Webcams and CCTV Cameras: Affordable Imaging Tools for the Amateur Astronomer Giles Hammond Institute for Gravitational Research University of Glasgow

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Webcams and CCTV Cameras: Affordable Imaging Tools for the Amateur Astronomer Giles Hammond Institute for Gravitational Research University of Glasgow Glasgow Astronomical Society 18 th September 2008 Slide 2 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 Slide 3 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 0.5eV Astronomical CCDs vs unmodified webcams 9.8x6.35.6x5.69.8x6.3 Pixel size ( m) 1/25s >1 hourMax Exposure 40 400-650Price NoYesYes/NoColour 8 Mono No 6.0x5.0 ICX255 AL 1004X 24 Colour (8 RGB)16 MonoBits NoYes (-30 o C ambient)Cooled 4.6x4.06.0x5.0CCD size (mm) ICX098 ALICX055 ALCCD type Toucam 840Starlight Express MX5Camera Slide 9 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 Slide 10 1004X Board Front view Back view 32mm +12V Video out 0V Timing generator Power supply Slide 11 Long Exposure Modification +5V Pin 20 Shutter control Pin 5: Vertical clock point 1 Vertical clock Point 2 Track linking clock pulses Slide 12 +5V Switch open: shutter operating Shutter Disable +5V Switch closed: shutter disabled (+5V) Slide 13 Switch open (clocking disabled) Switch closed (clocking enabled) 1s 2s 4s 10s Clock Disable/Enable Slide 14 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 Slide 15 Amp Off Modification Unsolder pin 9 and attach a wire 10k Switch open: CCD voltage 9V and amplifier shuts down Charge containment remains active 10k Switch closed: CCD voltage = 15V Pin 9 (CCD voltage) Slide 16 Astronomical CCDs vs modified webcams few minutesSeveral hoursMax Exposure NoYes/NoColour 8 (stacking can give 12) Can be!! 6.5x4.5 ICX255 AL 1004X Webcam 16Bits Yes (-30 o C)Cooled 6.5x4.5CCD size (mm) ICX055 ALCCD type Starlight ExpressCamera Astronomical Slide 17 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 Image Processing Slide 18 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 Image Processing Noise is random but signal is coherent. Therefore the image quality improves with increased number of exposures or exposure time If we obtain N photons/s during an exposure time, t, then Signal-Noise-Ratio (SNR) improves with increased exposure With webcams you typically take may short exposures (t