The largest digital camera ever built will shed new light on the universe

After more than 20 years’ work, the LSST (Lega­cy Sur­vey of Space and Time) cam­era is now com­plete and ready to be installed on its tele­scope at an alti­tude of 2,700 meters on the Cer­ro Pachón moun­tain in the Chilean Andes, one of the best astro­nom­i­cal sites in the world and already home to numer­ous instru­ments such as the VLT¹ and ALMA².

The 3200-megapix­el cam­era will be capa­ble of scan­ning the entire sky in just three days, tak­ing 800 images per night, each cov­er­ing an area 40 times the size of the Moon. It will thus be able to observe the uni­verse in unprece­dent­ed detail, but not only: it will also help to advance our under­stand­ing of dark ener­gy –  respon­si­ble for the accel­er­at­ed expan­sion of the uni­verse. To do this, it will look for signs of what is known as the weak grav­i­ta­tion­al lens­ing effect, in which very mas­sive clus­ters of galax­ies sub­tly bend the tra­jec­to­ries of light from back­ground galax­ies before it reach­es us. This process reveals impor­tant infor­ma­tion about how mass has been dis­trib­uted in the uni­verse over time.

The cam­era will also look for dark mat­ter, the mys­te­ri­ous sub­stance thought to make up 85% of all mat­ter in the uni­verse by observ­ing the dis­tri­b­u­tion pat­terns of galax­ies and how they evolved over time.

The cam­era was devel­oped and built by researchers and engi­neers at the SLAC Nation­al Accel­er­a­tor Lab­o­ra­to­ry in the US. The part­ner lab­o­ra­to­ries that con­tributed to the project are: Brookhaven Nation­al Lab­o­ra­to­ry in the Unit­ed States, which built the cam­er­a’s dig­i­tal sen­sor array; Lawrence Liv­er­more Nation­al Lab­o­ra­to­ry, also in the US, which built the cam­er­a’s lens­es; and Nation­al Insti­tute of Nuclear and Par­ti­cle Physics (IN2P3/CNRS)in France, which par­tic­i­pat­ed in the design of the sen­sors and elec­tron­ics, and built the cam­er­a’s fil­ter chang­er sys­tem. This sys­tem will enable the cam­era to take images in six dis­tinct bands of light, from the ultra­vi­o­let to the infrared.

“The cam­era is about the size of a car,” explains Johan Bre­geon from the Sub­atom­ic Physics and Cos­mol­o­gy Lab­o­ra­to­ry (LPSC)³ at IN2P3/CNRS, who has been work­ing on the project since 2019. It weighs around 3,000 kg and has three lens­es. The front lens mea­sures almost 160 cm, which is thought to be the largest high-per­for­mance opti­cal lens ever made.

“The LSST cam­era is tru­ly rev­o­lu­tion­ary and will do more than just take pret­ty pic­tures. It is an instru­ment capa­ble of detect­ing light and pro­cess­ing it as faith­ful­ly as possible.”

The three-lens sys­tem allows the field of view to be cor­rect­ed so that a larg­er por­tion of the sky can be observed with­out mak­ing the cam­era big­ger. At the focal plane of these lens­es are the CCD detec­tors, which are the com­po­nents that col­lect the light. Anoth­er impor­tant part of the instru­ment is the fil­ter changer.

“To do astron­o­my, and cos­mol­o­gy in par­tic­u­lar, we need to observe the sky in sev­er­al opti­cal bands (or wave­lengths) down to the near-infrared part of the elec­tro­mag­net­ic spec­trum. To do this, we use fil­ters, that is, pieces of glass that allow a cer­tain band of fre­quen­cies to pass through.”

IN2P3/CNRS con­tributed to the fil­ter chang­er sys­tem, which con­sists of a carousel con­tain­ing six fil­ters and a com­plex mechan­i­cal sys­tem, enabling them to be changed in less than two min­utes. “The fil­ters are 75 cm diam­e­ter discs. The light­est weighs 25 kg and the heav­i­est 38 kg, and we have to be able to posi­tion them with a pre­ci­sion of a few hun­dred microns. What’s also impor­tant to under­stand is that a fil­ter will be changed very reg­u­lar­ly dur­ing the obser­va­tions, sev­er­al times a night. So, over the 10 years that the tele­scope is expect­ed to oper­at­ed, the fil­ter chang­er will typ­i­cal­ly have to oper­ate over a hun­dred thou­sand cycles. From a mechan­i­cal point of view, this is dif­fi­cult to imple­ment because we obvi­ous­ly had to take into account wear problems.”

Asso­ci­at­ed with these fil­ters is the fil­ter load­ing sys­tem. “This sys­tem allows us to take a fil­ter from a box and insert it into the carousel of the fil­tra­tion cham­ber after remov­ing the exist­ing fil­ter. This loader was essen­tial­ly designed, test­ed, built and val­i­dat­ed by the teams at the LPSC in Greno­ble, where I work.”

The chal­lenge was not only to build the world’s largest dig­i­tal cam­era for astron­o­my, but also to be able to quick­ly read out the data from the CCD detec­tors. Cur­rent­ly, for exist­ing cam­eras that oper­ate in more or less the same way, read­ing a few hun­dred mil­lion pix­els takes around 30 sec­onds. “For the LSST, we want­ed to be able to make more than 1,500 expo­sures per night, so 30 sec­onds was too long.”

As a result, there was much opti­mi­sa­tion and design work on the read­out elec­tron­ics to ensure that they were effi­cient and able to read the 3 bil­lion pix­els in around just two seconds.

“As well as improv­ing the read­out elec­tron­ics, we also had to learn a lot about how the CCD detec­tors work, to make sure that once the raw data has been out­put by the cam­era, the images pro­duced are as faith­ful as pos­si­ble to the por­tion of the sky we are observing.”

Sev­er­al lab­o­ra­to­ries are work­ing on the cal­i­bra­tion and reduc­tion of the raw images to obtain the best pos­si­ble images. This part of the cam­era is still in the com­mis­sion­ing phase. “I’m cur­rent­ly analysing some of the data we took last year when the cam­era was at SLAC for its test runs. We have obtained data that will allow me to check the align­ment of the lens­es with the focal plane of the cam­era.” The first images are expect­ed in spring 2025.

Interview by Isabelle Dumé

Références:
Aaron J. Rood­man at al. Inte­gra­tion and ver­i­fi­ca­tion test­ing of the LSST cam­era. SPIE Astro­nom­i­cal Tele­scopes + Instru­men­ta­tion 2018, Jun 2018, Austin, Unit­ed States. pp.107050D, 10.1117/12.2314017. https://​hal​.sci​ence/​h​a​l​-​0​1​8​80806

Pierre Anti­lo­gus et al. Design, assem­bly and val­i­da­tion of the Fil­ter Exchange Sys­tem of LSST­Cam. In SPIE Astro­nom­i­cal Tele­scopes + Instru­men­ta­tion 2022, vol­ume 12182, page 121823A, Mon­tréal, Cana­da, July 2022. doi: 10.1117/12.2629336. https://​hal​.sci​ence/​h​a​l​-​0​3​8​38583