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EEV 2Kx4K 13.5um Pixel CCD

Contents

  1. The Device
  2. Use with AAO Instruments
    1. UCLES
    2. UHRF
    3. Others (including decommissioned instruments like Taurus and RGO)

This page contains information on the AAO's science grade EEV 2Kx4K device - hereafter EEV2. It was first commissioned on the AAT in April 2001. The information provided is based on tests made at the Epping Lab by John Barton (a copy of his comprehensive report can be found here), and observations made at the AAT with the RGO Spectrograph by R. Stathakis, and UCLES by S. Ryder.

1. The Device

Identity & Format

The CCD array has a 2048 (Horizontal) x 4096 (Vertical) pixel format (with an additional 4 unused vertical pixels). Each pixel is 13.5 x 13.5 microns in size, for a total active image area of 27.6 x 55.2 mm. The device has been thinned, and is used back-side illuminated. Its performance has been optimised for blue wavelengths.

This is the grade 1 (science grade) contract device (graded to Paul Jorden's revised specfications), received from RGO on 10/11/98. It has serial number 7461-16-7 and A6284 marked on the CCD header.

Binning

Binning in possible in the the vertical or horizontal directions.

Quantum Efficiency

We currently have no direct measurements of QE ourselves with this device. The QE as measured by EEV is shown below. These numbers would make it by far the AAO's most blue sensitive detector.

Wavelength (nm) 300 350 400 500 650 900
Qauntum Efficiency (%) 25 35.9 73.2 91.7 86.2 30.1
A larger version  of this figure is also available, as is a colour postscript file. The data used to produce it can be found here
NB: because the Postscript file was created by an Evil Empire Microsoft product, it may not preview with 
Ghostview version 1.5 or earlier. It will preview with Ghostscript version 4 or later, and it will print out. 

Limited comparisons of RGO spectrograph performance with the EEV and archive TEK data have been obtained.  We compared data for the standard LTT4364 through wide slits taken with the RGO centred at 6581 Å, 1.7 Å/pix, with archive TEK data with the RGO centred at 4778 Å, 3.0 Å/pix. Unfortunately, althrough the EEV data extends over the full wavelength range covered by the TEK data, the ends of the EEV data are significantly vignetted by the spectrograph camera. So we only get a decent comparison over the wavelength ranges of the EEV corresponding to  similar physical locations in the spectrograph focal plane as the TEK - roughly 4000-6000 Å.

The figure below summarises the photons/unit wavelength/time comparison. From 4000-6000 Å the EEV is 30-40% better than the TEK, as expected from the manufacturers numbers above. So although we don't have confirmation of the EEV's excellent blue response, we have no reason to disbelieve the manufacturers numbers. They are at least confirmed from 4000-6000 Å.

Fringing

Fringing with this device is much worse than seen in the MITLL large format CCDs, and even worse than that seen in the TEK CCD. Observations with the RGO spectrograph indicate spectroscopic fringing at a level of <0.5% p-p at 6000 Å, 0.5% p-p at 6500 Å, 4% p-p at 7000 Å, 22% at 8000 Å, 58% p-p at 9000 Å, and 60% at 10000 Å. This compares with 3% p-p at 8500 Å for the MITLL3, and 6.5% p-p for the MITLL2a.

The following GIF images shows a 10-pixel cut averaged down the centre of the EEV2 CCD, and very roughly normalised as a function of wavelength using a spline. The big wiggles near 6000 Å are not fringing - they're just my spline not matching the flat field's blaze function very well.

Cosmetics

The device has a number of hot pixels (see John Barton's report). The hottest pixel on the device at (1842,1525) generates a faint hot column of about 10 e-/pix as the rows are shifted through the hot pixel during the readout. Thus the hot column appears only on the "far" side of the hot pixel and extends right through to the last row of the readout. No other hot columns have been detected.

Flat field cosmetics appear to be very good. Anecdotal reports from the first spectroscopic commissioning run suggest a virtually featureless flat-field in the blue. Its hard to tell what the flat field performance is like in the red, as the observed flat fields are completely dominated by fringing effects, giving a dominant 'diseased zebra' pattern.

The 'diseased zebra' : the image below shows a sample 914x150 pixel spectroscopic flat-field sample. The left edge is ~8500 Å, and the right edge ~10100 Å at 1.7 Å/pixel. The stretch is such the the features have 60% peak-peak amplitude.

Lab results show regularly spaced lines across flat-fields. These occur every 512 pixels at the boundaries of the stitching process employed in the manufacture of these large CCDs. Each line appears as a pair of rows, one brighter and one fainter than the average for the flat field. The worst pair exhibits a row about 1% brighter and the next row about 4% fainter than the average, i.e. a 5% p-p effect. Hopefully these will flat-field out. This worst pair, in fact, form a limit to the full well for the CCD as this pair had a weaker charge handling capacity than the remainder of the CCD.

There are no light emitting defects. There are several trapping sites on this CCD but they are relatively small. See John Barton's report for more details.

BIAS
 

DARK
Click for larger image

Larger BIAS image (GIF) Gzipped FITS BIAS (7Mb)
Larger DARK image (GIF) - Gzipped FITS DARK  (16Mb)
EEV Full bias vertical cut EEV Dark frame cut
BIAS Frame this is a NORMAL speed bias created as median of 20frames. GIF Images are streched between -1 and 1 adu.  The vertical cut shows the sort of bias structure present in the image, and indicates over-scan subtraction is essential for low count images.
DARK Frame is a single 1800s dark at NORMAL speed created. It has been overscan subtracted, and had the bias frame to the left subtracted. GIF Images are stretched between -2  and 2adu. The horizontal cut through the image above shows the dark count rate in the image to be 0.5+-0.1 adu/1800s. A similar dark taken immediately before this one showed 0.9+-0.1 adu/1800s.

Cosmic Rays

A quick visual examination of an 1800s dark count image shows about 400 CR hits per 1800s exposure over the whole detector (where a 'hit' is a several hundred adu peak CR). This is much lower than the MITLL3 (about 1500 hits in the same time, though the detector has a 23% larger surface area).

Focus

The EEV detector sits in a standard AAO dewar, and will mount on all the AAO instruments - however it has a preferred orientation because it is rectangular. This means, for example, that on the RGO it has to be mounted ???? relative to the TEK.

Performance

NOTE: The readout characteristics presented in this table are appropriate only for the AAO-1 CCD controllers used up until ~August 2004. If your data has a header keyword FIRMVSEQ containing 'Sequencer: AAO2 CCD Controller', then you should refer instead to the CCD Performance with the AAO2 Controllers page.

SPEED  INTEGRATION
(us) 
GAIN
(e-/adu) 
READNOISE
(e-) 
ALPHA
(x1e-6)
SAT
(Ke-) 
READ
RATE
(us/pix) 
Read Time (s)
Full 2x2 5x5
NONASTRO  1+1 6.8 8.2 1.16 200 7.5 69 30 11
FAST 2.5+2.5 2.7 4.4 0.59 168 11.5 105 43 54
NORMAL 5+5 1.3 3.4 0.35 81 16.5 150 54 17
SLOW 20+20 0.32 2.7 0.11 21 46.5 416 123 31

Readout times include a reasonable overscan. Full is 2120 pixel (incl 72 pixel overscan) by 4096 rows unbinned. 2*2bin is 1060 binned (by 2) pixels (incl 48 overscan bins) by 2048 binned (by 2) rows. 5*5bin is 424 binned (by 5) pixels (incl 14 overscan bins) by 420 binned (by 5) rows.

Performance on the Telescope

NOTE: The readout characteristics presented in this table are appropriate only for the AAO-1 CCD controllers used up until ~August 2004. If your data has a header keyword FIRMVSEQ containing 'Sequencer: AAO2 CCD Controller', then you should refer instead to the CCD Performance with the AAO2 Controllers page.

The performance figures above were derived in the lab at Epping by John Barton. Some of these have been checked on the AAT in a commissioning run April 1-2, 2001 with the RGO spectrograph. Using the IRAF findgain task in a window [951:1150,2000:2100] of the full EEV CCD we confirm the following read noise and gain numbers, which give us confidence that the performance table above has been verified on telescope. The pairs of numbers are completely independent measurements from seperate bias and flat field frames.

SPEED  GAIN
(e-/adu) 
READNOISE
(e-) 
FAST 2.60,2.57 4.64,4.57
NORMAL 1.29,1.29 3.48,3.36
SLOW 0.29,0.32 2.36,2.62

On the telescope the dark current in unbinned mode has been measured as 1.2+-0.1 and 0.65+-0.1 e/pix/1800s  in a pair of consecutive exposures at the start of a run (the decrease is probably due to settling after power on as described from lab tests below).

Linearity tests in speed FAST confirm that the linear non-linearity correction alpha is actually a good approximation for this CCD (at least in speed fast, and probably in the slower speeds).  (This is not true for the MITLL CCDs where higher order corrections than alpha are needed - see the WFI linearity discussion for more details. )

The coefficient measured (+0.59e-6) is consistent with JRB's lab result in the table above, so these can be used with some confidence by picky observers.

Dark Current

Dark current is only detectable in binned images and then only if account is taken of the clock induced dark current - the latter dominating in short exposures. The dark current generation rate is about 0.15 e-/pix/2000sec or 0.27 e-/pix/hour. On the telescope the dark current in unbinned mode has been measured as 1.2 +/- 0.1 and 0.65 +/- 0.1 e/pix/1800s  in a pair of consecutive exposures at the start of a run (the decrease is probably due to settling after power on as described from lab tests below). This is negligible for most applications, unless exposures are longer than 500 seconds and a binning of 8 or more pixels is used.

Although only detectable in binned images (eg 5 x 5 binning), clock induced dark currents are comparatively strong on this CCD. The charge build-up is uniform across the CCD and independent of exposure time. It amounts to about 0.10 to 0.12 e-/pix, so is negligble unless binning more than 20 pixels together. It dominates shorter exposures if the CCD is properly cleaned out, i.e. not recently powered on.

This CCD recovers rapidly after power-on compared to others. After 1 hour the dark current rate is down to about 1 e-/pix/2000sec compared with the MITLL CCDs which require 8 hours to get to this level. The CCD cleanout rate following power off and on, with a cold CCD, as measured in dark frames are:

Time after Power on
Intensity
(e/pix/2000s) 
30s 
123 
5min 
12 
10 
6.3 
20 
2.9 
30 
1.3 
60 
1.2 
80 
0.7 
100 
0.5 
16 hours 
 0.35 

Residual Images

For the EEV, residuals from non-saturated images or from saturating overexposure should not be a problem.Tests were made for the residual cleanout rate by overexposing a small area of the CCD by a factor of 10 times saturation level, i.e. 2Me-/pix, and then taking a series of 100 sec exposures starting 30 secs after the end of the saturating exposure. This test indicated that the residuals accumulated at a rate less than 1 e-/pix/2500sec after 30 seconds and at 4 minutes were undetectable in a 5 x 5 binned image.

2. Use of the Device on AAO Instruments

2.1 UCLES

The EEV2 device was commissioned with UCLES in early July 2001 by Stuart Ryder. It should be the detector of choice for most programs when working blueward of H-alpha (6563 Å). At longer wavelengths, the significant fringing of the EEV could be a problem, and the MITLL3 has better QE in any case.

Windows
The same readout windows used for the MITLLL2A/3 detectors can also be used with the EEV2, as the array dimensions are the same. The smaller pixels give improved spatial and spectral resolution cf. the MITLL devices, at the expense of 10% less coverage of the echellogram. The EEV2 device shows no sign of the charge diffusion, which degrades the maximum resolution of UCLES with the MITLL2A (R < 70000 for even 1 pixel slits). By closing the slit down to 0.3", a resolving power R=115000 at 5500 Å is attainable with the EEV. More realistically, a 0.5" slit will yield R=85000 (though slit losses due to seeing will still be significant).

Format
When viewed on the XMEM display, EEV2 images have red (lower number) orders on the left, and wavelength increases going up each order. When saved to disk and viewed with figdisp/Ximtool/Skycat, the images are flipped vertically, so that wavelength increases from top to bottom along each order. This is the same orientation as the Tek, and the exact reverse (i.e. flipped in X and Y) of the MITLL2A/3 orientation.

Dark current
Just 36 hours after being installed, the dark current stabilises to a level of 0.9 e-/pix/hr. This is rather better than the MITLL2A can reach in the same time, but still higher than the lowest values reached in the lab. The difference is due to a faint glow from the coude room walls, which we hope soon to reduce by re-painting the walls.

Fringing
Fringing is apparent with the EEV2 at wavelengths beyond 6000 Å, and becomes worse as one goes further into the red. At 6000 Å, the fringing is at a level of <2% p-p; it is still <3% p-p at 6500 Å, but rises to 6% p-p at 7000 Å, and 36% at 8000 Å. However, careful flat-fielding can mitigate this; for instance, dividing the spectrum of a standard star by a flatfield image taken immediately afterward (see figure below) reduces the fringe amplitude at 7500 Å from 20% p-p, to 4% or less. Thus, unless the wavelengths of primary interest are redder than 6500 Å (in which case the MITLL3 should be favoured due to its superior quantum efficiency), the fringing should not be a serious hindrance to most UCLES programs.

Fringing with EEV2


Observing
An important point to note is that the current UCLES camera optics cannot illuminate the entire area of the EEV2 detector (27.6 x 55.2 mm). In fact the unvignetted (<10% vignetting) region which can be observed is more like 18.8 x 38.5 mm. The region covered at 50% vignetting is 60 x 34 mm, which is approximately the entire chip, however unless you are working in the very red, the echellogram will not put any light on much of the chip. You can see the effect of this by looking at the figures on the MITLL3 page, but note that the EEV CCD is physically 10% smaller than the MITLL3, so it sees 10% less of the echellogram than the figures indicate, and is therefore less subject to vignetting.

2.2 UHRF

The EEV2 device is now the standard large format device used on UHRF (MITLL3 is not generally useful, since it cannot be binned).

2.3 Others

Taurus

EEV2 has been used with Taurus in the past. Taurus is now decommissioned for use on the AAT. This information is provided for historical completeness only.

Field Sampling with Taurus II & EEV2
Detector f/8  f/15 
TEK  0.594"/pix  0.315"/pix 
MITLL2A & MITLL3  0.37"/pix  0.20"/pix 
EEV2  0.33"/pix  0.18"/pix 

The EEV2 detector allows an 7.5' (vertical) x 9.5' (horizontal) field to be charge shuffled and stored side by side on the detector. This has important uses for Taurus++ slit observations, and TTF imaging with band switching.

RGO Spectrograph

MITLL3 has been used with RGO in the past. The RGO spectrograph is now decommissioned for use on the AAT. This information is provided for historical completeness only.

The EEV2 device has been used on RGO spectrograph's 25cm camera. RGO users get smaller pixels than the MITLL devices (13.5um vs 15um), and superior blue sensitivity. Once again vignetting stops the whole chip from being used, with only about 3500 pixels being illuminated by the RGO. (This is still more than double the wavelength range covered by the TEK1K). You can play with the possible options using the WWW version of RGOANG.