Technical report: Focusing the DK-1.54m telescope (step-by-step user manual/guide)

FOCUSING THE DK-1.54m TELESCOPE WITH DFOSC IMAGING CCD A manual o ela i ely quickly ocus he elescope wi h DFOSC came a Ve sion his o y / change log: 20230701: Ve . 1 (Hinse). C ea ed documen using Lib eO ice (*od ile). 20251002: Ve . 2 (Hinse, Liu & Salinas San Ma ín). In oduc ion This li le manual should help you o ocus he elescope using he DFOSC came a in a s ep-by-s ep guide. I is assumed ha you know he TCS (Telescope Con ol Sys em unning on he Windows PC) and ARS (As o Robo ic Sys em unning on he Linux 1 PC). The DFOSC (Danish Fain Objec Spec og aph and Came a) came a h ps://www.eso.o g/public/sweden/ eles-ins /lasilla/danish154/d osc/ is a CCD came a moun ed on he back o he Danish 1.54m elescope. In he pho o, i 's he sil e - colo ed me allic cube wi h a an, kep cold using liquid ni ogen s o ed in he sil e cylinde (ESO/OHB echnicians e ill ha cylinde /dewa wi h liquid ni ogen e e y day cooling he CCD down o a ound -120 °C). The came a cap u es sky images using a se ies o il e s (U, B, V, e c.), which a e selec ed by o a ing a il e wheel posi ioned in on o he CCD. A he back end o he elescope, ligh is e lec ed h ough mul iple mi o s and con e ges a he ocal plane. To ge sha p images, he CCD mus si exac ly a his ocal poin — his ensu es he poin sp ead unc ion (PSF) is as small as possible (pe ec ocus). Howe e , he ocus can shi due o a ew ac o s: i) dome/en i onmen al empe a u e changes: e en a 1 °C change can sligh ly de o m he elescope s uc u e, al e ing he leng h o op ical pa h and hus mo ing he ocal plane and ii) il e changes: ligh o di e en wa eleng hs ha e di e en ocal leng hs (ch oma ic abe a ion). Page 1 / 14 The good news is ha he il e -induced shi in ocal leng h is au oma ically co ec ed by he TCS sys em. When you selec a new il e in he TCS (o om wi hin he ARS), i sligh ly adjus s he ocus as well. Bu empe a u e- ela ed ocus changes a e no co ec ed au oma ically, so you'll need o manually ecalib a e he ocus as en i onmen al condi ions change. The ocus p ocedu e will be done by ollowing he i e a i e “adap ing-sampling” me hod. The ocusing can be done a any empe a u e and using a b igh s a close o he zeni h. Chose a sui able b igh s a by clicking he “w ench” symbol in he TCS. B igh s a s a e always isible as hey a e p e-selec ed a all all Ras (00 – 24 h s). Focus will be done in he Bessel U- il e . This is because in U il e he b igh s a appea s dimme and hus will in mos cases no sa u a e he CCD. This is impo an o de e mine a good ocus. Usually, a good exposu e ime is 0.1 seconds. Op ing o e y sho exposu e imes (0.001 s) migh esul in ailu e o he DFOSC came a o acqui e a use ul image. The e a e wo me hods o ocus he elescope: i) uni o m sampling ( adi ional o b u e- o ce ocusing which is used in he Odin so wa e when ocusing he elescope wi h he LI / EMCCD came as) and ii) adap i e sampling, which is mo e ime-e icien achie ing he same ocus p ecision wi h only abou 10% as many images. Me hod 1: Uni o m sampling In his me hod, you ake images a egula in e als, o example e e y 0.05 mm, om 37.5 mm o 39.5 mm. This esul s in a o al o 41 images. As you go h ough his ange: • A sho e ocus leng hs, he s a looks like a donu o blu y ci cle. • I becomes inc easingly compac as you app oach he bes ocus. Then i becomes uzzy again as you mo e pas i . You ind he bes ocus by compa ing all he images and iden i ying whe e he s a appea s mos compac . The d awback is ha many o hese images a e unnecessa y. You don’ need high-p ecision sampling a om he bes ocus, so ime and e o a e was ed. Page 2 / 14 Me hod 2: Adap i e Sampling (Recommended) This me hod is mo e e icien . Ins ead o aking all images a once, you ake one image a a ime and na ow in on he bes ocus s ep-by-s ep. He e’s how i wo ks: • Take h ee images a equal in e als. • Iden i y he wo images whe e he s a appea s mo e compac . The bes ocus lies be ween hem. • Take a new image a he midpoin be ween hese wo. • Repea he p ocess: in e e y ound, always compa e h ee neighbou ing poin s and na ow he in e al. Each ime you do his, you hal e he s ep size and imp o e he ocus es ima e. A e 6 images, he p ecision is 0.0625 mm. A e 7 images, i eaches 0.03125 mm, which is mo e p ecise han wha he eye can dis inguish unde no mal seeing condi ions. Fo compa ison, using Me hod 1 o each his le el o p ecision would ake abou 40 images (0.05mm) o 60 images (0.03mm) - abou 10 imes mo e e o . Page 3 / 14 He e is he o e all p ocedu e: 1. Poin o a b igh s a . 2. Change o U il e , CCD binning o 1x1, window he CCD (!!), se exposu e o 0.1 s. 3. Follow his manual and de e mine a bes ocus alue in millime e s. 4. Reco d he empe a u e om he TCS (Me eo Da a). 5. Repo you indings o he nigh log on he wiki page. In he ollowing we desc ibe he i e a i e ocusing echnique. I is based on he assump ion ha he ocus changes symme ically a ound he bes in- ocus alue. We ha e es ed his and con i m ha he assump ion is co ec based on one ocusing es expe imen a empa a u e 15.2C (24. Sep. 2025). In he i s i e a i e le el (le el 1 = Round 1) you c ea e h ee images a h ee di e en ocus se ings. I e a ion Le el 1 (Round 1) Image1 @ 37.500 mm Image2 @ 38.500 mm Image3 @ 39.500 mm You will hen load he h ee images in o DS9 and adjus he zoom and scale o each image so hey appea iden ical. You job is hen o decide – om isual inspec ion -- which wo images a e he wo bes ocus images. Fo example you ind Image 2 and Image 3 o be bes in ocus. Tha means he bes - ocus is somewhe e in be ween ocus se ing o Image 2 and Image 3 (including he end poin s). Then you ind he (hal ) ocus in be ween image 2 and 3 and ake a new image (image 4) wi h he new (hal ed) ocus alue se in TCS. A his poin you a e in he i e a ion le el 2. New images a e w i en in bold ace. I e a ion Le el 2 (Round 2) Image2 @ 38.500 mm Image4 @ 39.000 mm Image3 @ 39.500 mm See he ollowing diag am / igu e. Page 4 / 14 The i e a ion p ocedu e con inues down o le el 4 / ound 4 (a o al o 6 images) o le el 5 / ound 5 (a o al o 7 images). Remembe ha he bes - ocus is a unc ion o empe a u e in he dome / La Silla moun ain. We judge ha i will be ha d o es ima e / de ec any imp o emen in ocus by eye a le el 5 / ound 5. A le el 4 / ound 4 i migh al eady be ha d o judge be ween he h ee s ella poin -sp ead- unc ions (PSFs) as displayed in DS9. The 1x1 binning will help o inc ease spa ial esolu ion. STEP-BY-STEP MANUAL S ep 01: In TCS (“Mo able Ca iage”): Posi ion o DFOSC imaging posi ion (click “pa k” = 300 mm bu on). S ep 02: In TCS (“CORRECTIONS”): Poin o a b igh s a nea /close o zeni h. Click he w ench icon. Slew he elescope o he b igh s a . S ep 03: In ARS (“Se ing”): Chose 1x1 binning ( emembe o click “se binning”). S ep 04: In ARS (“Se ing”): Window he CCD o only ead ou a cen al squa e a ea wi h he b igh s a loca ed nea o a ound he middle. The CCD dimension is 2148 pixels along x-axis and 2064 along he y-axis. The cen e is hus a (x,y) = (1124, 1032). We aim o window he CCD o a 400 pixel x 400 pixel sub window a ound he cen e . X begin = 924. X Size = 400. Y begin = 932. Y Size = 400. (The X and Y begin alues come om: hal he X o Y CCD axis dimension – 200 pixels). Click “se ” and double check in ARS o desi ed alues. Double check binning. S ep 05: Page 5 / 14 In ARS (“Se ing”): Se he exposu e ime o 0.1 seconds. S ep 06: In ARS (“Se ing”): Chose a sensible ilename. Fo example FocusU Fo each image he ARS so wa e will append a numbe . Fo example he i s image will ha e he ilename FocusU_000001. i s S ep 07: In ARS (“Se ing”): Se he image ype o “Ligh ”. S ep 08: In ARS (“Se ing”): Choose he U il e (Fil e wheel A: emp y. Fil e wheel B: U). S ep 09: In TCS (“Main Focus”): Se he “ABS” ocus o 37.500 mm and click “POSIT” S ep 10: In ARS: Take an exposu e. Focus image 1 (400 x 400 pixels) is s o ed in /da a/YYYYMMDD Page 6 / 14 S ep 11: In TCS (“Main Focus”): Se he “ABS” ocus o 38.500 mm and click “POSIT” S ep 12: In ARS: Take an exposu e. Focus image 2 is s o ed in /da a/YYYYMMDD S ep 13: In TCS (“Main Focus”): Se he “ABS” ocus o 39.500 mm and click “POSIT” S ep 14: In ARS: Take an exposu e. Focus image 3 is s o ed in /da a/YYYYMMDD A his s age you ha e now gene a ed h ee images a di e en ocus alues. You should now epea he abo e s eps and gene a e image 4, image 5, image 6 and e en ually image 7. To gi e you an idea wha o expec we will display ou esul s as ob ained om a 45 minu e es expe imen on 24. Sep embe 2025. To open he images in DS9 you need o open a command-line e minal on he DFOSC Linux 1 PC and issue he command: dk154@dk154:/da a/YYYYMMDD$ ds9 FocusU_000001. i s FocusU_000002. i s FocusU_000003. i s whe e YYYYMMDD is oday’s da e. Ou esul s look like as ollows: Page 7 / 14 Abo e sc eensho o DS9 (Linux 1 PC unning ARS) shows he b igh s a a h ee di e en ocus se ings a i e a ion le el 1. Focus I e a ion Le el 1 / Round 1 Le Image Middle Image Righ Image Image 1 (37.50 mm) Image 2 (38.50 mm) Image 3 (39.50 mm) F om isual inspec ion he bes ocus is be ween Image 2 and Image 3 Page 8 / 14 Abo e sc eensho o DS9 (Linux 1 PC unning ARS) shows he b igh s a a h ee di e en ocus se ings a i e a ion le el 2. Focus I e a ion Le el 2 / Round 2 Le Image Middle Image Righ Image Image 2 (38.50 mm) Image 4 (39.00 mm) Image 3 (39.50 mm) F om isual inspec ion he bes ocus is be ween Image 2 and Image 4 Page 9 / 14