CCD CHARGE TRANSFER EFFICIENCY (CTE) DERIVED FROM SIGNAL VARIANCE IN FLAT FIELD IMAGES The CVF method

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1 CCD CHARGE TRANSFER EFFICIENCY (CTE) DERIVED FROM SIGNAL VARIANCE IN FLAT FIELD IMAGES Th CVF mthod Fbrc Chrstn 1, Konrd Kujkn 1,, Dtrch Bd 3, Cyrl Cvdor 4, Sbstn Drs 3, Olf Iwrt 3. 1 Kptyn Astronomcl Insttut, Gronngn, Th Nthrlnds; Strrwcht Ldn, Ldn, Th Nthrlnds; 3 Europn Southrn Obsrvtory, Grchng, Grmny; 4 Thls, Frnc Abstrct: Ky words: Ths ppr dscrbs novl tchnqu for stmtng th CTE of CCD. It s bsd on th chng n vrnc wth CCD row or column n smpl flt fld mgs, nd uss th fct tht mprfct chrg trnsfr durng rdout hs smoothng ffct on th fnl mg. Th dt usd to tst th procdur r cqurd wth th ESO tst bnch, n th contxt of chrctrzng th OmgCAM CCDs. For nn CCDs rsults from th CTE msurmnts by v nd wth th vrnc bsd tchnqu dvlopd n ths ppr wr comprd. Rsults r promsng. Ths tchnqu, stll undr dvlopmnt, provd rlbl nd cn b usd for smpl nd ffcnt CTE msurmnts. Chrg-Coupld-Dvc chrctrzton, Chrg Trnsfr Effcncy. 1. INTRODUCTION Durng th tsts of th OmgCAM 1 CCDs (Chrstn 00), flt mgs hv bn cqurd usng th ESO tst bnch (Amco 1996) to chck out th chps cosmtcs. Ths ppr wll dscrb how to us ths dt to msur lso th chrg trnsfr ffcncy. Ths prmtr, rportd n unts of prcnt, chrctrzs th ffcncy to trnsfr corrctly chrgs from on 1 OmgCAM s on squr dgr wd fld mgr whch wll b mountd on th VST tlscop n Prnl, Chl, (Kujkn 004 & Iwrt 005, Scntfc Dtctor Workshop 005, Th OmgCAM 16k x 16k CCD Dtctor Systm for th ESO VLT )

2 Fbrc Chrstn1, Konrd Kujkn1,, Dtrch Bd3, Cyrl Cvdor4, Sbstn Drs3, Olf Iwrt3. pxl to ts nghbor. It wll b xtrctd from th study of th vrnc of th sgnl msurd n thm. Aftr dscrpton of th Chng n Vrnc n Flt fld (CVF) mthod n Scton, th frst rsults wll b prsntd n Scton 3. Th prlmnry conclusons r n scton 4.. CHANGE IN VARIANCE IN FLAT FIELD (CVF) METHOD Instd of studyng th loss of sgnl lk n th EPER mthod (Jnsck 001), th loss of vrnc cross flt fld mgs wll b nlyzd. Durng th trnsfr of chrgs, flt fld mgs wll b smoothd du to th chrg loss. Th sgnl n th flt fld rmns constnt durng th trnsfr but th vrnc from on ln (column) to th othr wll dcrs s soon s th CCD s rd. S blow th ffct of n mprfct trnsfr on th vrnc: ( ) = N + b N + + b N (1) 1 1 ( ) = N O( b ) () + ( ) : Vrnc of th sgnl n - of th ln N : Numbr of lctrons : Chrg Trnsfr Effcncy, CTE b = 1 b O b << N b : Chrg Trnsfr Inffcncy, CTI, ( ) O ( ) : Rsdul, ( ) By usng th CTI b = 1, nd consdrng tht b << 1, th trm b 1 b + O b nd Eq. () cn b wrttn s follows: ( ) 1 bcoms ( ) ( ) = N bn O( b ) (3) + Equton (3) cn b xprssd n Anlog Dgtl Unt (ADU) nstd of lctrons. Th sgnl nd th vrnc, msurd n lctrons, r rltd to th sgnl nd vrnc rspctvly n ADU by: N = gn nd g = (4)

3 CCD Chrg Trnsfr Effcncy (CTE) Drvd from Sgnl Vrnc In Flt Fld Imgs 3 g N : Gn n lctrons pr ADU : Numbr of ADU : rms nos n ADU Equton (3) bcoms: ( ) = b O( b ) (5) : rms nos n ADU bfor ny chrg shft Th Eq. (5) shows lnr dpndnc btwn th vrnc of th sgnl n ln (column) nd th ln numbr. To stmt ths quton, w wll us th bst ft ln of th st of ponts (, ( ) ). To msur th vrnc of ch ln, two flts (sm lvl) nd two bss r tkn. Th bs mgs r subtrctd from th flts nd ths two nw mgs r dvdd, on by th othr. Th rsult s multpld by th mn of on of th mgs. Th fnl rsult wll gv n mg (R) wth fxd pttrn nos (Photon Rspons Non Unformty, PRNU) flt fldd out. Th totl nos (n pxl unts, ADU) n ths mg R wll b composd of photon nos nd rd out nos ssntlly. To msur th photon nos, th rd out nos s subtrctd from th totl nos. To crt th plot vrnc of lns (columns) vrsus lns (columns), w msur th vrnc for ch ln (column) of R. Ths vlu s dvdd by to hv th vrnc of on ln nd th pont (, ( ) ) s plottd, s Fgur 1 for xmpl. Durng th clculton of th vrnc, bd pxls r lmntng by usng th sgm clppng mthod. Th bst ft ln wll gv us: ( ) = µ ν (6) µ : y-xs ntrcpt (n ADU ) ν : slop of th ln Th constnt trm µ wll gv us th vrnc of th sgnl bfor bng Ths prmtr s msurd by subtrctng two bs mgs. Th vrnc of th pxls n th dffrnc mg s qul to two tms th rd out nos squrd. Th vrnc from ths mg cn b drctly subtrctd from th vrnc msurd n th mg R to hv only two tms th photon nos squrd n ADU.

4 4 Fbrc Chrstn1, Konrd Kujkn1,, Dtrch Bd3, Cyrl Cvdor4, Sbstn Drs3, Olf Iwrt3. Fgur 1. Img A nd B show th plot (, ( ) ) t dffrnt CTEs wth th bst ft ln n rd. In A, th CTE s nd n B, Ths plots hv bn don wth smultd dt. Th dmnsons of th mgs r 1k x 1k. Th numbr of lctrons n th mgs s 1000 plus Posson nos. Th CTE s ppld ftr. Smlr pttrn r obsrvd wth rl dt. In th x-xs, th ln count ncrss wy from th rdout rgstr. ffctd by th chrg trnsfr nffcncy nd th slop,ν, dvdd by µ, th chrg trnsfr nffcncy. Th chrg trnsfr ffcncy,, s thn: ν = 1 (7) µ 3. RESULTS 3.1 Smultd Dt Th CVF mthod hs bn tstd on smultd dt. Two mgs (1k x 1k) wth mn ntnsty of nd photon nos of 1000 hv bn crtd. For ch st of mgs dffrnt CTEs hv bn ppld (s column on nd four of th Tbl I). Th procdur, bsd on th study of th vrnc nd dvlopd scton s usd to xtrct th horzontl nd vrtcl CTE. Th frst tsts wth ths smultd dt show, Tbl 1, tht th CVF mthod pproxmts xtrmly wll th CTEs. Th msurmnts r n ccordnc wth th orgnl vlus of th CTE.

5 CCD Chrg Trnsfr Effcncy (CTE) Drvd from Sgnl Vrnc In Flt Fld Imgs 5 Tbl I. CTE msurd wth th CVF mthod ppld to smultd dt. CTE CVF CTE CVF H & V CTE H-CTE V-CTE H & V CTE H-CTE V-CTE ± Th frst nd fourth columns r th thortcl CTE n th smultd dt. Columns two, thr, fv nd sx r th horzontl nd vrtcl CTE msurd wth th CVF mthod. 3. Rl Dt Th CVF mthod hs bn tstd on th dt of 9 dffrnt CCDs. For ch CCD two bs nd two flt fld mgs hv bn rcordd. For 5 CCDs, ths dt r cqurd usng rd out spd of 5kpx/s nd gn st t ~ /ADU. For 4 CCDs, th dt r tkn wth th sm rd out spd nd gn t ~.5 - /ADU. Th oprtng tmprtur s -10 dgrs. Th CTE from th v dt sht r ncludd n our tbl of rsults for comprson. Ev uss th 55 F mthod (Jnsck 001). Th workng tmprtur s -100 dgrs Clsus nd th rd out spd, 50kpx/s. Th rsults, rportd n Tbl IIA nd Tbl IIB r ncourgng. Almost ll th msurmnts crrd out wth th CVF mthod r consstnt wth th msurmnts from v. Ths rsults should not hd th dffrnc obsrvd for th dvcs nd For ths two CCDs, th CVF mthod s not n ccordnc wth th v rsults. Ths dffrnc my du to trps. In flt flds, th trps r full nd do not ctch chrgs durng th trnsfr whch s not th cs wth mgs tkn wth th 55 F stup. In tht cs th trps ctch mor chrgs nd rduc th CTE. 4. CONCLUSION Ths ppr hs dscrbd n ntrstng ltrntv tchnqu to stmt th Chrg Trnsfr Effcncy of CCD. Ths mthod s bsd on th study of th chng n vrnc wth CCD row or column n smpl flt fld mgs, nd uss th fct tht mprfct chrg trnsfr durng rdout hs smoothng ffct on th fnl mg. Th dt usd to tst th CVF mthod r smultd dt (n dl 1k x 1k CCD wth only chrg trnsfr nffcncy) nd rl dt from th OmgCAM CCDs. Th rsults r vry promsng. Th msurmnts don

6 6 Fbrc Chrstn1, Konrd Kujkn1,, Dtrch Bd3, Cyrl Cvdor4, Sbstn Drs3, Olf Iwrt3. Tbl II. Horzontl nd Vrtcl CTE msurd wth th CVF nd 55 F mthod. CCD H-CTE V-CTE CVF 55 F CVF 55 F A ± B In th prt A, th dt r tkn wth rd out spd of 5kpx/s nd wth gn st t ~ /ADU. In th prt B, th rd out spd s th sm but th gn s st t ~.55 - /ADU. wth smultd dt r n ccordnc wth th thortcl CTE. W obsrv th sm rsults wth lmost ll th msurmnts chvd wth rl dt. In tht cs th CVF mthod s comprd wth th msurmnts don wth 55 F st up. On top of tht, ths mthod ovrcoms th us of dngrous rdoctv mtrl, such s 55 F, tht nds spcl toolng nd ntrnc wndows to prform th msurmnts. Ths tchnqu s stll t ts prlmnry dvlopmnt howvr, convncd by ths frst rsults, th procdur wll b ntnsvly tstd nd mprovd. ACKNOWLEDGEMENTS Fbrc Chrstn would lk to thnk ll th mmbrs of th Optcl Dtctor Tm for thr hlp durng th tsts of th OmgCAM CCDs nd wthout whom h would not hv won th wrd of th Young Achvr durng th 005 Scntfc Dtctor Workshop. Fbrc Chrstn thnks lso OmgCEN dt cntr for provdng ts hlp nd fclts. Rfrncs Amco P. nd Böhm T., 1996, Optcl Dtctors for Astronomy, Kluwr Acdmc Publshrs, Dordrcht, Hollnd, pp Chrstn F. nd l, 00, Scntfc Dtctors for Astronomy, Th bgnnng of Nw Er, Kluwr Acdmc Publshrs, Dordrcht, Hollnd, pp Iwrt O., 005, ths procdngs, Sprngr. Jnsck J. R., 001, Scntfc Chrg-Coupld Dvcs, SPIE prss monogrph, Bllnghm Wshngton. Kujkn K. t l, 004, Procdngs of th SPIE, Volum 549, pp