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