. ▁Histor ically , ▁phot ographic ▁pl ates ▁were ▁widely ▁used ▁to ▁record ▁spect ra ▁until ▁electronic ▁detect ors ▁were ▁developed , ▁and ▁today ▁optical ▁spect ro graph s ▁most ▁often ▁employ ▁charge - cou ple d ▁devices ▁( CC D s ). ▁The ▁w avel ength ▁scale ▁of ▁a ▁spectrum ▁can ▁be ▁cal ibr ated ▁by ▁obser ving ▁the ▁spectrum ▁of ▁emission ▁lines ▁of ▁known ▁w avel ength ▁from ▁a ▁gas - d isch ar ge ▁lamp . ▁The ▁flux ▁scale ▁of ▁a ▁spectrum ▁can ▁be ▁cal ibr ated ▁as ▁a ▁function ▁of ▁w avel ength ▁by ▁comparison ▁with ▁an ▁observation ▁of ▁a ▁standard ▁star ▁with ▁corre ctions ▁for ▁atmos pher ic ▁absor ption ▁of ▁light ; ▁this ▁is ▁known ▁as ▁spect roph ot ometry . ▁ ▁Radio ▁spect ro sc opy ▁Radio ▁astronom y ▁was ▁founded ▁with ▁the ▁work ▁of ▁Karl ▁J ans ky ▁in ▁the ▁early ▁ 1 9 3 0 s , ▁while ▁working ▁for ▁Bell ▁L abs . ▁He ▁built ▁a ▁radio ▁anten na ▁to ▁look ▁at ▁potential ▁sources ▁of ▁inter ference ▁for ▁trans at l antic ▁radio ▁transm issions . ▁One ▁of ▁the ▁sources ▁of ▁noise ▁discovered ▁came ▁not ▁from ▁Earth , ▁but ▁from ▁the ▁center ▁of ▁the ▁Mil ky ▁Way , ▁in ▁the ▁const ell ation ▁S ag itt arius . ▁In ▁ 1 9 4 2 , ▁JS ▁Hey ▁captured ▁the ▁sun ' s ▁radio ▁frequency ▁using ▁military ▁rad ar ▁rece ivers . ▁Radio ▁spect ro sc opy ▁started ▁with ▁the ▁discovery ▁of ▁the ▁