Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Activity 2 : Use of CCD Cameras.In this activity some of the practical considerations of using and building CCD cameras are described.Simon Tulloch [email protected] SzymanekSpectral Sensitivity of CCDsThe graph below shows the transmission of the atmosphere when looking at objects at the zenith. The atmosphere absorbs strongly below about 330nm, in the near ultraviolet part of the spectrum. An ideal CCD should have a good sensitivity from 330nm to approximately 1000nm, at which point silicon, from which CCDs are manufactured, becomes transparent and therefore insensitive. Wavelength (Nanometers)Transmission of AtmosphereOver the last 25 years of development, the sensitivity of CCDs has improved enormously, to the point where almost all of the incident photons across the visible spectrum are detected. CCD sensitivity has been improved using two main techniques : ‘thinning’ and the use of anti-reflection coatings. These are now explained in more detail.Thick Front-side Illuminated CCDThese are cheap to produce using conventional wafer fabrication techniques. They are used in consumer imaging applications. Even though not all the photons are detected, these devices are still more sensitive than photographic film.They have a low Quantum Efficiency due to the reflection and absorption of light in the surface electrodes. Very poor blue response. The electrode structure prevents the use of an Anti-reflective coating that would otherwise boost performance.The amateur astronomer on a limited budget might consider using thick CCDs. For professional observatories, the economies of running a large facility demand that the detectorsbe as sensitive as possible; thick front-side illuminated chips are seldom if ever used.n-type siliconp-type siliconSilicon dioxide insulating layerPolysilicon electrodesIncoming photons625mAnti-Reflection Coatings 1n of air or vacuum is 1.0, glass is 1.46, water is 1.33, Silicon is 3.6. Using the above equation we can show that window glass in air reflects 3.5% and silicon in air reflects 32%. Unless we take steps to eliminate this reflected portion, then a silicon CCD will at best only detect 2 out of every 3 photons.The solution is to deposit a thin layer of a transparent dielectric material on the surface of the CCD. The refractive index of this material should be between that of silicon and air, and it should have an optical thickness = 1/4 wavelength of light. The question now is what wavelength should we choose, since we are interested in a wide range of colours. Typically 550nm is chosen, which is close to the middle of the optical spectrum. Silicon has a very high Refractive Index (denoted by n). This means that photons are strongly reflected from its surface.nt-nint+ninintFraction of photons reflected at the interface between two mediums of differing refractive indices = [ ]2Anti-Reflection Coatings 2The reflected portion is now reduced to :In the case where the reflectivity actually falls to zero! For silicon we require a material with n = 1.9, fortunately such a material exists, it is Hafnium Dioxide. It is regularly used to coat astronomical CCDs. With an Anti-reflective coating we now have three mediums to consider :ntAirninsAR CoatingSilicon[ ]nt x ni-ns2nt x ni+ns22nsnt2=Anti-Reflection Coatings 3The graph below shows the reflectivity of an EEV 42-80 CCD. These thinned CCDs were designed for a maximum blue response and it has an anti-reflective coating optimised to work at 400nm. At this wavelength the reflectivity falls to approximately 1%.Thinned Back-side Illuminated CCDThe silicon is chemically etched and polished down to a thickness of about 15microns. Light enters from the rear and so the electrodes do not obstruct the photons. The QE can approach 100% .These are very expensive to produce since the thinning is a non-standard process that reduces the chip yield. These thinned CCDs become transparent to near infra-red light and the red response is poor. Response can be boosted by the application of an anti-reflective coating on the thinned rear-side. These coatings do not work so well for thick CCDs due to the surface bumps created by the surface electrodes.Almost all Astronomical CCDs are Thinned and Backside Illuminated.n-type siliconp-type siliconSilicon dioxide insulating layerPolysilicon electrodesIncoming photonsAnti-reflective (AR) coating15mQuantum Efficiency ComparisonThe graph below compares the quantum of efficiency of a thick frontside illuminated CCD and a thin backside illuminated CCD.‘Internal’ Quantum EfficiencyIf we take into account the reflectivity losses at the surface of a CCD we can produce a graph showingthe ‘internal QE’ : the fraction of the photons that enter the CCDs bulk that actually produce a detected photo-electron. This fraction is remarkably high for a thinned CCD. For the EEV 42-80 CCD,shown below, it is greater than 85% across the full visible spectrum. Todays CCDs are very close to being ideal visible light detectors!Appearance of CCDsThe fine surface electrode structure of a thick CCD is clearly visible as a multi-colouredinterference pattern. Thinned Backside Illuminated CCDs have a much planer surface appearance. The other notable distinction is the two-fold (at least) price difference.Kodak Kaf1401 Thick CCD MIT/LL CC1D20 Thinned CCDComputer Requirements 1.Computers are required firstly to coordinate the sequence of clock signals that need to be sent to a CCDand its signal processing electronics during the readout phase, but also for data collection and the subsequent processing of the images.The CCD ControllerIn this first application, the computer is an embedded system running in a ‘CCD controller’. This controller willtypically contain a low noise analogue section for amplification and filtering of the CCD video waveform,an analogue to digital converter, a high speed processor for clock waveform generation and a fibre optic transceiver for receipt of commands and transmission of pixel data. An astronomical system might require clock signals to be generated with time resolutions of a few tens