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Institute for Gravitational Research

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Presentation on theme: "Institute for Gravitational Research"— Presentation transcript:

1 Institute for Gravitational Research
Webcams and CCTV Cameras: Affordable Imaging Tools for the Amateur Astronomer Giles Hammond Institute for Gravitational Research University of Glasgow Rugby Astronomical Society 22nd June 2008

2 Outline How a CCD works Astronomical CCD’s vs unmodified cameras
How to modify a CCTV camera Astronomical CCD’s 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

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<E<3eV (440nm<<1000nm) Typical well capacity is approximately 50k-150k electrons Pixel Structure H 656nm

4 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

5 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 216=65536 levels from black-white (smoother image) 8 bits means 28=256 levels from black-white (coarser image) 256 16 5 3

6 Astronomical CCD’s vs unmodified webcams
Yes Yes/No Colour ICX255 AL 1004X ICX098 AL ICX055 AL CCD type Toucam 840 Starlight Express MX5 Camera

7 Astronomical CCD’s vs unmodified webcams
9.8x6.3 5.6x5.6 Pixel size (m) No Yes Yes/No Colour 6.0x5.0 ICX255 AL 1004X 4.6x4.0 CCD size (mm) ICX098 AL ICX055 AL CCD type Toucam 840 Starlight Express MX5 Camera

8 Astronomical CCD’s vs unmodified webcams
9.8x6.3 5.6x5.6 Pixel size (m) 1/25s 1/5s several hours Max Exposure £40 £ Price No Yes Yes/No Colour 8 Mono 6.0x5.0 ICX255 AL 1004X 24 Colour (8 RGB) 16 Mono Bits Yes (-30oC ambient) Cooled 4.6x4.0 CCD size (mm) ICX098 AL ICX055 AL CCD type Toucam 840 Starlight Express MX5 Camera

9 How to modify a Camera
Webcams are limited to a maximum exposure of about 1/5s (okay for planets but not good for imaging nebulae/galaxies) The CCD is continually clocked (5-25 frames/s) 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) Quick Cam and Unconventional Imaging Astronomy Group

10 1004X Board 32mm Video out Timing generator Power supply +12V 0V
Back view Front view

11 Long Exposure Modification
Vertical clock Point 2 +5V Pin 20 Shutter control Pin 5: Vertical clock point 1 Track linking clock pulses

12 Shutter Disable +5V +5V Switch closed: shutter disabled (+5V)
Switch open: shutter operating +5V Switch closed: shutter disabled (+5V)

13 Clock Disable/Enable 1s 2s 4s Switch closed (clocking enabled)
Switch open (clocking disabled)

14 Hot Pixels/Amplifier Glow
When we take a long exposure (with lens cap on) we get this 10s s s 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

15 Amp Off Modification 10k 10k Pin 9 (CCD voltage) Unsolder pin 9 and
Switch open: CCD voltage  9V and amplifier shuts down Charge containment remains active 10k Switch closed: CCD voltage = 15V Pin 9 (CCD voltage) Unsolder pin 9 and attach a wire

16 Astronomical CCD’s vs modified webcams
Up to 10 mins Several hours Max Exposure No Yes/No Colour 8 (stacking can give 12) Can be!! 6.5x4.5 ICX255 AL 1004X Webcam 16 Bits Yes (-30oC) Cooled CCD size (mm) ICX055 AL CCD type Starlight Express Camera Astronomical

17 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 don’t worry about bias frames Final image = (I-D-B)/(F-D-B)  (I-D)/(F-D) for webcams 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<60s) and stack them together to reduce noise Signal=N*t Noise=(N*t) SNR= N*t / (N*t)= (N*t)

18 Image Correction 1 Raw image Dark corrected image (I-D) Raw flat
Dark corrected flat (F-D)

19 Image Correction 2 Flat corrected image (I-D)/(F-D)
Dark corrected image Single exposure Sum of 44

20 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

21 All Sky Camera at Glasgow

22 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)

23 The cameras can be used as weather monitors, satellite trackers, meteor detectors and will show plenty of interesting astronomical/meteorological 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

24 Example AVI’s 60s delay between images 900s delay between images

25 1004X Images (Prime Focus) Barnard 33 10” f4.8 Newtonian
44x16s (H filter) M1 10” f4.8 Newtonian 32x16s (H filter) M1 10” f4.8 Newtonian 40x16s (Meade UHC filter) M81 10” f4.8 Newtonian 40x16s (Meade UHC filter) M82 10” f4.8 Newtonian 50x16s (Meade UHC filter) M81 135mm lens f2.4 40x8s (Meade UHC filter)

26 Barnard 33 mosaic 10” f4.8 Newtonian 58 mins total (H filter) Barnards loop 50mm f2.4 35x8s (H filter) B33+flame 200 1/30s stacked M51 10” f4.8 Newtonian 30 mins total NGC 1499 50mm f2.4 300s total (H filter)

27 Useful Links 1004X distributor
1004X modifications Philips SPC900NC distributors Philips SPC900NC modifications

28 Useful Links Useful source of Electronic Components (can buy over the counter)) Maplin Electronics: Software AstroVideo: K3CCD Tools: Registax: Yahoo Group for modified cameras QCUIAG:

29 Conclusions Webcams and surveillance cameras offer a cheap alternative to astronomical CCD’s 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

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