CCD Imaging

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CCD Imaging

• David Richards2004-04-13
• All astronomical images taken by David Richards,
  2001-2004
• (Meade 8 LX200 SCT / SBIG ST-7E )

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CCD Imaging

• Introduction
• Example CCD Targets
• Typical CCD Results compared to Eyepiece View
• CCD Imaging Basics
• Components of a raw CCD Image
• Image Reduction and Processing (Light, Dark and
  Flat Frames)
• CCD Cameras
• CCD Chips and Cameras
• Considerations when choosing a CCD Camera
• Colour Imaging
• Comparison with Eyepiece View and Film
• CCD Images
• Moon, Planets
• Asteroids, Comets
• Stars, Clusters Nebula
• Galaxies, Supernova
• Science with CCD Camera
• Astrometry
• Photometry

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Example CCD Targets
Planets and other Solar System Objects
Stars and Clusters
Nebulae
Galaxies
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Typical CCD result compared with Eyepiece View
CCD (processed)
Eyepiece View
M51 (Ursa Major)15 x 1 min exposures
Simulated
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Longer Exposure Greater Magnitude Reach
Consecutive CCD images (star field in Milky Way
in Cygnus)2003-08-05  5.2 x 7.6 arc mins
(suburban site, Dorset, UK) The 10 sec exposure
reaches to mag 12.0 whilst the 40 sec exposure
reaches to 13.5
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Deep Sky - Abell 744 Galaxy Cluster
CCD Image, 3 x 60 sec exposure (summed) The
image records distant galaxies down to magnitude
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CCD Imaging The Basics
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CCD Imaging involves some work
Final Image
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Raw CCD Image
Noise
Noise
Noise
Lets examine the components of this image
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Stacking increases S/N
Single Raw Image (realtime contrast)
Single Raw Image (adjusted contrast)
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Cross-Section through a CCD Image (1)
Simulated image of light reaching camera in earth
orbit
Simulated image of light reaching camera at Sea
Level
Cross Section
Light from 3 Objects
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Cross-Section through a CCD (2)
Light from 3 Objects (after dispersion through
the atmosphere)
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Cross-Section through a CCD
Raw Image as recorded
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Sky brightness
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Cross-Section through a CCD (3)
Addition of Sky Glow /Light Pollution
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Effect of Vignetting and Dust and Pixel-to-Pixel
Variation in Sensitivity
Av. 40 x 0.5 sec flat frames (tee-shirt flats)
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Cross-Section through a CCD (4)
Vignetting at edge of frame
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Cross-Section through a CCD (5)
Absorption of light from dust on lenses and CCD
window/ chip and Variation in Pixel to
PixelSensitivity
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Dark Current(electrons counted due to heat,
even in the absence of light)
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Cross-Section through a CCD (6)
Addition of thermal electrons during
exposure(includes noise)
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Dark Current vs Time
All Frames -25 deg C and identical white-black
range(Black 0 ADU / White 1000 ADU)
10 sec
60 sec
120 sec
300 sec
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Dark Current vs Temperature
All Frames 60s exposureand identical white/black
range(Black 150 ADU, White 300 ADU)
-5 deg C
-15 deg C
-25 deg C
Colder
Astronomical Cameras typically cool CCD chips to
30 deg C below ambient (using Peltier cooling)
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Dark Current vs Camera
Simulated 60s exposuresshown with identical
white/black ranges
Low Spec Camera -15 deg C
Mid Spec Camera -15 deg C
High Spec Camera -15 deg C
High SpecCameras
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Cosmic Rays
Dark Frame
Light Frame
Dark Frame
Dark Frame
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Read Out Noise(Bias Frame a 0 sec exposure)
-15 deg C
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Cross-Section through a CCD (7)
Addition ofReadout Noise (/-)
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Cross-Section through a CCD (9)
Raw Image as recorded
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Cross-Section through a CCD (10)
Raw Image with Black Thresholdapplied Compare
with light from 3 objects
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Getting Good Images

• A principal aim during imaging (and subsequent
  reduction) is to maximise the
• Signal-To-Noise (S/N) in order to get the best
  image of the astronomical object.
• Techniques include
• Minimise noise from sky light by imaging from a
  dark site (if possible)
• Cool the CCD Chip as far as possible (temperature
  control important)
• Use longest exposure that telescope can track for
  without drifting, and without over-saturating the
  chip.
• Using on camera pixel binning (may decrease
  resolution but not if seeing limited)
• Use camera with low read out noise / low dark
  current
• Reduce images to remove dark current, allow for
  the varying response of each CCD pixel and remove
  the impacts of vignettting and dust on CCD chips
  or telescope optics
• Minimise read-out and dark noise (using Median of
  multiple Dark Frames)
• Use average (or median) of multiple Flat Frames
• Use stacking to add light from target, whilst
  cancelling noise thereby increasing the S/N

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Longer Exposure Higher S/N
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Reduction Steps (1)
Dark Reduced Frame
Raw Light Frame
Dark Frame

-
Removal of Dark Frame (an image with same
exposure length but taken with closed
shutter)Done in order to reduce read-out
thermal noise
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Reduction Processing Example
Raw Light Frame (60s)
Dark Frame (median of 9)
Reduced Light Frame
Final Image (15 frames stacked)
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Reduction Steps (2)
Raw Flat Frame
Even Light
Flat Frame (after dark subtraction)
Dark Frame (same exposure as flat frame)
Raw Flat Frame

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Creation of Flat Frame
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Flat Frame
Av. 40 x 0.5 sec flat frames (tee-shirt flats)
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Reduction Steps (3)
Flat
Normalised Flat
AverageFlat Field Value

/
Normalised Flat
Dark Reduced Frame
Final Image
/
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Final Processing
Final Reduced Image
Final Image (with Black Threshold Set)
Wavelet (assumed shape of atmospheric
dispersion)
Processed (Deconvolved) Image
Final Reduced Image
Deconvolved with

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The challenge of recording very faint objects
Attempt at imaging 2004 DW (a mag 19 Kuiper Belt
Object). Star field in Hydra with the predicted
position of Kuiper object marked by green circle.
2 x 5 min exposure (summed)Faintest visible
objects are mag 17.7
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Reduction/Stacking Example IC 434 (Horsehead
Nebula)
11 aligned frames summed
60s Raw
60s Reduced (dark subtract)
Final Image
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Reduction/Stacking Example NGC 2903
60s Raw
60s Reduced (dark subtract)
Average 10 x 60s
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CCD Cameras
SBIG (USA) e.g ST-7e, 1995 (US)
Starlight Express (UK) e.g HX-916 (Mono) 1395
Apogee (USA)
HX7-C (Colour) 995
WebCameg Philip ToUCam Pro II, 75
Low Light Videoe.g. Watec 120N, 579
e.g. Astrovid, 995 (US)
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Example range of CCD Cameras

• Cookbook CCD CamerasTC-211 (Mono) 13.8 x
  16um, 192 x 164 px, 2.6 x 2.6mm 50-100
• Electronic Eyepieces Meade Electronic
  Eyepiece TV/VCR/Camcorder connection 90
• WebCam Based CamerasPhilips ToUCam Pro ,
  Video 5.6 x 5.6um, 640 x 480 px, 4.6 x
  4.0mm 75
• Digital CamerasVarious 200 - 400
• Long Exposure Video CCD CamerasMinitron 29
  9Watec 120N 8.6 x 8.6 um, 752 x 582 px, 6.5 x
  5.0 mm, 0.00002 lx , 0.15 kg 579
• Smaller CCD CamerasStarlight Express MX5 (Mono)
  9.8 x 12.6um, 500 x 290 px, 4.9 x 3.6mm,
  495Starlight Express MX5C (Colour) 620
• Standard Size CCD CamerasStarlight Express
  MX716 (Mono) 8.6 x 8.3um, 752 x 580 px, 6.47 x
  4.83mm, 0.2kg, 895SBIG ST-7XME, 9 x 9 um,
  765 x 510 px, 6.9 x 4.9 mm, 0.9 kg, 1995
  (US)
• Large Format CCD Cameras Starlight Express HX916
  (Mono) 6.7 x 6.7um, 1300 x 1030 px, 8.71 x
  6.9mm, 0.25 kg, 1345SBIG ST-9X 20 x
  20um, 512 x 512 px , 10.2 x 10.2 mm 3195
  (US)SBIG ST-8XME, 9 x 9 um, 1530 x 1020 px,
  13.8 x 9.2 mm, 0.9 kg, 5995 (US)
• Very Large Format CCD CamerasStarlight Express
  SXV-M25 (Col) 7.8 x 7.8um, 3000 x 2000 px,
  23.4 x 15.6mm, Spring 2004SBIG STL-11000CM 9
  x 9 um, 4008 x 2745 px, 36 x 24.7mm (26 sec
  download) 8995 (US)

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Considerations when choosing a CCD Camera

• Chip Size / Pixel Size / Number of Pixels / Pixel
  Shape
• Match with Telescope Focal Length
• Sensitivity of CCD
• Dark Current / Read Noise
• Cooling / Temperature Regulation / Shutter
• Digitisation (12 bit/ 16 bit)
• Linearity of CCD / Capacity of a pixel
• Anti-Blooming (ABG vs NABG)
• CCD Quality / Defective Pixels
• Camera Weight / Size
• Binning / Windowing Capabilities
• Download Speed, USB / Parallel
• Self Guiding Capabilities
• Single Shot Colour / Filter Wheel attachment
• Software
• Cost
• Reliability / Support

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Example Spectral Response Curves
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CCD Chip Sizes Compared with 35mm Film
TC211
KAF0400
ST7
KAF1600
ST8
New Large Format Cameras
SLR Camera
35mm film
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Matching CCD and Telescope (1)

• Calculating Image Scale (arc secs per
  pixel)Image Scale 206 x pixel size (in
  um) ---------------------
  focal_length (in mm)
• e.g for SBIG ST-7 and 8 f/10 SCT Pixel Size
  9 um Focal length 25.4 x 8 x 10 2032
  mm Image Scale at 1x1 binning 206 x 9 / 2032
  0.9 arc sec/pixel Image Scale at 2x2 binning
  206 x 18/2032 1.8 arc sec /pixel
• Typical seeing is 2-4 arc sec, so 2x2 binning
  (1.8 arc sec/pixel) is about right (At 2x2,
  sensitivity is better and downloads are much
  faster, but images are only 382 x 255)1x1
  binning only really of benefit when imaging
  planets when there is benefit in sampling at lt1
  arc sec, and there is opportunity to benefit from
  brief moments of exceptional seeingWith Focal
  Reducer (63) 1x1 binning 1.3 arc sec/pixel,
  2x2 binning 2.5 arc sec/pixel
• General rule chose CCD (or choose Telescope)
  that gives around 2 arc sec /pixel

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Matching CCD and Telescope (2)

• Calculating Field Of ViewField (Horizontal) in
  arc mins Image Scale x No. pixels
  (horizontal) / 60Field (Vertical) in arc mins)
  Image Scale x No. pixels (vertical) / 60
• e.g for SBIG ST-7 and 8 f/10 SCT Pixel Size
  9 um, Focal length 25.4 x 8 x 10 2032
  mm Image Scale at 1x1 binning 206 x 9 / 2032
  0.9 arc sec/pixel (765 x 510)
• Field (Horizontal) 0.9 x 765/60 11.4 arc
  min Field (Vertical) 0.9 x 510/60 7.7 arc
  min
• With focal reducer (63) Image Scale at 2x2
  2.5 arc sec/pixel (382 x 255)
• Field (Horizontal) 2.5 x 382/60 15.9 arc
  min Field (Vertical) 2.5 x 255/60 10.6 arc
  min
• General rule Dependant of proposed Targets
  chose a Camera with a larger dimension CCD to
  gives a larger FOV (price will be a
  limitation).Alternatively select a low focal
  ratio telescope (eg f/4) or use a focal reducer

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CCD Cameras with ordinary Camera Lens

• CCD Cameras can also be used piggy-backed to a
  Telescope and fitted with ordinary camera lenses.
  This can provide wider fields of viewImportant
  to use Good Quality Lenses
• ST7e with 200mm lens

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Long Exposures / Guiding (1)

• Unless a scope is perfectly polar aligned and has
  perfect tracking, stars will trail on long
  exposures (at focal length of 2000mm this might
  be observed after only 2 mins exposure)
• Two main solutions to the problem- Take short
  (60 sec) exposures, then align stack- Guide
  the telescope during the exposure

Simulated unguided imageof M5112 min exposure
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Long Exposures / Guiding (2)

• CCD manufactures have developed several
  alternative guiding solutions
• Track and Accumulate (SBIG)
• Separate CCD Camera (e.g Meade)
• Self Guided (SBIG)
• Star2000 (Starlight Express)

Off-Axis
Finder
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Colour Imaging (1) Single-Shot Cameras
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Colour Imaging (2) Using Filters
Colour Filter Wheel
SBIG CFW-8A Red, Blue, Green, Clear
Filters Option to take and image in other
filter bandse.g UBRVI for photometry
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Colour Imaging with Filters
Blue (Av. 3x20s)
Red (Av. 3x10s)
Green (Av. 3x10s)
Colour Image (LRGB)
Luminance (Av. 6x10s)
M42(Orion)
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CCD Imaging compared with Eyepiece Viewing

• ve
• Can see fainter objects (i.e. can see objects
  impossible to see with the naked eye)
• Much easier to record and share what has been
  seen
• Can generally see more detail in objects
  (particularly nebula)
• Can find and locate objects more quickly (with
  appropriate software)
• Can even view from the leisure of indoors (with
  remote connection)
• Can playback /animate motion of slowly moving
  objects (eg Pluto)
• Can acquire the colour of faint objects (ones
  which look grey to naked eye)
• Can undertake more accurate (certainly easier)
  astrometry and photometry
• -ve
• Some objects more impressive with naked eye(eg
  red/blue double star , Jupiter moons)
• Loose some of that 3D effect feelings of awe
• Difficulty of claiming one actually saw /
  observed the object
• Realtime CCD images are often very noisy

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Typical realtime CCD image compared with
Eyepiece View
CCD (raw image on screen)
Eyepiece View
M51 (Ursa Major)1 min exposure
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CCD Comparisons with Film

• ve
• CCD Images immediately available (no waiting on
  film lab)
• Digital (no need to scan in order to process
  further),Easier manipulation - ability to stack
• Light record is linear (no recripicty)
• With suitable software the image can be used to
  automatically locate telescope position or to
  guide the telescope.
• -ve
• Smaller image area FOV (typically only 20 that
  of 35mm film)

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Comparisons of CCD Images with Film and Eyepiece
Observations
CCD
Film
Recording of naked eye observation
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Use and Sharing of CCD Images
Astronomical Records
World Wide Web
Presentations
Own records
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CCD Images (2001-2004)
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Moon
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Moon Apollo 17 Landing Site
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Planets
Venus 2004
Mars 2003
Jupiter 2003
Saturn 2001
Uranus 2002
Pluto 2003
Neptune 2002
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Jupiter / Saturn / Uranus Moons
Six of Saturn's moons appear in this CCD Image
(2 sec exposure)
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Asteroids (Minor Planets)
Animated Sequence of 10 CCD Images of Minor
Planet Kleopatra (216)The animation records 58
arc sec motion of the minor planet over a period
of 1 hr 56 min ( 30 arc sec/hour).
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Near Earth Asteroid
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Comets
Comet C/2000 WM1 (LINEAR) 2001-Nov(passing
through star field in Aries)
C/2002 T7 (Linear)2004-Feb(passing through
star field in Pegasus)
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Clusters in Gemini (CCD Mosaic)
M35
NGC 2158
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M45 Pleiades (CCD Mosaic)
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Double Cluster In Perseus (7 x 6 CCD Mosaic, 20s
exposures)
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Globular Cluster
M15 (Pegasus), 6 x 10s
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Extra-Solar Planets ?
HD 209458 (Pegasus) has a transiting Jupiter mass
short period extrasolar planet.(HD 209458-b).
Every 3.5 days, the planet produces a dimming of
the star of 1.7 that lasts for about 3 hours.
The dimming has been detected by Castellano and
Laughlin using almost identical equipment to me
(ie 8" telescope and ST-7E CCD camera), which
presents me the opportunity to also have a go at
trying to detect a extra-solar planet lying at a
distance of  1.45 x 1015 km (153 light years)
from Earth..
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Nebula
M57 Ring Nebula (Lyra)
M16 Eagle Nebula (Serpens Caput)
M27 Dumbbell Nebula (Vulpecula)
NGC 2261 - Hubble's Variable Nebula   (Monoceros)
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Galaxies
NGC 4567 / 4568(Virgo)
NGC 7331 (Pegasus)
M100 (Coma Berenices)
M105 (Leo)
M64 Black-eye Galaxy(Coma Berenices)
NGC 2903 (Leo)
NGC 2903 Spiral Galaxy
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Galaxy Cluster
NGC 7320 Galaxy Cluster (Stephan's Quintet,
Andromeda)
The 5 main galaxies range from magnitude 13.6 to
14.8 Faintest galaxy in image is 16.6
2002-10-02  2144 to 2151h UTCCD Image, 2 x 2
min exposure (2x2 binning)11.4 x 7.6 arc min 
(28003 28005)
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Supernova / Supernova Remnants
M1, Crab Nebula
SN 2001ib, 2001-Dec
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Colour Imaging - 2004
M42 Orion
NGC 2392 Planetary Nebula (Eskimo or Clown Face
Cluster)
NGC 2903 Spiral Galaxy, Leo
Jupiter
NGC 1857, Auriga
Saturn
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More Recent Images
NGC 3628 Spiral Galaxy, Leo
M63 Spiral Galaxy (Sunflower Galaxy)
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M65 Area
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Out-takes (1)
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Out-takes (2)