Webcams and CCTV Cameras: Affordable Imaging Tools for the Amateur Astronomer

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Webcams and CCTV Cameras Affordable Imaging
Tools for the Amateur Astronomer

• Giles Hammond
• Institute for Gravitational Research
• University of Glasgow

DACE 'Imaging the Night Sky' 31st January
2009 10am-4pm
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Outline

• How a CCD works
• Astronomical CCDs vs unmodified cameras
• How to modify a CCTV camera
• Astronomical CCDs vs modified webcams!!!!
• Basic Image Processing
• Dark, Flat and Bias frames
• Stacking
• All Sky Camera at the University of Glasgow
• Some results
• Useful Links
• Conclusions

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How a CCD works

• A CCD comprises an array of light sensitive
  silicon sites (pixels) of typical dimensions 6?m
  x 6?m
• A photon of suitable energy hitting the pixel can
  produce an electron that is constrained within
  the pixel using electric fields
• Typical energy response 0.5eVltElt3eV
  (440nmlt?lt1000nm)
• Typical well capacity is approximately ?50k-150k
  electrons

http//www.microscopyu.com/articles/digitalimaging
/ccdintro.html
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• At the end of the exposure the electric fields
  holding the charge are clocked at about 60Mhz
  to move the charge
• Vertical/Horizontal clocks shift the charge
  down/across to the charge amplifier

http//www.microscopyu.com/articles/digitalimaging
/ccdintro.html
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• The resulting voltage output from the charge
  amplifier is then digitised into a number of
  bits by the analogue-to-digital converter (ADC)
• 16 bits means 21665536 levels from black-white
  (smoother image)
• 8 bits means 28256 levels from black-white
  (coarser image)

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Astronomical CCDs vs unmodified webcams
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Astronomical CCDs vs unmodified webcams
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Astronomical CCDs vs unmodified webcams
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How to modify a Camera

• Webcams are limited to a maximum exposure 1/25s
  (okay for planets but not good for imaging
  nebulae/galaxies)
• The CCD is continually clocked and a shutter
  operates in bright conditions
• In order to take long exposures we need to
  disable the shutter/CCD clocks by applying
  suitable voltages/adding switches which can be
  opened/closed (Steve Chambers/Jon Grove etc.
  1996)
• We can control the switch with software which
  then picks the exposed frame when the switch is
  closed (ASTROVIDEO, K3CCD)
• Lets look at the 1004X CCTV camera (the
  modification principle is similar for the Toucam
  840 or SPC900NC)

http//www.qcuiag.co.uk Quick Cam and
Unconventional Imaging Astronomy Group
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1004X Board
?32mm
Power supply
Video out
Timing generator
12V
0V
Back view
Front view
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Long Exposure Modification
Vertical clock Point 2
5V
Pin 20 Shutter control
Pin 5 Vertical clock point 1
Track linking clock pulses
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Shutter Disable
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Clock Disable/Enable
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Hot Pixels/Amplifier Glow

• When we take a long exposure (with lens cap on)
  we get this
• 10s 40s 60s
• Hot pixels are due to an uncooled CCD
• The bright corner is due to an on-chip amplifier
  that produces infra-red radiation
  (electroluminesence)
• The amplifier can be disabled by reducing the
  voltage to the CCD chip during integration

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Amp Off Modification
Unsolder pin 9 and attach a wire
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Astronomical CCDs vs modified webcams
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Image Processing

• Typically we need 3-4 types of exposure per
  object
• The image frame
• Dark frame (hot pixels)D
• Flat frame (non-uniform pixels, vignetting)F
• Bias frame (0s exposure for readout noise)B
• Typically I dont worry about bias frames
• Final image (I-D-B)/(F-D-B) ? (I-D)/(F-D) for
  webcams

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Image Processing

• Typically we need 3-4 types of exposure per
  object
• The image frame
• Dark frame (hot pixels)D
• Flat frame (non-uniform pixels, vignetting)F
• Bias frame (0s exposure for readout noise)B
• Typically I dont worry about bias frames
• Final image (I-D-B)/(F-D-B) ? (I-D)/(F-D) for
  webcams

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Image Correction 1
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Image Correction 2
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All Sky Camera at Glasgow

• Developed from a project run for two 3rd year
  students studying Physics at the University of
  Glasgow
• Aim is to develop an all sky camera based on the
  1004X modification capable of taking images
  throughout the day/night
• The camera uses a 1.6mm fisheye lens attached to
  the front of the 1004X board to produce a
  1800x1400 field of view

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All Sky Camera at Glasgow
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Control Software

• Camera control is based on MATLAB (a powerful
  mathematics software which also includes image
  acquisition tools)
• The control program has been turned into a
  standalone executable (quite large) which can run
  on any Windows PC

• Begin loop
• Take an image of 0.1s
• Sum up the intensity in the image
• Determine whether the sun is out (saturated
  pixels)
• Adjust exposure length
• Take 10 images
• Remove hot pixels using a pixel map
• Sum up images
• Save as a JPEG
• End loop
• Combine images into an AVI after 90 frames (and
  start again)

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• The cameras can be used as weather monitors,
  satellite trackers, meteor detectors and will
  show plenty of interesting astronomical/meteorolog
  ical events
• The hardware is now at a fairly stable design but
  the software needs some work (a windows
  executable would be much better than the MATLAB
  executable)
• The plan is to apply for research council funding
  to produce some cameras in kit form and
  distribute them to schools etc
• The next stage is to install one on the western
  coast of Scotland on Islay

?100 miles
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Example AVIs
60s delay between images
900s delay between images
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1004X Images (Prime Focus)
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(No Transcript)
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Useful Links
1004X distributor http//www.rfconcepts.co.uk/boa
rd_camera.htm 1004X modifications
http//www.geocities.com/jgroveuk/ExViewMod.html
http//www.leadbeaterhome.fsnet.co.uk/1004xcam.htm
http//www.saao.ac.za/wpk/1004x/index.html Phi
lips SPC900NC distributors http//www.amazon.co.u
k/gp/product/B000BEY4DE?ieUTF8tagkm_10000-21gc
lidCIK-84eZipQCFQ8gQgodGBYgXA http//www.kikatek.
com/product_info.php?products_id19218sourcefroo
gle http//www.pcworld.co.uk/martprd/store/pcw_pag
e.jsp?BV_SessionID_at__at__at__at_1823582525.1214211267_at__at__at__at_B
V_EngineIDccffadeegkihhffcflgceggdhhmdgmh.0page
Productfmnullsmnulltmnullsku638873categor
y_oid Philips SPC900NC modifications http//hom
e.zonnet.nl/m.m.j.meijer/D_I_Y/spc900nc.htm
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Useful Links
Useful source of Electronic Components (can buy
over the counter)) Maplin Electronics
http//www.maplin.co.uk/ Software AstroVideo
http//www.coaa.co.uk/astrovideo.htm K3CCD
Tools http//www.pk3.org/Astro/ Registax
http//www.astronomie.be/registax/ Yahoo Group
for modified cameras QCUIAG http//www.megastron
omy.150m.com/New-QCUIAG/ Webcam adaptor
http//www.telescopehouse.com/cgi-bin/sh000001.pl?
REFPAGEhttp3a2f2fwww2etelescopehouse2ecom2f
acatalog2fOnline_Catalogue_Camera_Adaptors_152eh
tmlWDwebcamPNWebcam_Adaptor_1_25__2ehtml23aA
C378aAC378 http//www.moggadapters.com/astro/ada
pter.asp
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Conclusions

• Webcams and surveillance cameras offer a cheap
  alternative to astronomical CCDs
• The results can be comparable
• The only limitation is bit depth which can be
  partially overcome by stacking/mosaicing
• The exposures are usually short which makes
  guiding/accurate polar alignment less critical
• Narrow band imaging can offer a method of imaging
  under light polluted skies (H?, SII and OIII)
• Faint nebula are still within reach under
  suburban skies
• Galaxies are harder targets due to their broad
  emission