Orthogonal Transfer CCD Arrays OTAs: Pros and Cons for LSST

1
Orthogonal Transfer CCD ArraysOTAs Pros and
Cons for LSST

• A collaborative effort between
• MIT Lincoln Laboratory (Burke)
• Semiconductor Technology Associates (Bredthauer)
• Univ of Hawaii / PanSTARRS (Tonry, Luppino)
• WIYN Observatory (Jacoby)
• Univ of Arizona / Imaging Technology Laboratory
  (Lesser)

2
MotivationImproved Image Quality Over 1

• Based on Tip/Tilt Performance at WIYN (Claver)
• Tip/tilt improves seeing by 15 in FWHM
  (typically about 0.14)
• RIZ medians become 0.52, 0.43, 0.35
• Atmosphere decorrelates at 2 arcmin degrades
  0.32 images by 10

3
OPTIC The OTAs Ancestor

• Example 300s R-band image
• 3 of 4 guide regions selected
• Read at 10-50 Hz
• Tip/tilt correct remaining pixels

• CCD Format
• 2 2Kx4K OT CCDs
• 4 high-speed read zones
• 4 science zones

4
OPTIC A Time Domain Application

• Quick readout of selected regions
• Enables the 10-50 Hz time domain for CCDs
• Example planet transits
• 0.3s samples binned to 60s
• Relative accuracy 6E-4
• Approaches HST (1E-4)

Planet Transit
5
Orthogonal Transfer Array (OTA)
OTCCD pixel structure
OTA 8x8 array of OTCCDs
Basic OTCCD cell
6
Summary of OTA Properties

• 64 independent 500 x 500 CCDs
• Individual addressing of CCDs
• 2 arcmin field of view at LSST
• Bad columns confined to cells
• Point defects are tolerated
• Cells with bright stars ? guide stars, or read
  fast, up to 30 Hz, to avoid blooming, or for time
  studies
• 8 video channels 2s readout
• Intercell gaps (0.1-0.3 mm 1-3) dithering
  required
• Inter-OTA spacings 2 mm (20)

5cm
12 um pixels 0.23 at LSST
7
Pinout Configuration
99 pins include some redundancy (69
required) Symmetry allows single package for
front or backside devices
Right side
Left side
8
STA CCD Array ConfigurationLincoln is similar

• 8x8 Array of elements
• 480x494 Subcells allow for increased busing area
• 3840 x 3952 pixels per OTA
• 336 micron x 132 micron streets
• Data sheet available

49.48 mm
9
Dalsa Fabrication Status

• Foundry run divided into 3 silicon groups to
  guard against flaws
• 8 wafers of 40 ?-cm material (thin to 15 ?m)
• 8 wafers of 150 ?-cm material
• 8 wafers of 500 ?-cm material (thin to 30 ?m)
• Production started mid-January 10 week
  fabrication time ? first wafers in mid-March
• Wafer level testing in late March (STA) and April
  (ITL)
• Package (ITL) 1 thick OTA in April for lab and
  on-sky testing at WIYN.
• Package 1 thin OTA in July for lab and on-sky
  testing at WIYN
• Revisions to masks, second foundry run to start
  in the Fall 2004

10
Packaging Status

• Designed by G. Luppino and M. Lesser
• Components being fabricated by Kyocera for
  delivery in April

11
Expected Device Properties

• Pixel rates gt 1 Mpix/s
• Read noise 6-8 e- (degraded by long video lines
  on chip)
• Pixel size is 12 ?m MITLL testing 10 ?m
• Thinning depends on resistivity (thus, so does
  QE, MTF vs ?)
• Blooming is different than usual CCD with channel
  stops puddling of charge instead of trails is
  cosmetically cleaner

12
Advantages For LSST

• Improved device yield
• Resilient to shorts (lose a cell, not an entire
  detector)
• Bad columns delimited by cell size
• Tip/tilt image enhancement (smaller pixels are
  better)
• Control of blooming from bright stars
• OT pixels lead to puddling of charge (N
  affected pixels) rather than trailing (gt1K pixels
  in a column)
• Fast readout of cells with bright stars avoids
  saturation
• Extreme saturation is delimited by cell size
• Improved observing efficiency
• Can start exposures while telescope settles
• Can observe in windy conditions
• Science array provides guider signal
• Development underway by other groups available
  with licensing agreement
• Reduces demands on telescope structure and control

13
Disadvantages For LSST

• Detectors dont exist yet
• Fill factor is 90 due to circuitry between
  cells (though future developments can greatly
  reduce this loss)
• With OT shifting, each cell has unique
  astrometric solution (dominated by simple XY
  offset)
• With OT shifting, each cell has a unique PSF that
  may vary slightly across a cell
• Due to the 64 amplifiers per OTA, heat load is
  about 0.7 watt per 50 mm of silicon
• More complex electronics
• NOAO MONSOON
• IFA IOTA
• Pixel traps may be present

14
Other Considerations for LSST

• Consider OTA geometry but without OT pixels
  control bright stars
• Consider OTA but with smaller pixels (10 ?m
  0.20)
• Consider OTA but with type 2 pixels
  half-pixel tip/tilt control

• Replace logic on the detector with bump-bonded
  ASIC having front-end electronics
• improves fill factor
• Improve readout speed by 8X (to 0.25 sec)
• Reduces read noise by eliminating long video
  lines

15
More Considerations for LSSTAlternative
Observing Modes

• Use OT to track moving objects instead of
  telescope motions

• Use OTA geometry to obtain high speed time series
  data on many stars at once

16
Simulated Image Improvement

• Adopt real centroid sequence from a WIYN
  observation
• Native seeing 0.65
• 300 sec exposure
• Data from OPTIC
• Pixels are 0.14

17
Simulated Image Improvement

• Assume Gaussian PSFs of 0.33, 0.55, 0.77
• Apply shift sequence
• Trail from guider telescope shake atmosphere

No tip/tilt Correction
With tip/tilt Correction