Santa Barbara Instrument Group CCD Camera Operating Manual

Contents

Santa Barbara Instrument Group CCD Camera Operating Manual Manual pdf 9 page image

Section 2 - Introduction to CCD CamerasPage 52. Introduction to CCD CamerasThis section introduces new users to CCD (Charge Coupled Device) cameras and theircapabilities and to the field of CCD Astronomy and Electronic Imaging.2.1. Cameras in GeneralThe CCD is very good at the most difficult astronomical imaging problem: imaging small, faintobjects. For such scenes long film exposures are typically required. The CCD based systemhas several advantages over film: greater speed, quantitative accuracy, ability to increasecontrast and subtract sky background with a few keystrokes, the ability to co-add multipleimages without tedious dark room operations, wider spectral range, and instant examination ofthe images at the telescope for quality. Film has the advantages of a much larger format, color,and independence of the wall plug (the SBIG family of cameras can be battery operated inconjunction with a laptop computer, though, using a power inverter). After some use you willfind that film is best for producing sensational large area color pictures, and the CCD is best forplanets, small faint objects, and general scientific work such as variable star monitoring andposition determination.2.2. How CCD Detectors WorkThe basic function of the CCD detector is to convert an incoming photon of light to an electronwhich is stored in the detector until it is read out, thus producing data which your computercan display as an image. It doesn't have to be displayed as an image. It could just as well bedisplayed as a spreadsheet with groups of numbers in each cell representing the number ofelectrons produced at each pixel. These numbers are displayed by your computer as shades ofgray for each pixel site on your screen thus producing the image you see. How this isaccomplished is eloquently described in a paper by James Janesick and Tom Elliott of the JetPropulsion Laboratory:"Imagine an array of buckets covering a field. After a rainstorm, the buckets aresent by conveyor belts to a metering station where the amount of water in eachbucket is measured. Then a computer would take these data and display apicture of how much rain fell on each part of the field. In a CCD the"raindrops" are photons, the "buckets" the pixels, the "conveyor belts" the CCDshift registers and the "metering system" an on-chip amplifier.Technically speaking the CCD must perform four tasks in generating an image.These functions are 1) charge generation, 2) charge collection, 3) charge transfer,and 4) charge detection. The first operation relies on a physical process knownas the photoelectric effect - when photons or particles strikes certain materialsfree electrons are liberated...In the second step the photoelectrons are collected inthe nearest discrete collecting sites or pixels. The collection sites are defined byan array of electrodes, called gates, formed on the CCD. The third operation,charge transfer, is accomplished by manipulating the voltage on the gates in asystematic way so the signal electrons move down the vertical registers fromone pixel to the next in a conveyor-belt like fashion. At the end of each columnis a horizontal register of pixels. This register collects a line at a time and then