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1、CharacterizationofaCCDThephysicalpropertiesandfunctionofaCCDcameraaredescribedbythetermsreadnoise,darkcurrent,signal-to-noise,bias,quantumefficiency,gain,dynamicrangeandafewothers.Thefollowingwillattempttodemystifythesetermsandgiveabetterunderstandingofwhattheymean.Readou
2、tNoise(orReadNoise)Readnoisecomesfromtheelectronicsusedto“read”theCCD.Therearetwocomponentstoreadnoise:1)theconversionfromvoltage(ananalogsignalthatisactuallywhatisreadfromeachpixel)toadigitalnumber,and2)passageofthesignalthroughanamplifierandanalogtodigitalconverter.Asan
3、exampleoftheconversionfromvoltagetodigitalnumbers,letssayyouarereadingthesamepixeltwice,eachtimewithanidenticalcharge.Youwillgetaslightlydifferentdigitalnumbereachtime.Thisisbecauseyoucannevermeasureanythingexactly.Thereisastatisticaldistributionofpossibleanswerscenteredo
4、nameanvalue.Second,theelectronicsthemselveswillintroducespuriouselectronsintotheentireprocess,generatingunwantedfluctuationsintheoutput.Thesetwoeffectscombinetoproduceanuncertaintyinthefinaloutputvalueforeachpixel.Theaverageofthisuncertaintyiscalledthereadnoise.Soiftherea
5、dnoiseofyourCCDis20electrons,eachpixelwill,onaverage,contain20extraelectronseachtimetheCCDisread.GainTheoutputvoltagefromagivenpixelisconvertedtoadigitalnumberduringreadout.Theamountofvoltageneeded(whichtranslatesintothenumberofcollectedelectronsorreceivedphotons)toproduc
6、e1count(alsocalledananalog-to-digitalunit,ADU)iscalledthegainoftheCCD.Atypicalgainmightbe10electrons/count,whichmeansthatforevery10electronscollectedwithinaphotosite,thatpixelwillproduce,onaverage,1count.TaketwobiasframesandtwoflatfieldframeswitheithertheAp7portheST-2000X
7、MCCD.1)Determinethemeanpixelvaluewithineachbiasandeachflatfield.2)Nowcropthetwoflatfieldstoremovethedarkcorners.Calculatethemeanpixelvaluesforthetwocroppedflatfields.ImageMeanPixelValueFlat1Flat2Bias1Bias2Flat1croppedFlat2croppeda.Whatisthedifferencebetweenthemeanpixelval
8、uesforeachuncroppedandcroppedflatimage?b.Howdoyouexplainthisdifference?2)Nowcreatethreedifferenc
