons can be attached to its value. It is simply a zero-point, on which the CCD's read-noise is superimposed. Wether the bias level is 50 ADU or 5000 ADU will not affect the read-noise.
In the past, variations in the bias levels with time have been noticed. Generally, the bias level has fluctuated slowly plus or minus one or two ADUs - though on rare ocassions the bias level had been seen to jump by several ADUs between two readouts (or extremely rarely during a readout).
Considerable effort has been spent to remedy this but although some changes have been made and suspect components replaced, some drifting may still occur. The drifts are made worse (compared with the earlier GEC and RCA CCDs) by the fast readout time of the Thomson and Tektronix CCDs, which requires an extra gain of 2.5 to be placed after the DCS and this is where it appears most of the drifts take place. The setting of a "gain balance" facility in the DCS circuit has been found to be readout rate dependant and the reason for this is not clear. The setting has been optimised for the NORMAL and the SLOW readout rates - FAST and NONASTRO are not so well compensated for general system drifts and may show a poorer bias level stability.
Astronomers should note that the bias level will be affected by all
of the following:
All CCDs are non-linear to some extent. For all of the AAO CCDs at the most commonly used speeds this non-linearity is very small, and can be ignored. Only at the fastest readout speeds, and at very high count levels do non-linearity corrections need to be even considered.
The extent of non-linearity is parametrised by Alpha, where
Nm are the measured (i.e. non-linear) counts in ADU above the bias level, and Nt are the `true' (i.e. linear) counts in ADU above the bias level.
In practice, 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
(where t is the exposure time at constant illumination). From the slope m and y-intercept c of this fit, you find
Unfortunately, no constant illumination source is currently available at the AAT. However, exposures taken of the dome flat lamps can be calibrated for slow changes in the lamp's flux over periods of about an hour enabling observers to measure Alpha themselves if necessary.
Typical values of Alpha for the fastest speeds used for science
exposures are :
for the Thomson CCDs at speed NONASTRO, Alpha=0.08e-6.
This means that even at 40,000 ADU, the non-linearity correction (Nm-Nt)/Nt is only ~ 0.3%.
for the Tektronix CCD at speed FAST, Alpha=-0.14e-6.
This means that even at 40,000 ADU, the non-linearity correction (Nm-Nt)/Nt is only ~ 0.3%
At slower speed the corrections required are even smaller.
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. There are only a small number of fairly specific applications for which any of the other CCDs are prefereable. Table 2.3 summarises the performance performance of the array. Readout times are only approximate. Alpha is the non-linearity parameter, as defined in §2.1.
Table 2.3: Readout parameters of the TEK CCD (for a 1050x1024 window).
|Readout Speed (TEK)|
|Readout time (s)||394||120||75||52||33|
|Readout noise (e-)||2.3||3.6||4.8||7.2||11|
|Alpha x 1.e-6|
The CCD comprises 1028 columns and 1024 rows of 24 micron square pixels. There appears to be no aluminised covered columns or rows so that there is a sharp edge to the image area permitting the bias level to be estimated from either the H and V overscan regions once a sufficient margin is allowed for the effects of HCTE or VCTE (vertical change transfer efficiency). The first and last columns have about a 60% increased light sensitivity. The first row may have 5% extra "sensitivity". It is probably advisable to avoid the first and last columns in any critical observations. Also, they should not be included with others in windows binned along the row direction if they are not to affect the bins into which they fall. The readout register has 48 overhanging pixels between the first column and the readout amplifier. The readout register is cosmic ray sensitive so that it is possible for single row cosmic events to occur.
The QE of the TEK chip is a strong function of its operating temperature. Therefore it is offered to observers at two operating temperatures - 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 ~0.1e-/pix/sec. The 170K mode is therefore necessary for spectroscopic or narrow-band imaging observers who nead to minimise detector noise, while the 200K mode is suitable for direct imaging, where even very short observations are sky noise limited.
At 200K there is evidence to indicate that the dark current is a rising function of integration time. The dark current is about 0.13e-/pix/s for a 2000s exposure, but only 0.05e-/pix/s for a 30s exposure. 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 first 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, residuals from saturation or high light level illumination.
The TEK chip also has an extra readout speed XTRASLOW, which gives a read noise of 2.3e-. By using smaller windows and/or binning the readout times can be made not excessively long. XTRASLOW requires the preamplifier offset to be accurately set - 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.
XTRASLOW is superfluous for direct imaging - the above discussion is included only for completeness.
The quantum efficiency for the TEK at both "cold" and "warm" temperatures is shown in Table 2.2 and Figure 2.1.
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 band filter shows fringes at about the 2% level. No fringing is seen at bandpasses shortward of I.
At 200K six hot pixels have been found. Their locations are referenced to (1,1) as first pixel in a 1028 X 1024 image.
At 170K only the four brighter hot pixels in the above list are seen and at a very much reduced magnitude. In NORMAL readout there are no visible trails
In bias frames made with the slower readout speeds there is evidence
of trails from these hot pixels. These trails can seen clearly when the
frame is summed in the column direction and may be visible in image displays
especially of XTRASLOW frames. The table summarises their importance
|TRAIL INTENSITY (e/pix)|
Readouts in the XTRASLOW, SLOW, NORMAL and FAST modes are all ADC limited and (except for FAST) at 65K the CCD is nowhere near saturation, which occurs at about 450000e-/pixel. Readouts in NONASTRO are limited to a maximum of about 35K ADU because of saturation of the electronics amplifiers ahead of the DCS stage, and not that of the CCD.
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 currents 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 (recall the 'rest' value at 170K is 0.55e-/pix/2000s).
It takes about 2 hours after filling the dewar with LN2 for the CCD temperature to fall to 170K and commence temperature regulation. Powering up the CCD as it cools provides a dark current of less than 7 e/pix/2000sec only 4 minutes after it attains 170K temperature regulation.
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 final optical alignment measurements indicated that the CCD has:
Two Thomson (THX) CCDs are available for use at the AAT. Both are unthinned 1024 x 1024 pixel format devices with 19um pixels. One of these devices (the "Blue" Thomson) has a Metachrome coating which produces enhanced performance (over the uncoated, or "Red" Thomson) at wavelengths shortward of 4500Å.
Unlike the 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 following discussion (unless a specific chip is mentioned) applies equally to both devices.
We have no detailed description of the physical make-up of the Thomson CCDs. The following is a description of the chip as gleaned from several weeks of testing.
The pixels within the 1024x 1024 image area are very uniformly responsive to illumination. Outside this area there are:
The following are the ``final'' read-noise, gain and Alpha values. Read times are approximate only. Alpha is the non-linearity coefficient defined in §2.1. For all practical purposes the CCDs are linear.
Table 2.4: Readout parameters of the Red Thomson CCD.Figures in parentheses are guestimates.
|Bin by 2|
|Bin by 4|
Table 2.5: Readout parameters of the Blue Thomson CCD. Figures in parentheses are guestimates.
After powering on the CCD, it takes some time for the chip to clean itself out and for the dark current to become sufficiently low for astronomical applications. Power-on dark current is flat and featureless but takes several hours to fall to negligible levels, as shown in the following table. This table is based on NO strong light being applied to the CCD as it cleans itself (see the section on residuals). The power-on dark current extends into the ten columns either side of the 1024 x 1024 image area and the two rows above and below this area.
|Table 2.6: Thomson power-on dark current decay|
|Time after power on||Dark Current|
|> 48 hr||2|
Poor horizontal charge transfer efficiency (HCTE) was found using the fast H clock rates recommended by the CCD manufacturer. Major improvements were made by providing longer (slower) H clocks - the longer the clock time the better the result. A total H shift time of 50 us looked pretty good, but the readout noise does suffer and readout times are increased by about 60 seconds.
Observers should use the readout speeds SLOW_BESTCTE or NORMAL_BESTCTE for the best HCTE or the FAST_GOODCTE for rapid readout with not quite the best HCTE. NONASTRO speeds provide fairly poor HCTE and will show low level charge trails in the row direction. This speed is intended for use only on images with high backgrounds of several hundred e/pixel.
Because the Thomson CCD suffers seriously from residual-image-generated dark currents, its users must be careful when using small CCD windows to test flat field exposures (see §2.1 above).
It is important to ensure, when testing dome- or sky-flat field exposures with the Thomson CCDs and small windows, that the window reads an entire column - i.e. the window covers the entire vertical direction of the CCD. A sample suitable window (11 x 1024 pixels) called THOMSON_F1_TEST can be found in DISK$USER:[OBSERVER.WINDOWS].
Bias frames and zero background images show faint traces of striations attributed to mains injected noise. These are not so apparent on NORMAL or SLOW readouts but can be seen on FAST and NONASTRO bias frames where the readout noise is digitisation limited. The effect is at a very low level and made more noticeable by high contrast images which span only a few ADUs. This noise is made worse (compared with the GEC and RCA systems) by the 2.5 times gain after the DCS and the use of long cable lengths at the focal reducer and at the CES.
Measurements in the lab put the cosmic ray event rate at about 220 per 2000 seconds.
See Table 2.2 for the quantum efficiencies of the two Thomson CCDs.
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 700,000e-/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.
Residual images after bright illumination appear to be of two
Table 2.7: Residual Images on the Red Thomson (see text below)
|Time after read|
to start of
60s dark frame
|Residual Intensity (e-/s) in 60s Dark Frame|
The data in Table 2.7 were obtained with the Red Thomson, but similar behaviour is seen in both devices. `Above' and `below' saturation area refers to the trailing appearing above the residual and the trails below the saturation area. `Overscan' refers to that trailing into the vertical overscan. `Gen DK' refers to the general background well away from the spot image.'
Higher CCD operating temperatures do not affect the strength or decay rate of the residuals.
Some problems of data conversion and data corruption in the serial link to the LEM were experienced in the first couple of months of use of the Thomson CCD at the telescope. These have been cleared up. However, it seems that cutting down the analog delay times in the Thomson system to facilitate fast readout times has led to the possibility that the analog to digital conversion may be less than perfect. Astronomers should keep an eye open for recurring odd data values or histograms of data that show the presence of missing ADC codes.
These occur fairly frequently at random locations and may appear as single pixel events or in horizontal or vertical pairs with amplitudes up to several thousand electrons. They look very much like cosmic ray events except they are distinguished by not showing the low-level cloud of electrons found in adjacent pixels of a cosmic ray event. In two Thomson frames, during lab tests, 9 single events and 2 pairs occurred. Magnitudes were up to several 1000e-. These ``events'' are definitely on the CCD as they show trailing when the HCTE is deliberately made poor.
|Table 2.8: Red Thomson Hot Pixel Summary|
The only repeatable "hot pixels" found in 22 frames (11 bias and 11 500sec dark on 21 and 26/9/91) were several groups of pixels at the image area to readout register interface. These occur only in long dark frames and only in two rows, one of which is two rows ahead of the first row of an EXACT window and the other two rows after the last row of the same window. Thus they will not be seen when using the EXACT window but will be seen in the sixth of the eight rows of under-scan used in some windows to estimate the bias level or in window that includes vertical overscan (none are known to me). Thus only the top five rows of the under-scan should be used to estimate the bias level for the frame. The intensity of these hot pixels is variable - in a 500sec DF they are commonly less than 100e but occasionally peaks to 500e have been seen. They do not smear into the EXACT window and should be no worry.
Table 2.9: Red Thomson Trapping Site Summary
|422||833||340||54||>191 (to bottom of CCD)|
|589||846||610||91||>178 (to bottom of CCD)|
These column and row numbers are based on a 1024 square image. The intensity is a measurement of the trailed charge level on the third overscan row, (i.e. the third row below the last row of the 1024 square image) with the CCD illuminated with a flat field of about 70,000e-/pix. The trap length is an estimate of the number of rows beyond the trapping site that the dark column extends (the dark column being produced by the trapping site on the low-level flat field). These measurements were done at flat-field intensities of approximately 100 e-/pix and 20 e-/pix.
Table 2.10: Blue Thomson Trapping Site Summary
|462||1026||Off screen. No image area counter-|
The column and row numbers are based on a 1024 square image. The intensity is based on the third vertical overscan column, i.e. is the third row below the last row of the 1024 square image. The trap length is an estimate of the number of rows beyond the trapping site that the dark column is produced in a low light level flat-field. These measurements (made on the 3/7/91) were done at intensities of approximately 100 e/pix and 20 e/pix.
Columns 264 and 269 appear as dark columns in low light images read out at the default NONASTRO rate. "Trapping sites" in the H readout register were located by reducing the horizontal charge-transfer efficiency until the dark columns (extending top to bottom in low-level FFs) were more evident. The corresponding columns where these "traps" are located are as follows (determined on 6/7/91) -
Table 2.11: Blue Thomson Readout Register Traps
Introduction The Telescope & Optics The Detectors
The Imaging Cameras An Imaging Cookbook The Data you Take Away
Exposure times OFFSET_RUN files CCD Windows Data Catalogs
On-line Reduction Filters Flat-fields Blank Fields Orientation Shutters
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This Page last updated: 30 May 1997, by Chris Tinney