SMC-Last extracted photometry

SMC-Las Ex ac ed Pho ome y T. A. Kucha 1 , G. C. Sloan 2,3 , D. R. Mizuno 1 , Ka hleen E. K aeme 1 , M. L. Boye 2 , Ma in A. T. G oenewegen 4 , O. C. Jones 5 , F. Kempe 6,7,8 , Iain McDonald 9 , Joana M. Oli ei a 10 , Ma a Sewiło 11,12,13 , Sunda S ini asan 14 , Jacco Th. an Loon 10 , and Albe Zijls a 15 1 Ins i u e o Scien ific Resea ch, Bos on College, 140 Commonweal h A enue, Ches nu Hill, MA 02467, USA 2 Space Telescope Science Ins i u e, 3700 San Ma in D i e, Bal imo e, MD 21218, USA 3 Depa men o Physics and As onomy, Uni e si y o No h Ca olina, Chapel Hill, NC 27599-3255, USA 4 Koninklijke S e enwach an België, Ringlaan 3, B–1180 B ussels, Belgium 5 UK As onomy Technology Cen e, Royal Obse a o y, Black o d Hill, Edinbu gh, EH9 3HJ, UK 6 Ins i u de Ciències de l’Espai (ICE, CSIC), Can Mag ans, s/n, E-08193 Ce danyola del Vallès, Ba celona, Spain 7 ICREA, Pg. Lluís Companys 23, E-08010 Ba celona, Spain 8 Ins i u d’Es udis Espacials de Ca alunya (IEEC), E-08034 Ba celona, Spain 9 Jod ell Bank Cen e o As ophysics, Uni e si y o Manches e , Manches e , M13 9PL, UK 10 Lenna d-Jones Labo a o ies, Keele Uni e si y, ST5 5BG, UK 11 Exoplane s and S ella As ophysics Labo a o y, NASA Godda d Space Fligh Cen e , G eenbel , MD 20771, USA 12 Depa men o As onomy, Uni e si y o Ma yland, College Pa k, MD 20742, USA 13 Cen e o Resea ch and Explo a ion in Space Science and Technology, NASA Godda d Space Fligh Cen e , G eenbel , MD 20771, USA 14 Ins i u o de Radioas onomía y As o ísica, UNAM, An igua Ca e e a a Pá zcua o 8701, Ex-Hda. San José de la Hue a, Mo elia 58089, Mich., México 15 Jod ell Bank Cen e o As ophysics, The Uni e si y o Manches e , Manches e , M13 9PL, UK Recei ed 2023 No embe 22; e ised 2024 Feb ua y 2; accep ed 2024 Feb ua y 2; published 2024 Ma ch 11 Abs ac We p esen poin -sou ce pho ome y om he Spi ze Space Telescope's final su ey o he Small Magellanic Cloud (SMC). We mapped nea ly 30 deg 2 in wo epochs in 2017, wi h he second ex ending o ea ly 2018 a 3.6 and 4.5 μm using he In a ed A ay Came a. This su ey duplica es he oo p in om he SAGE-SMC p og am in 2008. Toge he , hese su eys co e a nea ly 10 y empo al baseline in he SMC. We pe o med ape u e pho ome y on he mosaicked maps p oduced om he new da a. We did no use any p io ca alogs as inpu s o he ex ac o in o de o be sensi i e o any mo ing objec s (e.g., o eg ound b own dwa s)and o he ansien phenomena (e.g., ca aclysmic a iables o FU O i– ype e up ions). We p oduced a poin -sou ce ca alog wi h high- confidence sou ces o each epoch as well as a combined-epoch ca alog. Fo each epoch and he combined-epoch da a, we also p oduced a mo e comple e a chi e wi h lowe -confidence sou ces. All o hese da a p oduc s will be made a ailable o he communi y a he In a ed Science A chi e. Unified As onomy Thesau us concep s: Small Magellanic Cloud (1468);In a ed pho ome y (792);Celes ial objec s ca alogs (212) 1. In oduc ion The Small Magellanic Cloud (SMC)is a nea by, me al-poo dwa galaxy. I s dis ance (62.44 ±0.94 kpc; G aczyk e al. 2020)and me allici y (Z=0.1–0.2 Z e ; e.g., Russell & Dopi a 1992; Choudhu y e al. 2018)make i an ideal a ge o s udying he e olu ion o bo h he in e s ella medium and s a s wi h well-cha ac e ized dis ances in a mo e p imi i e chemical en i onmen han he Milky Way. Consequen ly, he SMC has been a ge ed by many su eys co e ing a wide wa eleng h ange, om X- ays o adio (e.g., Za i sky e al. 2002; Cu i & 2MASS Team 2004; Ka o e al. 2007; Udalski e al. 2008; I a e al. 2010; Cioni e al. 2011; Go don e al. 2011; Habe l e al. 2012; Meixne e al. 2013; Joseph e al. 2019). The Spi ze Space Telescope (We ne e al. 2004)co e ed he SMC in whole o in pa in mul iple epochs. The Spi ze Su ey o he Small Magellanic Cloud (S 3 MC; Bola o e al. 2007)mapped 2.8 deg 2 in he co e o he galaxy (ou lined in Figu e 1)in all se en pho ome ic fil e s in he In a ed A ay Came a (IRAC; 3.6, 4.5, 5.8, and 8.0 μm; Fazio e al. 2004) and he Mul i-band Imaging Pho ome e o Spi ze (24, 70, and 160 μm; Rieke e al. 2004). A second Spi ze p og am, Su eying he Agen s o Galaxy E olu ion in he Tidally S ipped, Low Me allici y Small Magellanic Cloud (SAGE- SMC; Go don e al. 2011), ollowed. I used he same fil e s bu co e ed he en i e galaxy and i s en i onmen , including he ba , wing, and ail ( hese egions a e labeled in Figu e 1). Spi ze ’s Las Look a he SMC (o SMC-Las )duplica ed he sky co e age o SAGE-SMC. Bo h su eyed he same 30 deg 2 , and bo h obse ed in wo epochs spaced 3–4 mon hs apa . SMC-Las was comple ed du ing Cycle 13, which ook place du ing he wa m phase o he Spi ze mission. Thus, he only bands a ailable we e o he 3.6 and 4.5 μm IRAC fil e s. The S 3 MC p og am obse ed he co e o he SMC ( he denses egions o he ba )wi h IRAC in 2005 May. The wo IRAC epochs o SAGE-SMC ollowed in 2008 June and Sep embe , and he wo SMC-Las epochs we e ob ained in 2017 (Augus –Sep embe )and om 2017 No embe o 2018 Feb ua y. In be ween SAGE-SMC and SMC-Las , he SAGE- Va p og am mapped an a ea o 2.9 deg 2 in he co e o he SMC in ou epochs om 2010 Augus o 2011 June du ing he wa m Spi ze mission (Riebel e al. 2015). Thus, SMC-Las ex ends he empo al co e age in he cen e o he SMC by Spi ze o nine epochs a 3.6 and 4.5 μm co e ing a empo al baseline o o e 12 y . The minimum co e age in he SAGE- The As onomical Jou nal, 167:149 (16pp), 2024 Ap il h ps://doi.o g/10.3847/1538-3881/ad2601 © 2024. The Au ho (s). Published by he Ame ican As onomical Socie y. O iginal con en om his wo k may be used unde he e ms o he C ea i e Commons A ibu ion 4.0 licence. Any u he dis ibu ion o his wo k mus main ain a ibu ion o he au ho (s)and he i le o he wo k, jou nal ci a ion and DOI. 1 SMC oo p in is ou epochs wi h a baseline o o e 9 y . Table 1lis s he epochs om Spi ze su eys o he SMC. The Wide-fieldIn a edSu eyExpe imen (WISE; W igh e al. 2010)has p o ided addi ional co e age o he SMC in wo epochs in 2010 and, s a ing in 2014, wo epochs pe yea as he Nea -Ea h Objec WISE-Reac i a ed (NEOWISE-R)mission (Mainze e al. 2014). WISE scans he en i e sky once e e y 6 mon hs. The fi s epoch in 2010 was ob ained in all ou WISE fil e s. All subsequen epochs ollowed he loss o c yogens and include jus he 3.4 and 4.6 μmfil e s. The 2023 elease o he mul iepoch NEOWISE-R ca alog includes a o al o 20 epochs om 2010 h ough 2022. While WISE p o ides be e empo al co e age han he Spi ze su eys, i is no as deep, and i is a lowe esolu ion (∼6″, as discussed in Sec ion 5). Sec ion 2desc ibes he mosaics om which he poin -sou ce ca alogs we e gene a ed. Sec ion 3desc ibes he ex ac ion Figu e 1. The SMC-Las IRAC 3.6 μm(uppe panel)and 4.5 μm(lowe panel)composi e images combining bo h epochs o obse a ion. The majo mo phological ea u es o he SMC ( he ba , wing, and ail, he la e being a pa o he Magellanic B idge)a e labeled. Red squa es in he 3.6 μm image indica e he wo es fields ( he spa se field in he wing and he c owded field cen e ed on NGC 346, as defined in Sec ion 5). In he 4.5 μm image, he colo ed ou lines show he su ey a eas o S 3 MC, SAGE-SMC, and SAGE-Va . 2 The As onomical Jou nal, 167:149 (16pp), 2024 Ap il Kucha e al. p ocess and de ails how he flux and posi ional unce ain ies we e calcula ed. Sec ion 4desc ibes he con en s o he ca alogs and a chi es. Sec ion 5compa es he SMC-Las poin sou ces o SAGE-SMC and WISE in wo es fields. Sec ion 6 summa izes he SMC-Las p ojec . 2. Da a The pho ome y p esen ed he e we e ex ac ed om mosaic images p oduced om he SMC-Las su ey (Mizuno e al. 2022). The mosaics consis o 52 pla es, each 1°.06×1°.06, a bo h 3.6 and 4.5 μm. These da a co e ∼30 deg 2 in he SMC, including he main ba o he galaxy, he wing ex ending o he eas , and he ail ex ending u he in ha di ec ion owa d he La ge Magellanic Cloud. Al hough he wo IRAC bands ha e sligh ly di e en esolu ions, he pla es we e mosaicked o ha e he same pixel size, 0 6, and esolu ion, ∼2″. The obse a ions we e aken wi h high dynamic ange exposu es a 0.4 and 10.4 s in eg a ions o co e bo h ain and b igh sou ces and minimize sa u a ion in he images. Two comple e maps we e made o e wo epochs sepa a ed by 70 days. The fi s epoch was ob ained om 2017 Augus 25 o Sep embe 13 and he second om 2017 No embe 24 o 2018 Feb ua y 12. Scheduling logis ics equi ed he ex ension o epoch 2 o 78 days o co e he gaps le in he o iginal scheduling. A hi d map was gene a ed ha combined obse a ions om bo h epochs, and hese da a a e he basis o Figu e 1. 3. Sou ce Ex ac ion We used he open sou ce SEx ac o (Sou ce-Ex ac o e sion 2.14.2; Be in & A nou s 1996) o pe o m ape u e pho ome y on ou mosaicked images. The so wa e is flexible in se ing a wide a ie y o pa ame e s needed o pho ome y. In gene al, he so wa e calcula es a model o he sky backg ound and hen p oduces a backg ound-sub ac ed image on which o pe o m he pho ome y. De ec ions a e hen pho ome ically ex ac ed, “cleaned”o a i ac s (i.e., eex- amined o con ibu ions o nea es neighbo s), and hen sa ed o a sou ce lis . The documen a ion wi h he so wa e explains ha sou ces ha o e lap pa ially, so ha hey show wo clea peaks, a e ex ac ed as sepa a e sou ces. 3.1. Ape u e Selec ion The selec ion o ape u e size depends la gely on he unce ain ies expec ed o he candida e ape u es. The p ima y sou ce o unce ain y o he ain e sou ces is he backg ound noise in he ape u e. Fo pixel- o-pixel noise σ BG , he unce ain y in he in-ape u e flux will be p opo ional o NBG s, whe e Nis he numbe o pixels in he ape u e, o equi alen ly, p opo ional o dσ BG , whe e dis he ape u e diame e . The backg ound unce ain y in he o al flux, hen, is p opo ional o C(d)dσ BG , whe e C(d)is he ape u e co ec ion o diame e d. We es ima e C(d)a small ape u es om he 3.6 and 4.5 μm poin - esponse unc ion (PRF)images supplied by he In a ed P ocessing and Analysis Cen e (IPAC). 16 C(d) ises s eeply below abou 4″diame e , and we find ha he backg ound unce ain y is minimized below a 3″ape u e diame e o bo h bands. The second sou ce o ape u e-dependen unce ain y is wha we e m “small-ape u e”unce ain y, which a ises om he unce ain ies in he cen oid de e mina ion om he ex ac o , pu ing a ying po ions o a sou ce's PRF inside o ou side he ape u e. This e ec inc eases wi h dec easing ape u e diame e , and i also inc eases (in absolu e flux unce ain ies) wi h flux, because i is a ac ional e ec , bu i is pa ially mi iga ed by he esul ha he cen oid unce ain y gene ally dec eases wi h inc easing flux. Ou app oach is o apply he ape u e ha minimizes he o al unce ain y o each sou ce indi idually, pa icula ly wi h espec o hese wo compe ing unce ain ies (o he sou ces o unce ain y a e ei he independen o o depend only weakly on he ape u e). As he op imum ape u e will a y wi h backg ound noise and sou ce flux, we se he sou ce ex ac o o epo he in-ape u e fluxes wi h a ange o ape u e diame e s: 4″,5″,6″,8″, and 10″. Fo each ape u e, we calcula e he o al unce ain y and selec he esul s o he ape u e ha p oduces he minimum unce ain y. See Sec ion 3.2 o de ails on he calcula ion o unce ain ies. One excep ion o his scheme is ha o sou ces wi h FWHM >25, o elonga ion >1.25, a minimum ape u e-size c i e ion is applied ega dless o he ape u e wi h he minimum unce ain y, because he ape u e co ec ions (and he esul ing unce ain ies)a e inaccu a e o such sou ces. The ca alog fluxes a e he ex ac o - epo ed in-ape u e fluxes ( o he op imum ape u e)scaled by he ape u e co ec ions C(d). The ape u e co ec ion o he 10″ape u e o each band is in e pola ed om he co ec ion alues shown in he IRAC Use 's Manual ( hei Table 4.8 p o ides c yogenic and wa m IRAC ape u e co ec ions). The co ec ions o he emaining ape u es a e de e mined as he median a io o he 10″in-ape u e fluxes o he in-ape u e fluxes a he o he ape u es o e all he epoch 1 sou ces be ween 1 and 100 mJy (∼47,000), hen scaled o he 10″ape u e co ec ion. We es ima e ha he unce ain y in he ape u e co ec ions epo ed in he IRAC Use 's Manual, abou 1%–2%, should hold o he ape u e co ec ions applied o ou da a. Table 2shows he ape u e co ec ions we apply o he wo bands. Table 1 Spi ze and WISE Obse a ions o he SMC Spi ze Su eys o he SMC All-Sky WISE b Name Region a A ea (deg 2 )Epoch Epoch S 3 MC Co e o he SMC c 2.8 2005 May 2010 Ap SAGE-SMC Full SMC 30 2008 Jun 2010 Oc SAGE-SMC Full SMC 30 2008 Sep 2014 May SAGE-Va Co e 2.9 2010 Aug 2014 No SAGE-Va Co e 2.9 2010 Sep 2015 May SAGE-Va Co e 2.9 2010 Dec 2015 No SAGE-Va Co e 2.9 2011 Jun 2016 May SMC-Las Full SMC 30 2017 Aug 2016 No SMC-Las Full SMC 30 2017 No 2017 Ap LLLL2017 No No es. a The S 3 MC, SAGE-SMC, and SAGE-Va su ey a eas a e indica ed in he 4.5 μm image in Figu e 1. b WISE epochs con inue om 2018 o p esen , wo pe yea . c The “ba ”and pa o he “wing”; see Figu e 1. 16 h ps://i sa.ipac.cal ech.edu/da a/SPITZER/docs/i ac/calib a ionfiles/ ps p / 3 The As onomical Jou nal, 167:149 (16pp), 2024 Ap il Kucha e al. 3.2. Unce ain y in Flux Wi h all such so wa e packages, i is necessa y o unde - s and exac ly wha he so wa e is measu ing when i ou pu s a measu emen . Fo example, he ex ac o es ima es he flux unce ain y gi en he ape u e size(s). Howe e , he unce ain y calcula ion, pa icula ly o he pho oelec on coun ing s a is- ics, assumes a fixed in eg a ion ime o he en i e image. Bu since he images a e mosaics, hey ha e a ying co e ages. Also, we ha e subs i u ed 0.4 s da a o he e y b igh sou ces ha sa u a ed he de ec o s in he 10.4 s in eg a ion. Fu he - mo e, he Spi ze p ocessing pipeline o he aw da a includes addi ional unce ain ies, pa icula ly o b igh sou ces, such as unce ain ies associa ed wi h he nonlinea i y co ec ion and co ec able sa u a ions. Fo hese easons, we ha e chosen o calcula e unce ain ies sepa a ely om he ex ac o ou pu . Fo he ape u e pho ome y, we iden i y ou sou ces o unce ain y om he pixel alues hemsel es: (1)unce ain y in he backg ound sub ac ed om he da a in he ape u e, (2) unce ain y in he ape u e in eg a ion om pixel- o-pixel ms in he ape u e, (3)pho oelec on coun ing unce ain y and o he unce ain ies o b igh sou ces, and (4) o small ape u es, unce ain y in he e ec i e ape u e co ec ion due o unce ain ies in he posi ioning o he cen oid o he sou ce. Fo ain sou ces (below abou 0.1 mJy), he e is also an unce ain y associa ed wi h he co ec ion o a small back- g ound-le el e o . 3.2.1. Backg ound-le el Unce ain y (σ LEV ) The sou ce ex ac o p oduces a backg ound ms map. Wi h he pa ame e s o ou ex ac ions, his map is gene a ed by de e mining he ms in 14 ×14 pixel bins, wi h some agg essi e ou lie dele ion. The ms alues o each bin a e smoo hed o e 10 bins and hen in e pola ed o gi e an ms es ima e o each pixel. While he ms is o mally unique o each pixel, i is basically he a e age backg ound ms in he su ounding 80″×80″ egion (10 pixels ×14 bins ×06pixel −1 ). Fo each sou ce, he backg ound ms σ BG is de e mined om he ms map a he sou ce loca ion. The ex ac o epo s he backg ound flux sub ac ed o he selec ed ape u e, which is con e ed o a backg ound b igh ness le el by di iding by he ape u e a ea. The backg ound le el is de e mined o each ape u e in a 24 pixel wide “squa e annulus”su ounding he ape u e, o oughly a 48 ×48 box o pixels. Fo a fla backg ound, σ BG is simply he pixel- o-pixel noise, and he backg ound-le el unce ain y would be app oxima ely σ BG /48. Bu i σ BG con ains significan backg ound a ia ions on a leng h scale la ge han he sou ce size bu smalle han 14 pixels, hen he backg ound unce ain y would be close o σ BG i sel . We ha e no p ac ical way o di ec ly measu e backg ound a ia ions a ound a sou ce. Howe e , as an app oxima ion, we use he backg ound b igh ness alues o each ape u e: he ms o he le els o e he di e en ape u es is aken, and he g ea e o his ms (deno ed BG s ¢)o σ BG /48 is used as he backg ound-le el unce ain y. Fo each ape u e, hen, he es ima ed backg ound-le el unce ain y is scaled by he numbe No pixels in he ape u e o gi e he unce ain y in he in-ape u e flux (in in eg a ed MJy s −1 pixel uni s), hen scaled by he solid angle Ao he mosaic pixels o gi e he in-ape u e flux unce ain y in mJy (A=8.461595 ×10 −12 s imes he 10 9 MJy- o-mJy con e - sion). This esul is scaled by he ape u e co ec ion C k o ape u e k(Table 2) o gi e he unce ain y in he o al flux. This unce ain y is adjus ed by wo addi ional ac o s. Fi s , he alues in he backg ound ms images a e smalle han he expec ed alues o he backg ound ms in he indi idual IRAC ames di ided by he squa e oo o he co e age. Thus, he mosaicking p ocess p oduces some amoun o e ec i e smoo hing. This smoo hing educes he measu ed pixel- o- pixel ms bu should no a ec he backg ound unce ain ies o e ex ended egions, so we scaled he backg ound ms alues upwa d by 20% o app oxima ely ma ch he expec ed unsmoo hed ms in he mosaics. Second, he IRAC ames ha e a pixel size o 1 2, while he mosaics ha e 0 6 pixels, so gene a ing he mosaics e ec i ely ebins he da a by a ac o o 2×2. Fo he unce ain y calcula ions o e ensembles o pixels, his ebinning inco ec ly inc eases he n 1 2 - ms benefi o sums and a e ages by a ac o o 2, so we scale he calcula ed unce ain ies by 2 o accoun o he ebinning. The backg ound-le el unce ain y (in mJy)is hen ( )()()()()NAC48 1.2 2.0 . kLEV BG ss= Fo he case whe e he empi ical backg ound ms BG s ¢is used, he unce ain y is ()()()NAC kLEV BG ss=¢ because BG s ¢is a di ec measu e (albei an app oxima ion)o he backg ound-le el unce ain y wi hou he smoo hing and ebinning adjus men s. 3.2.2. Ape u e Flux Unce ain y Due o Backg ound Noise (σ SUM ) The backg ound unce ain y σ SUM in he ape u e in eg a ion is also de e mined om he ex ac o backg ound ms image. We ake he alue o he backg ound ms σ BG a he sou ce loca ion as cons an o e he ape u e. Wi h his assump ion, he unce ain y o he sum o image pixel alues o e he ape u e is N B G 1 2s, whe e Nis he numbe o mosaic pixels in he ape u e. Wi h he con e sion o mJy and applying he adjus men s o smoo hing and ebinning, we ha e ()()()()NAC1.2 2.0 . kSUM BG 1 2 ss= 3.2.3. Pho oelec on Coun ing Unce ain y (σ PH ) The pho oelec on coun ing unce ain y (and ela ed b igh - sou ce unce ain ies)is de e mined om he unce ain y images gene a ed by Spi ze 's Mopex mosaicking so wa e (Mako oz & Khan 2005). These images con ain he o al unce ain y o each mosaic pixel (in MJy s −1 )as es ima ed om he ini ial p ocessing o he aw IRAC ames. While hese unce ain y images con ain backg ound unce ain y alues, he alues do no accu a ely ep esen he pixel- o-pixel ms alues in ou images (Mizuno e al. 2022)and also include o e all le el unce ain y es ima es om he da k sub ac ion, which do no a ec he unce ain y o he ape u e pho ome y. Fo each Table 2 IRAC Ape u e Pho ome y Co ec ions Band Ape u e Size 10″8″6″5″4″3″ 3.6 μm 1.070 1.100 1.148 1.205 1.329 1.649 4.5 μm 1.078 1.100 1.141 1.211 1.357 1.685 4 The As onomical Jou nal, 167:149 (16pp), 2024 Ap il Kucha e al. sou ce, we sub ac he “backg ound”o hese unce ain y images by aking a 12 ×12 box o pixels su ounding he sou ce and sub ac ing he median o he pe ime e o he box as he backg ound. Any esul ing nega i e- alue pixels a e eplaced wi h ze os. The pho oelec on coun ing unce ain y σ PH is hen ⎜⎟ ⎛ ⎝ ⎞ ⎠()()()pAC2.0 , i ikPH 2 1 2 å s= whe e he summa ion is o e he mosaic pixels in ape u e k and p i a e he backg ound-sub ac ed pixel alues o he unce ain y image. No e ha he mosaic smoo hing desc ibed abo e does a ec he unce ain y image esul s bu no in a s aigh o wa d way, and i is igno ed in he p esen calcula ion. 3.2.4. Small-ape u e Unce ain y (σ SA ) Fo small ape u es, unce ain y in he loca ion o he cen oid o he sou ce p oduces addi ional unce ain y in he calcula ed flux due o a ia ion in he ac ion o he flux con ained wi hin he ape u e depending on he accu acy o he cen oid. We ha e modeled his small-ape u e e ec wi h he PRFs supplied by he Spi ze Science Cen e . Fo each band, he a e age o he 25 images (sampling he ocal plane)is aken, wi h he backg ound sub ac ion and scaling se o ma ch he ape u e co ec ions supplied in he IRAC Use ’s Manual (Table 4.8). A Mon e Ca lo simula ion o he cen oid unce ain ies was hen pe o med o each ape u e, wi h he cen e o he ape u e shi ed by andom amoun s in xand y ( ela i e o he PRF cen e )wi h a specified ms o simula e he cen oid accu acy, and he flux in he ape u e was hen measu ed. The esul ing sca e in in-ape u e fluxes is a model o he ac ional unce ain y o he sou ce flux as a unc ion o he ape u e size and he cen oid ms unce ain y. Fo each sou ce, he cen oid unce ain y is es ima ed om an empi ical model (see Sec ion 3.3 below)and hen applied o he Mon e Ca lo model o es ima e he ac ional flux unce ain y o each ape u e o ha sou ce. These ac ional unce ain ies a e hen scaled by he ape u e-co ec ed fluxes (in mJy) o gi e he unce ain y σ SA o each ape u e. This unce ain y is a maximum o he ain es sou ces in he ca alog, a ound 5%–10%, and is a pe cen o less abo e abou 0.1 mJy. 3.2.5. Ins umen al Unce ain y (σ INST ) The IRAC Use 's Manual (IRAC Ins umen & Ins umen Suppo Teams 2021)desc ibes wo sou ces o ins umen al unce ain y. The fi s is he “A ay Loca ion”unce ain y and is due o he fla -field co ec ion being pe o med wi h he zodiacal backg ound, and so he e will be a ia ions wi h di e en sou ce spec a. IPAC supplies a co ec ion image ( o each band) o a Vega spec um. We use hese o es ima e he esul ing expec ed sca e in flux alues o ou sou ces by calcula ing he ms o he co ec ion alues in hese images. This is an es ima e o he unce ain y o a single co e age. Because ou da a a e mosaicked om se e al IRAC ames, his unce ain y is adjus ed by di iding by he squa e oo o he co e age o ha sou ce. The second sou ce is he “Pixel Phase”unce ain y, which a ises om di e en esponses o he image pixels depending on he p ecise loca ion o he sou ce cen oid on he pixel. IPAC supplies IDL so wa e o calcula e a co ec ion o he loca ion o he sou ce wi hin a pixel. 17 This co ec ion is calcula ed o a g id o loca ion alues on he pixel, and he expec ed unce ain y is es ima ed om he ms o he co ec ion alues and again scaled o each sou ce by he squa e oo o he co e age. These wo e ec s di e o each band bu oge he gi e an expec ed unce ain y o abou 2.4% o bo h bands. Howe e , hese fixed-pe cen age unce ain ies ha e been adjus ed o ma ch an empi ical analysis o he unce ain ies. See Sec ion 3.2.7 below. 3.2.6. Low-flux Backg ound-le el Unce ain y (σ Δ ) Sys ema ic a ia ions in he a ios be ween he fluxes measu ed a he a ious ape u es (4″,5″,6″, and 8″, compa ed wi h 10″) o sou ces wi h fluxes below ≈0.3 mJy a e consis en wi h a small backg ound-le el e o , inc easing wi h dec easing flux. Figu e 2shows he esul ing modeled flux co ec ions a he h ee smalles ape u es ( he 4″ape u e was used o mos o he sou ces in his flux ange). The le el e o is posi i e, p oducing a flux defici , and so he flux co ec ion is also posi i e. No e ha he backg ound e o seems o be a Figu e 2. Modeled flux co ec ions due o backg ound-le el e o o he h ee smalles ape u es and o he 3.6 μm band. The esul s o he 4.5 μm band a e simila . 17 h ps://i sa.ipac.cal ech.edu/da a/SPITZER/docs/i ac/calib a ionfiles/ pixelphase/pixel_phase_co ec _gauss.p o 5 The As onomical Jou nal, 167:149 (16pp), 2024 Ap il Kucha e al. small cons an abo e abou 0.3 mJy, bu we a e omi ing he co ec ion when i d ops below abou 1%. I we deno e he co ec ion as Δ k ( ), o ape u e kand flux , hen he associa ed unce ain y o he flux co ec ion is es ima ed as ()d d , k ss=D D whe e σis he flux unce ain y o he sou ce om all o he causes. 3.2.7. To al Unce ain y and Empi ical Analysis These unce ain ies a e s a is ically independen (wi h he possible excep ion o σ Δ , as we do no know he mechanism p oducing he unde lying e o , and while σ LEV and σ SUM bo h depend on he backg ound noise, any associa ed e o s a e de i ed om sepa a e ensembles o pixels). The o al unce ain y o a sou ce is hen . 2LEV 2 SUM 2 PH 2 SA 2 INST 22 ss s sss s= + +++ + D The sepa a e unce ain ies a e also s a is ically independen ac oss he wo bands. The e m σ LEV could ha e some co ela ion i o a gi en sou ce ha unce ain y is domina ed by s uc u ed backg ound, which would likely be simila in bo h bands, bu σ LEV is ne e he dominan e m in he o al unce ain y. The calcula ed unce ain ies can be checked empi ically wi h he sou ce ma ches be ween he wo epochs. No ing ha he backg ound noise le els a he IRAC ame le el o a gi en band a e app oxima ely he same be ween he wo epochs, and conside ing only sou ces wi h wo co e ages in he mapping scans, he flux unce ain ies o a gi en sou ce be ween he wo epochs should be app oxima ely he same. The flux di e ences be ween he epoch-ma ched sou ces, hen, should in he agg ega e eflec he ac ual unce ain ies. Specifically, he ms o he flux di e ences o an ensemble o sou ces in a na ow flux ange ( o which he unce ain ies should also gene ally all in a na ow ange)should be app oxima ely 2 imes he median unce ain y in he ensemble. Figu e 3shows he 12 -scaled flux-di e ence ms alues o he epoch-ma ched sou ces o e a se o flux bins o bo h bands, he e limi ed o he co e age =2 sou ces (filled blue squa es). The c i e ion o inclusion in a flux bin is ha he mean o he epoch 1 and epoch 2 fluxes o a gi en sou ce alls wi hin he flux bin. Also shown a e he median o mal σ(as calcula ed abo e)in each bin o he epoch 1 sou ce (open ed squa es). In he 3.6 μm band in pa icula he e is clea ly a disc epancy be ween he o mal and empi ical unce ain y alues, he o mal being abou a ac o o 2 oo small abo e a ew mJy, bu he e is also a defici in he 4.5 μm band o abou 20%–30% (in bo h cases igno ing he small numbe s a is ics a he e y high fluxes). Abo e a ew mJy, he only significan con ibu o o he unce ain y is he ins umen al unce ain y (Sec ion 3.2.5), so he disc epancy may be a ibu ed o an inaccu a e accoun ing o ha componen . Wha e e he cause, we ha e a bi a ily adjus ed he ins umen al unce ain y o app oxima ely ma ch he empi ical esul s, om 2.4% in bo h bands o 4.2% and 2.8% in he 3.6 and 4.5 μm bands, espec i ely. Figu e 3. Empi ical e o es ima e de i ed om ma ching sou ces ac oss epochs, calcula ed o e flux bins (filled blue squa es). Fo each bin, he ms o he c oss- epoch flux di e ences is calcula ed and di ided by 2. The open ed squa es a e he median nominal σ o mal unce ain y in he bin, and he filled ed squa es a e he σ alues a e he adjus men desc ibed in he ex . 6 The As onomical Jou nal, 167:149 (16pp), 2024 Ap il Kucha e al. C owding o sou ces was no conside ed in he unce ain y calcula ions. When ano he de ec ed sou ce ell wi hin a gi en sou ce's ape u e, ha o he sou ce was masked (pixels se o 0 a e backg ound sub ac ion), so ha o a fi s app oxima ion, he nea by sou ce does no a ec he gi en sou ce's flux o unce ain y. Howe e , o sou ces wi h a e y close de ec ed neighbo , less han 3″o so, we see e idence ha he masking does educe he measu ed flux, al hough his e ec is no quan ified and is no accoun ed o in he epo ed unce ain y. Fo he mos “poin like”sou ces (below specified h esholds o FWHM and elonga ion), he dominan unce ain ies a e he in-ape u e backg ound noise (σ SUM ) o fluxes below abou 0.2 mJy. Fo fluxes abo e 0.2 mJy, he ins umen al unce ain y (σ INST )domina es. As hese a e he only wo ape u e- dependen unce ain ies, o he poin like sou ces, he flux is he p ima y de e minan o he ape u e. Fo sou ces ha a e less poin like, he ape u e is usually inc eased om he o mal unce ain y minimum, la gely because he ape u e co ec ion is likely o be less accu a e o such sou ces, and a la ge ape u e should educe he associa ed e o . 3.3. Unce ain y in Posi ion The ex ac o de e mines he posi ion o he sou ce om he pixel posi ion o he emission cen oid a e backg ound sub ac ion, which can in oduce unce ain ies as he ex ac o subdi ides pixels du ing his p ocess. The accu acy o he measu ed coo dina es o he sou ces, i.e., he cen oid unce ain y in he sou ce ex ac ions, is es ima ed wi h he sou ce ma ches be ween he wo epochs. Fo each band and he R.A. and decl. sepa a ely, he posi ional di e ences be ween he ma ched sou ces a e aken, and he ms o he di e ences a e calcula ed o e flux bins. I we assume ha he posi ional unce ain ies o gi en sou ces will be equi alen o bo h epochs (as bo h epochs ha e simila co e age and noise cha ac e is ics), hen he ms o he di e ences will be app oxima ely 2 imes he posi ional unce ain y o he indi idual sou ces ( o sou ces wi h simila unce ain ies).We find ha he posi ional unce ain ies a e flux-dependen and a e essen ially equi alen o bo h bands in bo h R.A. and decl. Figu e 4shows cen oid unce ain ies calcula ed in flux bins. Figu e 4includes an exponen ial unc ion fi ed o he da a o he o m () ae c, b CENT s=+ - whe e is he log o he sou ce flux. This unc ion is he cen oid unce ain y used o calcula e he small-ape u e unce ain y (Sec ion 3.2.4). In p inciple, hese esul s include he as ome ic unce ain y in he images as well as he cen oid unce ain y specifically. Howe e , hese da a a e domina ed by sou ces in egions o which he e a e hund eds o sou ces used o he as ome ic co ec ions o each IRAC ame (using ma ches o he Two Mic on All Sky Su ey, 2MASS). The IRAC as ome y is co ec ed (wi h a ansla ional adjus men )by ma ching he measu ed cen oids o he sou ces in he IRAC ame o 2MASS sou ces (see Appendix Band Mizuno e al. 2022 o de ails). Fo ha p ocedu e, he cen oid accu acy was app oxima ely 0 5. In he c owded fields, a ypical da a ame includes abou 400 sou ces and wo co e ages, so he as ome ic accu acy ( ela i e o he mean 2MASS as ome y o e he IRAC ame)is abou 0 5/20/1.4 =0 018, gene ally a small componen o he abo e esul s. This es ima e assumes pu ely ansla ional unce ain ies in he IRAC as ome y ela i e o he 2MASS as ome y o any gi en da a ame. We do, howe e , see a small field- o a ional e o o abou 1′in he 3.6 μm a ay as ome y in all he da a (see Appendix B). This e m gi es an e ec i e addi ional sca e o abou 0 03 in he 3.6 μm coo dina es, which is implici ly included in he σ CENT calcula ion, al hough i is small enough ha di e ences in posi ional accu acy be ween bands can be igno ed. Fo he posi ional accu acy epo ed in he ca alog, an adjus men o he as ome ic accu acy in he spa se fields is included due o less accu a e ansla ional co ec ions wi h ewe sou ces. This is app oxima ely 0 5/NC , whe e Nis he mean numbe o 2MASS sou ces used o he as ome ic co ec ion and Cis he co e age numbe , so he o al posi ional Figu e 4. Posi ional unce ain y (single axis)calcula ed om c oss-epoch ma ches o sou ces. Fo each flux bin, he posi ional di e ences be ween epochs in bo h R.A. and decl. and o bo h bands a e aken. The plo ed alues a e he ms o hese bin ensembles di ided by 2. The epoch 1 3.6 μm sou ce is used o he flux indexing. 7 The As onomical Jou nal, 167:149 (16pp), 2024 Ap il Kucha e al. unce ain y o a sou ce is ()NC0. 5 . POS 2 CENT 22 ss=+ Again, his is ela i e o he local mean 2MASS as ome y o e an IRAC ame. This posi ional unce ain y is o a single sou ce de ec ion along a single axis (R.A. o decl.). Fo sou ces de ec ed in bo h bands, he epo ed R.A. and decl. a e he unce ain y-weigh ed a e ages o he coo dina es de e mined o each o he bands, which p o ides oughly a 2benefi in he posi ional accu acy. The associa ed single-axis unce ain y o he weigh ed a e ages is hen scaled upwa d by 2 o gi e a adial unce ain y. The epo ed posi ional unce ain ies a e andom de ia ions om he nominal 2MASS as ome y. The absolu e unce ain y depends on he accu acy o he 2MASS as ome y in he SMC a he epoch o he SMC-Las obse a ions. The global p ope mo ion o he SMC is app oxima ely 1.5 mas y −1 (Zi ick e al. 2018; see also Niede ho e e al. 2021), so o he ∼20 y be ween he su eys, we expec ∼30 mas o posi ional shi . Also, Zi ick e al. (2018)obse e p ope mo ions o up o abou 0.3 mas y −1 ela i e o he global SMC p ope mo ion in each o hei ∼35 sepa a e analysis egions, so his can con ibu e up o ∼6 mas u he shi (al hough Niede ho e e al. 2021 epo somewha highe isola ed local p ope mo ions). In addi ion, we ha e some e idence o a sys ema ic disc epancy o abou 0 03 in he measu ed coo dina es o sou ces be ween ou wo epochs, possibly due o a small esidual o se e o in he poin ing co ec ions, which esul s in a coo dina e shi a di e en oll angles. The ne sys ema ic e o we expec is he e o e a mos ∼007. The b igh es objec s in he su ey, howe e , a e mos likely o eg ound objec s and will ha e much la ge p ope mo ions. Cau ion should be applied wi h hese objec s when ying o ma ch ac oss su eys and/o epochs. 4. Da a P oduc s 4.1. Ca alogs and A chi es Following he p ac ice o he SAGE p ojec (Meixne e al. 2006), SAGE-SMC (Go don e al. 2011), and many o he Spi ze -based sou ce lis s, we ha e sepa a ed he final lis s o ex ac ions in a file con aining he mo e eliable sou ces, e e ed o as he “ca alog,”and a mo e comple e “a chi e,” which con ains he ca alog sou ces as well as less eliable ex ac ions. Toge he , hese sou ce lis s balance comple eness and eliabili y. Each ca alog and a chi e is published in h ee e sions o a o al o six files. We applied he sou ce ex ac o sepa a ely o he mosaics om each obse a ional epoch, and we also ex ac ed sou ces om he combined-epoch mosaics. The sou ce ex ac o gene a ed measu ed flux densi ies and p elimina y es ima ed unce ain ies o all o he selec ed ape u es. Once he o al unce ain ies we e calcula ed as desc ibed abo e, he ape u e wi h minimum unce ain y was ini ially selec ed o he sou ce lis . Fo sou ces de ia ing om “poin like”(FWHM >25 o elonga ion >1.25), a la ge ape u e was gene ally selec ed depending on he pa icula alues o FWHM and elonga ion. C owding has no been conside ed o he ape u e selec ion. The sou ce lis s include he selec ed ape u e. Fo he poin like sou ces, below abou 2.0 mJy, he in- ape u e backg ound noise unce ain y (σ SUM )domina es, and he smalles ape u e (4″)is usually selec ed. Abo e 2.0 mJy, he “small-ape u e”unce ain y (σ SA )begins o domina e o e σ SUM (as i is a ac ional unce ain y), and he selec ed ape u e gene ally inc eases wi h inc easing flux. Abo e ∼0.3 mJy, he ins umen al unce ain y (σ INST )domina es he o al e o , bu his unce ain y does no depend on ape u e. The c i e ia de e mining whe he sou ces belonged in he ca alogs o he a chi es di e significan ly om p e ious Spi ze -based su eys such as SAGE-SMC. Sou ces we e included in he ca alog only i he signal- o-noise a io (S/N) was 3 o g ea e in each band. To main ain independence be ween he obse a ional epochs, sou ces a e no ma ched be ween he epochs in ei he he ca alog o he a chi e. In a gi en epoch, sou ces we e ma ched ac oss bands i hey we e wi hin 1 6 o each o he . Al hough mos sou ces in he a chi e ha e been ma ched ac oss bands, ha was no a necessa y c i e ion o be in he a chi e. The e o e, i is possible o an S/N=3 ex ac ion in IRAC band 1 o be lis ed in he a chi e wi hou a co esponding band 2 de ec ion, and ice e sa. We limi ed he size and shape o he sou ces o cull ex ended sou ces and any emaining a i ac s in he images as well as alse de ec ions. The ex ac o uses he dis ibu ion o he sou ce pixels o calcula e he shape o he sou ce om he second-o de momen s o he emission. These momen s define he ellip ici y o he sou ce. We p ima ily used he FWHM and elonga ion ( a io o axes). Fo he ca alog, we limi ed he size and shape o sou ce o emo e non-poin sou ces. Sou ces in he ca alogs had o ha e FWHM 3″and elonga ions 2. These c i e ia we e elaxed o he a chi e, wi h FWHM 5″ and elonga ion 3.5. Thus, sou ce ypes ha can be sligh ly ex ended such as young s ella objec s a e mo e likely o appea in he a chi e han he ca alog (see also he discussion by Sewiło e al. 2013). We applied flux c i e ia o bo h he ca alog and a chi e o he h ee epochs. Fain e sou ces (<0.1 mJy)nea b igh e ones (>1 mJy) ended o be spu ious de ec ions i hey we e wi hin 12″o each o he (e.g., om di ac ion spikes o Ai y ings). The e o e, we emo ed hese ain sou ces om bo h he ca alog and a chi e. This s ep culled abou 700 sou ces. Finally, we mo ed 19 sou ces ha we e b igh e han 3 Jy a 3.6 μm o he a chi e. These objec s a e badly sa u a ed e en in he sho e in eg a ion imes and hus ha e less eliable flux densi ies han equi ed o inclusion in he ca alog. The appendix p o ides a able wi h hese sou ces. 4.2. Modified Julian Da e The ca alogs and a chi es con ain a column gi ing he mean Modified Julian Da e (MJD)o he obse a ion, along wi h maximum and minimum obse a ion imes. These alues a e based on he da a o a gi en epoch and a e p o ided sepa a ely o 3.6 and 4.5 μm. The combined-epoch da a also include he mean MJD, calcula ed in he same manne . The anges in imes a e wide han hose in a single-epoch ca alog, since he s a s o he wo epochs a e sepa a ed by 70 days and epoch 2 was obse ed o e a pe iod o 78 days. 4.3. Comple eness Figu e 5shows he numbe o sou ces binned by magni ude o he wo epochs and he combined-epoch ca alog o bo h IRAC bands. To es ima e comple eness, we ex end he linea inc ease in he log o he numbe o sou ces as a unc ion o 8 The As onomical Jou nal, 167:149 (16pp), 2024 Ap il Kucha e al. magni ude and epo he magni ude whe e he cumula i e numbe o sou ces coun ed eaches 95% and 90% o he expec ed o al. Table 3gi es he esul s. The imp o ed S/Nin he mosaics o he combined epochs esul s in significan ly be e comple eness es ima es bu a he cos o empo al p ecision. Table 4compa es SMC-Las o SAGE-SMC, showing he numbe o sou ces and minimum and maximum flux densi ies and magni udes o he a ious i e a ions o he ca alogs and a chi es. He e and in he ollowing sec ions in his pape , compa isons o SAGE-SMC a e based on he SAGE-SMC IRAC “Single F ame +Mosaic Pho ome y”Ca alog and A chi e, 1.5, 18 which a e based on pho ome y ex ac ed om he combined epoch 1 and epoch 2 mosaics (Go don e al. 2011). Fo bo h SMC-Las and SAGE-SMC, he a chi e includes all sou ces in he ca alog. The sou ce coun s e eal a di e ence in how he ca alogs and a chi es we e sepa a ed in he wo su eys. In SAGE-SMC, only abou 10% o he sou ces a e unique o he a chi e, while in SMC-Las , a mo e conse a i e app oach has led o mo e sou ces unique o he a chi e han a e in he ca alog. The o al sou ce coun s o he a chi es in bo h su eys a e compa able. 5. Compa ing SMC-Las o O he Mid-in a ed Su eys As ou lined in he In oduc ion, mul iple su eys wi h Spi ze and WISE ha e co e ed he SMC. The WISE mission has led o he AllWISE ca alog (Cu i e al. 2013), which is based on he o iginal wo epochs ob ained in 2010, and he Ca WISE ca alog (Ca WISE2020; Ma occo e al. 2021), which coadds he fi s 12 WISE, NEOWISE, and NEOWISE-R epochs ( om 2010 h ough 2018). The AllWISE and Ca WISE ca alogs equi e S/N>5 o inclusion o a sou ce, bu only in a single band, as opposed o he SMC-Las ca alogs, which equi e S/N>3inbo h bands. The abili y o ex ac a pho ome ic sou ce om he da a depends on he complexi y o he field. We examined wo es fields, as shown in Figu e 6(also indica ed in Figu e 1). One field, e e ed o as he “c owded”o “NGC 346 field,”is 12 12¢´ ¢ and cen e ed on NGC 346 (α,δ=14°.7721, −72°.17589). The o he field samples a spa sely popula ed egion 2 020¢´ ¢ in size and cen e ed a 21°.70, −73°.32. We e e o i as he “spa se field.” To compa e he co e age o hese su eys, we pe o med wo es s in he spa se and c owded fields, using he SAGE-SMC IRAC “Single F ame +Mosaic Pho ome y”A chi e, 1.5, as he basis o compa ison. Fi s , we ex ac ed all sou ces om hese egions o SAGE-SMC, AllWISE, Ca WISE, and all h ee epochs (1, 2, and combined)o he SMC-Las a chi e. Table 5 gi es he sou ce coun s in each field o he SAGE-SMC a chi e, Figu e 5. Cumula i e sou ce coun s o he a chi e (blue)and ca alog ( ed) o epoch 1 and he combined-epoch mosaics a bo h 3.6 and 4.5 μm. Epoch 1 beha es simila ly o epoch 2. Table 3gi es he limi ing magni udes o 95% and 90% comple eness. Table 3 Limi ing Magni udes and Comple eness o he SMC-Las Su ey Epoch P oduc 3.6 μm 4.5 μm 95% 90% 95% 90% 1 o 2 Ca alog 15.6 16.2 15.8 16.4 Combined Ca alog 15.7 16.7 16.5 16.9 1 o 2 A chi e 17.5 17.6 17.4 17.5 Combined A chi e 17.8 17.9 17.8 17.9 18 A ailable om he In a ed Science A chi e (IRSA);h ps://i sa. ipac.cal ech.edu. 9 The As onomical Jou nal, 167:149 (16pp), 2024 Ap il Kucha e al. in bo h bands, he sepa a e coo dina e co ec ions a e applied as-is. This h eshold is somewha a bi a y, bu wi h an ms o abou 0 5 o he cen oid p ecision in he 2MASS ma ches, he expec ed esidual e o o 50 sou ces is abou 0 07, well unde he ypical posi ional sca e in he sou ce ex ac ions om he mosaics (Figu e 4), so combining he e o s a is ics o bo h bands o his numbe o sou ces (o g ea e )would ha e li le benefi . Fo ewe han 50 sou ces in ei he band, he medians o he R.A. and decl. e o s a e aken o e bo h a ays, and he fixed ocal plane o se is con e ed o o se s in R.A. and decl. and hen appo ioned o he sepa a e a ays as an adjus men o he o e all co ec ion depending on he ac ion o he o al sou ces on each a ay. This p ocedu e equi es ha he sepa a e R.A. and decl. e o dis ibu ions o he wo a ays su ficien ly o e lap so ha he median is close o he a e age, and his is in ac he case ( he sca e o he indi idual e o s is a ound hal an a csecond). Finally, wi h he poin ing efinemen applied as abo e, i was ound ha he subsequen sou ce ex ac o coo dina es measu ed om mosaics c ea ed om he co ec ed ames di e ed sys ema ically om he 2MASS coo dina es by a small amoun , abou 0 055, also fixed in ocal plane coo dina es, which is likely due small di e ences in he cen oiding p ocedu es used and possibly he sligh ly asymme ic PRFs. As he ex ac o coo dina es a e he ones ul ima ely epo ed, his o se is also added o he calcula ed poin ing co ec ions. B.1. Scaling and Ro a ion The co ec ions applied a e ansla ions in he ocal plane, because he da a show ha he poin ing e o s a e la gely ime- a ying e o s in he bo esigh , which o easonable magni udes o e o will only cause ansla ions in he 5′-size IRAC a ays. Fo ha eason, i is highly unlikely ha he e will be any ue ame-by- ame a ia ions in scaling o field o a ion. Thus, i is easonable o in es iga e he p esence o fixed e o s in scaling and o a ion in he IRAC ame as ome y. Fo his pu pose, we ha e de eloped a χ 2 minimiza ion p ocedu e o op imize he mapping o he measu ed x,y cen oids o he sou ces on a gi en ame o he x,ycoo dina es o he co esponding 2MASS sou ces, using he ame’s nominal as ome y o map he 2MASS sou ces in o ocal plane space. Fi e pa ame e s a e op imized: xand yscaling, a o a ion, and xand y ansla ion. Op imized pa ame e s a e de e mined o each IRAC ame o an AOR (using AORs in ai ly c owded egions o be e s a is ics), hen examined ac oss he AOR o pe sis ence. While he e is conside able ame- o- ame sca e , pa icula ly in he o a ion, we find no significan sys ema ic e o s in he scaling pa ame e s o he 4.5 μm o a ion pa ame e in any o he AORs examined. Howe e , he op imized 3.6 μm o a ion shows a median e o o abou 1′, and his e o is p esen a a nea ly iden ical alue o e all o he AORs. A 1′field o a ion e o gi es a peak ue angula e o o abou 0 07 in he 5′ IRAC a ays and an e ec i e ms e o o abou 0 03 o an ensemble o sou ces dis ibu ed ac oss he a ay. ORCID iDs T. A. Kucha h ps://o cid.o g/0000-0003-1955-8509 G. C. Sloan h ps://o cid.o g/0000-0003-4520-1044 D. R. Mizuno h ps://o cid.o g/0000-0003-0947-2824 Ka hleen E. K aeme h ps://o cid.o g/0000-0002- 2626-7155 M. L. Boye h ps://o cid.o g/0000-0003-4850-9589 Ma in A. T. G oenewegen h ps://o cid.o g/0000-0003- 2723-6075 O. C. 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