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

Johan Bregeon
Johan Bregeon
Research Fellow in Observational Cosmology at Grenoble Laboratory of Subatomic Physics and Cosmology
Key takeaways
  • After 20 years of development, the LSST camera has been finalised.
  • Thanks to the technology and the expertise of various laboratories, it will be able to observe the universe in unprecedented detail.
  • The 3,200-megapixel camera will offer a new understanding of the universe by studying dark matter and dark energy.
  • The efficiency of LSST is guaranteed by the innovative technologies that make it up: six filters, filter changer, fast readout electronics, CCD detectors, etc.
  • Optimisation of the readout electronics will enable the camera to process data efficiently, paving the way for major discoveries in astronomy.
  • While certain aspects are still in the commissioning phase, the first images are expected in spring 2025.

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.

A truly revolutionary camera

“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.”

Front view of the LSST cam­era, show­ing the 3,200-megapixel focal plane inside. Cred­it: Jacque­line Ram­sey­er Orrell/SLAC Nation­al Accel­er­a­tor Laboratory

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.”

Six filters and a complex mechanical system

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.”

Schemat­ics of the main com­po­nents of the LSST cam­era. Cred­it: Chris Smith/SLAC Nation­al Accel­er­a­tor Laboratory

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.”

Fast readout of CCD detector data

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

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

3The LPSC is a joint CNRS/Université Greno­ble Alpes lab­o­ra­to­ry

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