AAO Wide Field Imager & Prime Focus Unit


Performance Information

(including Information for Proposers).

 
 
 

Version 1.1 - 19 February 2002
 
 

Chris Tinney


Useful WFI links


 

Scope

This document is a detailed report on the measured performance of WFI and PFU on the Anglo-Australian telescope. It has grown out of the WFI+PFU Commissioning Plan and Status Report.

It also includes a summary of performance measures suitable for use by proposers.


History

Version 1.0  - 21 Feburary 2002. Chris Tinney

Contents

Shortcuts
  1. Wide Field Imager and the Prime Focus Unit at the AAT
    1. General
    2. Layout
  2. WFI - Science CCD Performance
    1. Read Noise & Gain
    2. Full-well depth / saturation
    3. Linearity
    4. Serial Charge Transfer Efficiency
    5. Read Times
    6. Effects of Windowing & Binning
    7. Quantum efficiency
    8. Dark Count Rates
    9. BIAS Performance
    10. Example Flat Fields
  3. WFI Guider Performance
    1. When to Guide?
    2. Guide CCD Performance
  4. PFU Performance
    1. SCATTERED LIGHT / LIGHT LEAKAGE.
    2. SHUTTER
  5. WFI+PFU Performance
    1. Image Quality
    2. Focus
    3. Ghost / Light Concentration Images.
    4. Standards & Sensitivities
    5. Astrometry.
  6. Information for Proposers


 
 

Shortcuts to Results.

WFI
PFU

NEWS: WFI suffered a major vacuum failure in June 2004, which was repaired by RSAA. However, it was found that on completion of the repair, two of WFI's 8 CCDs were not working. One CCD has since been fixed, but another remains out of use. The inoperative CCD is No. 7, where the layout of CCDs is given below.


1.Wide Field Imager and the Prime Focus Unit at the AAT

1.1 General

WFI is an CCD imaging mosaic, consisting of eight 2KxK MIT Lincoln Labs 15um edge-buttable CCDs, arranged to give an 8Kx8K imaging format. It is mounted at the triplet-corrected f/3.3 prime focus of the AAT on the AAO Prime Focus Unit (PFU), and has a field of view of roughly 33' on a side. The WFI focal plane also includes 8 small guide CCDs.

WFI is shared facility which was constructed as a collaboration between the Research School of Astronomy & Astrophysics (RSAA) of the Australian National University, the Anglo-Australian Observatory, the University of Melbourne, and Auspace. Its use is shared between the RSAA 40" telescope on SSO and the AAT. While the WFI mosaic itself moves between the AAT and the 40", each telescope has its own "exposure controller" (ie shutter and filter wheel).

Prime Focus Unit (PFU) is the AAT's exposure controller.  It contains a six position filter wheel able to hold filters up to 165mmx165mm in size, and 10mm thick. It also contains a two-travelling-blade shutter, able to uniformly expose the entire focal plane at better than 1% for exposures longer than 2s.

The entire 128Mb WFI 8K image is read from the CCDs using 8 parallel controller channels, displayed in real time, and transferred to disk in 58-60s (depending on computer load). The data are written to disk as multi-extension FITS files.
 

Layout

NEWS: WFI suffered a major vacuum failure in June 2004, which was repaired by RSAA. However, it was found that on completion of the repair, two of WFI's 8 CCDs were not working. One CCD has since been fixed, but another remains out of use. The inoperative CCD is No. 7, where the layout of CCDs is given below.

 

2. WFI - Science CCD Performance

2.1 Read Noise and Gain

Only on speed (FAST) is available. This reads the entire 8K mosaic in 60s, with a read-noise of ~ 5e in each detector and gains between 1.5 and 2 e/adu.

For the most recent Read Noise and Gain measurements see the long-term records of read-noises and gains.

 

2.2 Full-well depth / saturation

Linearity calibrations were acquired in small 1024x1024 pixel windows in the centre of each CCD using a flat field lamp over a range of exposure times. 5s and 10s exposures over the course of the sequence were used to calibrate the lamp to ~0.2%. From these data we made plots of 'Calibrated RAW ADU above bias' versus 'Exposure Time' for each CCD. From these we can derive where the full well limits lie in raw adu averaged over a window in the center of the 1024x1024 window in the center of each CCD.

IN  ALL CASES SATURATION IS DUE TO REACHING FULL WELL, RATHER THAN A/D CLIPPING. This means data obtained NEAR the saturation limit may be subject to severe non-linearity. Observers are advised to always stay several thousand ADU below these limits, and if linearity is critical to their science requirements, to stay below 30,000adu.
Results from multiple runs are consistent giving confidence they are generally applicable to WFI observations. Look at the plots to see how I derived these full-well limits. All CCDs have full wells over 80kph.
 



December 2000 & February 2001 - Saturation Results
CCD  Satur'n in      Bias   Satur'n  Gain   Full Well
   (adu above bias)  (adu)  (raw adu)(ph/adu)  (kph)
1    56,000          3450    59,450  1.45    81.2    Plot
2    53,000          4100    57,100  1.71    90.6    Plot
3    42,000          3992    45,992  1.96    82.3    Plot
4    52,000          4549    56,549  1.72    89.4    Plot
5    44,000          2848    46,848  2.02    88.9    Plot
6    56,000          3368    59,368  1.67    93.5    Plot
7    54,000          4002    61,000  1.88    101.5   Plot
8    55,000          4514    59,514  1.69    92.9    Plot

2.3 Linearity

Linearity data has been acquired with the a dome flat lamp calibrated as a function of time using repeated 5s and 10s exposures. The resulting 2nd order polynomial calibration has lamp fluctuation residuals of +-0.2%.

We determine a mean count rate for the linear part of the linearity curve (or at least for the middle part of the linearity curve), from which we can predict the 'true' counts at all times. By plotting the measured counts Nm against the true counts Nt we can derive a very approximate linearity correction alpha.

Nm = Nt ( 1+alpha*Nt )
or equivalently if alpha << 1,
Nt = Nm ( 1-alpha*Nm )
The non-linearity at a count level Nt~Nm is then just alpha*Nm, and when alpha is positive you need to SUBTRACT counts from the measured signal to get a linear signal.

Unfortunately, plots of Nm vs Nt are incredibly difficult to analyse - the deviations of interest are tiny, and so invisible on a plot. Much more useful is to examine Nm/Nt vs Nm. When this is done you can (a) see whether particular data points are outliers from the general trend and should be deleted, and (b) actually examine different parametrisations to see which works best.

When we do that we find that the 'standard' parametrisation above in terms of a single 'alpha' term is in fact pretty lousy. It will get you an approximate linearity correction, but not a very good one. For CCDs like CCD4 and CCD7 where the CCD's non-linearity is highly non-linear, it is especially poor. For comparison with other AAO CCDs we therefore provide the alpha number, but I can't recommend you actually use it.

To see examples of how bad the alpha parametrisation is, view the following Postscript files :   CCD1, CCD2, CCD3, CCD4, CCD5, CCD6, CCD7, CCD8 . The general trends in behaviour are similar to those seen in other MITLL CCDs. CCD4 has the linearity profile which is most discrepant from that of the other CCDs. It is also the only Phase I device in the mosaic


Linearity measurements and Rough alpha (Linear) parametrisations. 
(Do not use these to correct data - use the polynomials below.)

 
      24AUG00, V filter, 1024x1024 central window, 3% `lamp' stability.
      25DEC00, g filter, 1024x1024 central window, 0.2% 'lamp' stability
      02FEB01, i filter, 1024x1024 central window, 0.2% 'lamp' stability
        CCD  Useful range    AUG00           DEC00        FEB01
       (adu above bias) Alpha*10^6     Alpha*10^6  Alpha*10^6
      1    0-53,100    -0.24 (+-0.05)   -0.154        -0.121
      2    0-54,000    -0.05      "     -0.090        -0.098
      3    0-49,500    -0.29      "     -0.203        -0.217
      4    0-52,400    +0.53      "     +0.534        +0.160
      5    0-53,000    -0.38      "     -0.145        -0.099
      6    0-56,350    -0.47      "     -0.347        -0.219
      7    0-53,100    -0.38      "     -0.235        -0.189
      8    0-52,800    -0.47      "     -0.256        -0.157


 

Linearity Measurements and Polynomial Parametrisations


Much better fits are obtained with a polynomial parametrisation
 

Nt/Nm = 1 + A1*Nm + A2*Nm*Nm + A3*Nm*Nm*Nm .....
The term A0 is usually fixed at 1, and A1 is usually fixed at 0.0. CCD7 requires higher order terms than A2. CCD4 is the most unusual device, as its non-linearity does not follow the general trend (which is to be asymptotically more linear at lower counts, and to deviate at high counts).  CCD4 requires a second order fit, with all parameters being free. With the execption of  the zeoth order term (which essentially just adjusts the CCD gain) we derived identical fits in December and February for CCD4. The remaining CCDs all also showed very similar fits on both tests.

We have some confidence therefore that these polynomial calibrations can be generally applied.

To apply these corrections, your data reduction procedure would be

  • Overscan subtract and trim each image
  • Bias (or zero) subtract each image
  • Run a linearity correction program to replace the value in each pixel (Nm(x,y)) with

  • Nm*(1 + A1*Nm + A2*Nm*Nm + A3*Nm*Nm*Nm .....)
  • Proceed with dark subtraction, flat fielding, etc as per usual.
  •  
    Recommended Polynomial Linearity Corrections for WFI.
    (A log of  past measurements is available here).
    CCD
    A0
    A1
    A2
    A3
    Residuals
    (%)
    Max ADU
    above bias
    Uncorrected Non-linearity
    < 0.5% below 
    (adu above bias)
    1
    1
    0
    3.86e-12
     
    0.4
    52,000
    36,000
    Plot
    2
    1
    0
    3.18e-12
     
    1.0
    54,000
    35,000
    Plot
    3
    1
    0
    8.43e-12
     
    0.9
    42,000
    24,000
    Plot
    4
    1.02753
    -1.6455e-6
    1.7791e-11
     
    0.8
    52,000
    <10,000
    Plot
    5
    1
    0
    4.26e-12
     
    1.3
    44,000
    32,000
    Plot
    6
    1
    0
    5.93e-12
     
    1.3
    57,000
    28,000
    Plot
    7
    1
    0
    -5.50957e-12
    2.39334e-16
    0.6
    54,000
    36,000
    Plot
    8
    1
    0
    5.37e-12
     
    0.4
    53,000
    30,000
    Plot


    Note that for most CCDs these non-linearities are quite significant. CCD4, for example, is non-linear at all count levels! Non-linearity corrections are therefore strongly recommnded for WFI reductions.

    The non-linearities observed are not dissimilar to (though in some cases more extreme than) those seen in the AAO's MITLL2a and MITLL3 detectors.

    As a rule of thumb, try to always keep your targets peaking at < 30,000 adu. Non-linearity will then be  ~ few per cent (except in CCD4) without correction. 


    For most observations however, linearity correction is recommended. Applying the above corrections should get the linearity <0.2% below the MAX ADU number above in each CCD.

     

    2.4 Serial Charge Transfer Efficiency

    Images are compared below which show the horizontal (i.e. serial) charge smearing in data acquired with the original ~160K operating temperature, and the current 183K operating temperaure. At the penalty of poorer dark current performance the SCTE has been greatly improved and is now acceptable in all CCDs.
    CCD1
    GOOD

    CCD2
    OK

    CCD3
    GOOD

    CCD4
    GOOD

    CCD5
    OK

    CCD6
    GOOD

    CCD7
    GOOD

    CCD8
    GOOD

    2.5Read Times


      Binning   Size      Time to read     Time to read+transfer
    Full mosaic windows
        1x1     2098x4136x8     53s          56s-59s depending on load
        2x2     1049x2068x8     23s          23s-??s depending on load
        3x3      700x1379x8     12s          ~15s
    Centre of mosaic windows
        1x1     2048x2048x4     26s          26s
        1x1      256x256 x4      3s           3s
    Centre of each detector windows
        1x1     1024x1024x8     11s          11s
        1x1      512x512 x8      6s           6s
    One whole single detector
        1x1    2098x4138        53s          53s
    Two whole detectors (one per controller)
        1x1    2098x4138        53s          53s

    Notes
  • x4 sizes are windows in the centre of the mosaic (eg.256x256x4 is the central 512x512 pixels of the mosaic).
  • It takes the same time to read 1 detector or 8. The only time saving is in transferring data, and once the AAT WFI computer's CPU and DISKs are optimised even this will be negligible.

  • 1.1.7   Effects of windowing - does windowing produce any noticeable impact on any of the other CCD parameters (other than read time of course)?

    2.6 Effects of Windowing & Binning

    Windowing : Presently windowing can produce small 'edge' effects on the up to 50-70 pixels near the top/bottom of the window boundary (near the top of CCDs read through the upper controller, near the bottom fo CCDs read through the bottom controller).  These are not always present, and not always present on both controllers, but are most commonly seen as a ~10% flux deficit in the affected rows.

    Eg. you can look at GIF images of 1024x1024 pixel windows in the center of each chip illuminated by a flat field lamp and the V filter. On the 24aug0055 frame, CCDs 1, 2, 3, and 4 all show a bottom edge effect. CCDs 5, 6, 7 and 8 do not show any effect. On the other hand when the exposure time was increased for 24aug0060, edge effects were seen at the top of CCDs 5, 6, 7 and 8 , but not CCDs 1, 2, 3, and 4.

     
    Don't trust the bottom/top 70 pixels of windowed data.
    This effect is presumably caused by saturation of the readout register by the rows which are not being used, and the readout register requiring time to recover once rows stop being skipped, and readout of thw window starts.

    The major problem with windowing is that it does not, at present, provide an overscan. This complicates reduction. Also the reduction pipeline based on IRAF cannot handle data sets based on several windows - they have to be reduced separately. This means it is HIGHLY ADVISABLE TO TAKE ALL DATA IN ONE WINDOW. The seconds saved at the telescope in read-time do not justify the pain in reduction.

    Binning : No noticeable effect on gain and read-noise. The comments above on windowing and data processing overheads apply also to binning. Seconds saved in read-out will rarely justify the extra pain in data reduction.

    2.7 Quantum efficiency

    No laboratory measurements for WFI were possible before it was shipped to the telescopes.

    The following figure compares the QE Curves measured for 4 of the the devices in WFI (w16c2 is CCD1, w90c1 is CCD4, ll 10-15-5 is CCD5, w90c2 is CCD6) with the MITLL2 and MITLL3 CCDs also in use at the AAO. w19c1, MITLL3 and MITLL2 (before coating) all share a common red-optimised AR-coating, producing poor blue QE. The QE in the blue for the Phase 2 devices is better, though not as good as that seen in (for example) EEV devices. On the other hand, these devices generally have excellent red QE, good fringing performance (twice as good or better than the AAO's TEK1K) and the best CCD amplifiers ever constructed.

    Comparison of the photometric throughput in B and I can also be used to compare the relative QEs of the WFI CCDs.
     

    Using

     
     

    2.8Dark Count Rates

    The dark current rates if WFI reflect the warm temperature it is run, with dark counts of ~30e per 1800s being counted (or a noise contribution of about 5.5e-). This means in a typical 300-600s exposures, dark current will contribute about half as  much noise again, as read noise.

    At least one of the exposure below showed anomalously high dark currents  (25DEC0035). The cause is not known.
     
     
    26DEC0017 1800s
    Guider CCDs on
    (183K)
    27DEC0051 1800s
    Guider CCDs OFF
    (183K)
    25DEC0035 1800s
    Guide CCDs on
    (183K)
    25DEC0037 1800s
    Guide CCDs on
    (183K)
     02feb0155/156
    Guide CCDs on
    (183K)
     
    CCD
    ADU above
    bias
    e/1800s
    ADU above
    bias
    e/1800s
    ADU above
    bias
    e/1800s
    ADU above
    bias
    e/1800s
    ADU above
    bias
    e/1800s
     
    1
    17
    25
    18
    26
    56
     
    23
     
     20
     28
     
    2
    16
    27
    10
    17
    37
     
    18
     
     20
     34
     
    3
    10
    20
    11
    21
    30
     
    13
     
     17
     33
     
    4
    29
    50
    30
    52
    72
     
    37
     
     33
     54
     
    5
    12
    24
    7
    14
    26
     
    15
     
     19
     38
     
    6
    10
    17
    7
    12
    20
     
    14
     
     20
     34
     
    7
    11
    21
    9
    17
    32
     
    15
     
     21
     39
     
    8
    14
    24
    9
    15
    36
     
    18
     
     22
     37
     

    The dark levels also have considerable structure. This means dark frames must be acquired and subtracted from all data. The following  images compare a 1800s dark frame taken with the guide CCDs turned on (after zero-subtraction, trimming and overscan subtraction), with an 1800s dark frame taklen with the guide CCDs turned off  (after zero-subtraction, trimming and overscan subtraction), and with a zero(or bias) frame (trimmed and overscann subtracted).

    CCD4 shows considerable structure on very large scales (as well as the worst dark current). CCD1 shows a bright region on its right edge. CCDs 1,2,5,7 and 8 all show "warm blobs" where dark currents are elevated by 5-50 adu per pixel over several hundred pixels.

    Turning the Guide CCDs on produces  four bright spots at the edges of the mosaic peaking at 100-200adu/hr, and slightly elevated overall dark currents.

    26dec0017 1800s dark - Guide CCDs on. Stretch=-100 to 200
    27dec0051 1800s dark - Guide CCDs off. Stretch=-100 to 200
    Zero frame (7 frames medianned). Guide CCDs on. -100 to 200
     
     

    2.9 BIAS Performance

    Overscan subtraction is important  : Examination of the bias levels in frames taken over several nights on a run show that bias levels fluctutate up and down by +-2adu on all timescales - between subsequent exposures, over hours, and between nights.

    Typical bias levels :

    CCD     Mean    
    im1     3452 
    im2     4080 
    im3     3992 
    im4     4550 
    im5     2848 
    im6     3369 
    im7     4002 
    im8     4514
    BIAS Flatness : Most of the CCDs show one of the following; bad columns, trapping sites, small leds. In general the BIAS frames seemed quite flat away from these regions. Bad columns and trapping sites generally don't subtract well with bias frames (or as IRAF calls them zero frames). Nonetheless, with SO much data in each frame, it will make sense to obtain and subtract BIAS frames always, so that you don't have to rely on the detectors always being flat, since it will be hard to ensure they are!

    Subtraction of cosmetic defects : Most of the bad pixels and/or bad columns don't  subtract.

    Pickup noise : The bias frames do show evidence for pickup noise, though not at a level significantly higher than the read-noise. Given the most common high background applications in which WFI will be used, this is probably not worth worrying about.

    Recommended bias procedure : Fitting a constant to the overscan region and subtracting is recommended, followed by the subtraction of a zero/bias frame made up from 9-15 biases taken during your run.

    The recommended parameters for the MSCRED version of ccdproc (related to overscan subtraction are) are

    ccdtype="" interactive=no function=legendre order=1 sample=* naverage=1 niterate=2 low_reject=3.0 high_reject=3.0  grow=0.0

    Overscan correction can also be done in PIPELINE, where you should be able to see the same parameters set in the /opt/cicada/config/iraf_table file.

    You should follow this by doing a zerocombine using the following parameters in IRAF.

    cl> zerocombine input=@zerofiles.lis output=yourzerofile combine=median reject=sigclip lsigma=5 hsigma=5 mclip=yes scale=none

    You can then use the file yourzerofile.fits for zero correction of your data in either PIPELINE, or directly in IRAF.

    2.10 Example Flat Field Images

    Click on the small images to see a largely version in GIF format.
     
     
    U-band Flat Field B-band Flat Field g-band Flat Field
    R-band Flat Field i-band Flat Field z-band Flat Field

    3. WFI Guider Performance

    The most significant 'feature' of the performance of the WFI guider CCDs is that they contain some component which is injecting light pollution into the edges of the WFI mosaic. The effects can be clearly seen in a pair of 1800s darks acquired on 24aug (24aug0004,5) - click on the images to see larger versions. As is shown above for the 183K warm operating temeperature.
    Examination of these images shows that when the guide CCDs were turned off the dark current levels for cold operation (prior to december 2000) were  typically 1.5-3 adu/1800s, but when the guider CCDs are turned on the general dark current level increases to 5-10adu/1800s, and in the bright regions at the edges of CCDs1,4,5 and 8 the contamination peaks at 50-100adu/1800s.

    These effects will be much smaller in the typical exposures expected for AAT broad band imaging (5-10min), but nonethless dark exposures will be critical if guiding is to be used.

     

    3.1 When to Guide?

    The AAT tracks adequately in 300-600s exposures. You only need to guide when your exposures are not sky limited in such an exposure. The only time when this happens is in the U passband or narrow-band passbands. There are significant overheads to guiding, so it is not recommended unless necessary.

    3.2 Guide CCD Performance

    Each guide CCD delivers an image 320x240 10um pixels (though they are best operated binned x2, for guiding), corresponding to a field of view 48x32" on the sky at a scale of 0.15"/pix (or 0.3"/pix if binned). Orientation on the sky is shown above. Imaing in the guide CCDs indicate that the ideal focus for the guide CCDs is usually 0.05-0.1mm less than that for WFI - so the guide CCDs are not perfectly confocal with the mosaic, but close enough for guiding.

    Guiding can be carried out at rates as short as one exposure per second, or as slow as one exposure per 10-20s.

    The faintest star which can be guided on is ?? For more details on guiding see the relevant section of the WFI/PFU Cookbook.


    CCD   Gain      Read Noise  Bias   Satun'n   Dark Counts
         (e/adu)      (e)       (adu)  (adu)      (e/300s)
    1     7.967     135.233     383    
    2     6.178     129.074     216              124 
    4     8.13      154.188     258              154
    3  DEADDEADDEADDEADDEADDEADDEADDEADDEADDEADDEADDEADDEADDEAD
    5    11.287     163.332     339
    6     6.89      128.802      83
    7     7.226     127.832     194
    8     6.966     124.919     121              111

     The flat fields below  below show the 7 working guide CCDs, illuminated by a flat field lamp though an r filter. All are pretty good, with a few noticeably bad spots, but most of the area being quite usable. The results above show none to have particularly bad read noise. Dark current is equal to or less than read noise for all conceivable guiding exposures.

    G8 
    G1
    G7 
    G2
    G6 
    DEAD
    G5 
    G4

    A distinctive pattern is produced when a guide CCD reaches full well. The left image below is a 4s image with G8 of a dome flat field lamp (~5400adu). The right image is a 6s exposure with the same lamp, and we see that at 8500adu full well has been reached.
     

    G8: 4s,5400adu 
    G8:  6s,8500adu

     
     

    4. PFU Performance

    4.1 SCATTERED LIGHT / LIGHT LEAKAGE.

    Low General Light Test
     An incandescent lamp was used to evenly illuminate the entire room, including the floor at which the PFU+CCD looks. A 100s exposure allowed 55,897 adu through the shutter onto the CCD. A 100s dark produced no detectable counts above bias. From this I conclude the unit is light tight to 'over-all' illumination to better than 0.005%.

    Light Incident on Shutter Test
     An incandescent lamp was shone directly 'under the skirt' of  PFU. A 2s exposure collected 37,182 adu. A 60s dark (with the lamp still on) collected 5 adu above bias, or 0.0005% of the photons incident on the closed shutter. This seems an acceptable level of leakage.

    Light injection by filter wheel movement
    During a 60s dark frame, the PFU filter wheel was advanced by 1 slot 9 times. The resulting exposure showed no evidence for light leakage or contamination at the 1-2 adu level.  It is probably safe to move the filter wheel while WFI is being read out, but for the time being until more experience is gained we will continue to recomend no wheel motions while the detectors are being read.

    4.2 SHUTTER

    Timing
    Data was acquired on 27 December, with the telescope at zenith.  Some data was also acquired with an AAO CCD (so the CICADA bug was not relevant) on 9 August before PFU went on the telescope, and poor data was acquired in August 2000 with the CICADA system.

    In all cases, the results are parametrised as

    Tactual = Trequested - t0
    The sign of t0 is in the sense that t0 positive implies the exposure was short of the requested time.
     

    August 9, 2000 : Estimated shutter 'dead time' was 31+-1ms (ie actual exposures were 31ms shorter than requested).

    August 24, 2000 : This data was poor, but indicated the dead time was 50+-50ms (ie actual exposures were 50ms shorter than requested).

    December 27, 2000 : A more comprehensive data set  wasacquired. The lamp was calibrated to be constant to within +-0.1% over time. The data has been processed in two ways; (M1) by plotting the observed counts as a function of requested exposure time, and making a linear least squares fit; (M2) by plotting the requested exposure time as a function of observed counts, and measuring the extrapolated exposure time when the counts become zero. You can view the data as Postscript files M1: CCD 1,2,3, 4, 5, 6, 7, 8  and M2: CCD 1, 2, 3, 4, 5, 6, 7, 8
     


    27DEC00 Exposure sequence Zenith 
    CCD  t0 : M1         t0 : M2
           (ms)            (ms)
    1    18+-4             18
    2    17+-4             18
    3    17+-4             18
    4    18+-5             24
    5    16+-7             13
    6    16+-7             13
    7    18+-6             18
    8    17+-5             15

    Mean t0 = 17.1+- 2 ms.


    The December data would seem to robustly determine the shutter delay as being 17 ms (each exposure is 17ms too short). This says that exposures longer than 2s will have absolute timing to better than 1%. Observers seeking precise timing information from short exposures are advised to measure the shutter delay for themselves, and correct their exposure times until we gain enough to experience to guarantee the delay is constant from run-to-run and with telescope position.

    February 2, 2001 : Another comprehensive data set was acquired. The lamp was calibrated to be constant to within +-0.1% over time. The data has been processed in two ways; (M1) by plotting the observed counts as a function of requested exposure time, and making a linear least squares fit; (M2) by plotting the requested exposure time as a function of observed counts, and measuring the extrapolated exposure time when the counts become zero. You can view the data as Postscript files M1: CCD 1, 2, 3, 4, 5, 6, 7, 8  and M2: CCD 1, 2, 3, 4, 5, 6, 7, 8
     


    02FEB2001 Exposure sequence Zenith 
    CCD  t0 : M1         t0 : M2
           (ms)            (ms)
    1    22+-2             23
    2    20+-3             21
    3    20+-2             22
    4    22+-5             28
    5    20+-4             23
    6    20+-3             22
    7    23+-3             25
    8    22+-3             25

    Mean t0 = 22.4 +- 2 ms.


    The February data  seems to provide a robust t0 estimate, though one which is about 5ms different from that obtained in December.  So far no tests have been done to determine the shutter delay as a function of telescope position (all were done with the telescope at zenith).
     
    We  conclude that 
  • Exposures of longer than 5s will always have uncorrected exposure times in their headers good to 0.4%
  • Observers for whom absolute shutter timing is important should correct their exposure times to be 20ms shorter than the time requested (though they should also assume a residual uncertainty of at least 5ms in their exposure length). Resulting shutter timing for exposures of longer than 5s will be good to 0.1%.
  • Observers for whom shutter timing is hyper-critical should obtain a set of shutter timing data during their run to check that the 20ms offset above still holds for their instrumental set-up.
  • Uniformity

    It would appear that PFU meets its specification of delivering better than 1% uniformity for exposures of > 2s.

    In fact 1s exposures seemed to be uniformly exposed at the 0.7% level in August 2000, and to the 0.5% level in December 2000. At this sort of level, flat fielding difficulties limit our ability to probe shutter uniformity more closely.
     

    December 2000

    Shutter uniformity was examined by acquiring long (10-40s) exposures and using them to flatten short (0.05-1s) exposures. Any resulting non-uniformity should be due to the shutter no uniformly exposing the field of view. In particular we searched for variations in shutter illumination in the E-W (ie up-down) direction - the direction of shutter travel.

    26DEC0006 1s flattened with 40s This image (click on it for a larger version) shows a 1s exposure obtained on 26DEC00 through a Gunn g filter. It has been bias subtracted and flattened with a 40s exposure taken directly before it.

    The count level in the image is ~420adu. You can view vertical cuts  through CCD3 and CCD6, which show the  residual peak-peak  non-uniformity is ~0.5%.

    The following exposure (with the shutter blades travelling back in the  reverse direction) was also analysed to find a similar result for both CCD3 and CCD6.

    We also acquired a similar set of data on 27DEC00 through a V filter  with a 0.05s short exposure and a 10s long exposure. In this case we saw a more marked non-uniformity,  in both CCD3 and CCD6. However its form is that of a central doughnut, so it is through this is due to flat fielding problems in dealing with light reflected of the CCDs and off the V filter to form an out of focus image of the sky. This effect would be worse in V than g as the V filter is not AR coated.

    In this exposure the non-uniformity is about 1.4% in a 50ms exposure. This is still well within the <1% for a 2s exposure specification.

     

    5. WFI+PFU Performance

    5.1 Image Quality

    Examination of images shows good image quality accross the mosaic and in alls filter in 1" seeing.

    We found we could obtain near 1" images right accross the field. Detailed examination of focus frames has revealed a slight trend indicating some non-alignment of the WFI and AAT focal planes. But this is slight, and we need to determine whether its repeatable (so that WFI should be shimmed) or is just a run-to-run mounting difference.
     

    5.2 Focus

    In the following table we adopt the V (WFI Schott) filter as our reference (ie 0.0 focus offset).
    Telescope Focus values in available WFI passbands
    Filter
    Telescope
    Focus
    (mm)
    Focus Offset
    (V=0.00)
    (mm)
    FOCOFF NAME
    U (WFI Schott #48)
    39.72
    +0.77
     U
    B (WFI Schott #49)
    39.60
    +0.65
     B
    V (WFI Schott #50)
    38.95
    0.00
     V
    R (WFI Schott #51)
     38.95
     +0.04
     R
    g (WFI SDSS #90)
    39.26
    +0.31
    GG
    r (WFI SDSS #91)
    39.21
    +0.26
    GR
    i (WFI SDSS #92)
    39.12
    +0.17
    GI
    z (WFI SDSS #93)
    39.13
    +0.18
    GZ

    5.3 Ghost / Light Concentration Images.

    No obvious ghost images have become apparent, though there are hints at the 1% level in shutter uniformity observations made with the V filter.
     

    5.4 Standards & Sensitivities

    December 2000 : Photometric standards were observed in conditions believed to be photometric on several nights. Detailed flat-fielding and processing is still under way. The following sensitivites were measured from unflattened data. They compare the sensitivities of a number of passbands in CCD6, and also compare CCD6 with the other CCDs at B and I. These numbers have been used to update the Direct Imaging Calculator.  As expected at V the performance is very slightly worse than the TEK. At R and I it is similar or better.  CCD4 (the only Phase I device in the mosaic) has very similar sensitivity to the MITLL2A. The remaining chips are better at r and i than the MITLL2A.

    CCD6 was chosen as the 'best' single device based on its cosmetics. CCD7 has better blue sensitivity, but similar red sensitivity to CCD6.

     
    Sensitivites of CCD6 in Available WFI passbands
    Filter
    Object
    Ph/s for 22.5 mag star
    at AM=1.25
    Sky
    Ph/s per pixel 
    (0.2295"x0.2295")
    Comments
    U (WFI Schott #48)
    1.3
    0.18
     
    B (WFI Schott #49)
    17.5
    1.7
     
    V (WFI Schott #50)
    28.5
    5.5
     
    R (WFI Schott #51)
         
    g (WFI SDSS #90)
    34.4
    5.0
    for V=22.5 star
    r (WFI SDSS #91)
    37.8
    10.3
    for R=22.5 star
    i (WFI SDSS #92)
    27.6
    20.0
    for I=22.5 star
    z (WFI SDSS #93)
    Sensitivities measured using Landolt standard stars. These data were acquired over an airmass range of 0.15. 
    Given usual extinction coeffs for SSO this will produce errors of at most 5% in U and 1.5% in I. Sensitivies
    were estimated for neutral (B-V=V-I=0.0) colour stars.

    Average value accross the mosaic have been used in the Direct Imaging Calculator - you will not derive precisely
    the same sensitivities as those above. Please however, always use the Direct Imaging Calculator when preparing
    proposals.


     
    CCD
    Photons/s for a  22.5 mag star at AM=1.25
    Comments
     
    B (WFI Schott #49)
    i (SDSS #92)
     
    1
    20.2
    17.7
     Worst at I
    2
    20.1
    26.5
     
    3
    20.4
    26.3
     
    4
    15.8
    27.7
     Worst at B (Phase I CCD)
    5
    19.2
    25.2
     
    6
    17.5
    27.6
     Best cosmetics
    7
    22.6
    28.5
     Best at B and I
    8
    20.5
    26.3
    Best and worst CCDs are highlighted. These data were acquired
    over an airmass range of 0.13. Given usual extinction coeffs for
    SSO this will produce errors of at most 3.5% in B and 1.5% in I.

    6. Astrometry.

    There will be significant astrometric distortion in the WFI focal plane - the plate scale at the edge will be several percent different to that in the centre.
    5.1 Determine radial distortion correction for triplet corrector (and for doublet corrector at some future date). Compare with 'a priori' radial distortions. Does it depend on filter, or on focus?

    5.2 Determine CCD positions within focal plane.

    5.3 Determine stability of radial distortion and CCD positions within focal plane.

    5.4 Explore how to take data, and determine parameters needed to map mosaic into a single image, in MSCRED.

    7. Information for Proposers

    WFI proposers should prepare their observing proposals for the following assumptions




    This page created by Chris Tinney.