Engineers will start cryogenic testing next year of the James Webb Space Telescope's instrument module, the heart of the observatory's unprecedented imaging capabilities, without two of the mission's core sensors as the units contend with delays, according to scientists.

Artist's concept of the James Webb Space Telescope. Credit: NASA
 
The instrument units were built by contractors and science teams in Europe, the United States and Canada, and they will be assembled inside the observatory's scientific module at NASA's Goddard Space Flight Center in Greenbelt, Md.

Two of the sensors - the Mid-Infrared Instrument from Europe and the Fine Guidance Sensor from Canada - were delivered to Goddard in May and July.

Shipment of the telescope's other instruments, a near-infrared camera and spectrometer, is now expected in mid-2013, a year later than planned when officials revamped JWST's budget and schedule in 2011.

The late arrivals mean engineers will conduct the first phase of cryo-vacuum testing on JWST's instrument module without half of the telescope's science payload.

The Integrated Science Instrument Module, or ISIM, is the frame in which technicians will install the mission's four observing instruments. Officials plan three rounds of cryogenic and vacuum testing on the ISIM to ensure its structure, electronics, and detectors will function in cold temperatures in space.

The first cryo-vacuum test of the ISIM will begin in May, according to Matt Greenhouse, project scientist for JWST's instrument payloads.

Greenhouse said engineering units for the telescope's near-infrared camera and spectrograph will be used for the first of three cryo-vacuum test cycles. The last two cycles will use the flight instruments.

The JWST test plan calls for each of the mission's instruments, the telescope's 18 primary segments, and other components to be tested individually at the conditions the units will experience in space. As pieces come together, the assemblies will be tested again to make sure they perform as expected.

Once the instruments arrive at NASA's Goddard Space Flight Center, they will be installed into the Integrated Science Instrument Module, pictured here. Credit: NASA
 
After the ISIM cryo-vacuum tests, the module will be attached to the telescope's mirrors and structure and shipped to NASA's Johnson Space Center in Houston for another exposure to cold temperatures.

The $8.8 billion mission is the successor to the Hubble Space Telescope. NASA is targeting launch of the observatory in October 2018, and officials do not expect the instrument delays to trigger any slip in JWST's launch date.

NASA announced a four-year delay to the telescope's launch last year after independent reviews showed the project needed more time to finish development and testing.

The Near-Infrared Camera, or NIRCam, will be shipped to Goddard between April and June of next year. NIRCam is being assembled and tested at the Lockheed Martin Advanced Technology Center in Palo Alto, Calif.

Earlier this year, officials anticipated NIRCam's delivery in late 2012, but several factors pushed back the completion of the instrument's testing.

"The slip was the result of several factors with the biggest problem being the telescope simulator that Lockheed built to use during testing of NIRCam," said Marcia Rieke, NIRCam's principal investigator at the University of Arizona. "This simulator's hardware was OK, but the control software was not OK and we lost over a month during our first cryogenic test just getting it debugged and running smoothly."

NIRCam will detect infrared light from the earliest galaxies in the universe, observe young stars in the Milky Way, glimpse planets orbiting other stars, and acquire images of objects within the solar system.

"The next biggest time sink came as a result of the testing from the first round where we learned that there were some sneak stray light paths, and we have had to retrofit the flight instrument with some baffles," Rieke wrote in an email to Spaceflight Now. "This was not particularly surprising as one often has issues like this the first time an instrument is tested."

Rieke said NIRCam will start its second cryogenic test in November.

Engineers allocated time for two cold temperature tests of NIRCam before its delivery to Goddard.

"The optical performance requirements on NIRCam are so tight that we needed to test at cryogenic temperature, measure the optical behavior, and then warm up to install shims to trim the focus positions of NIRCam's detector arrays," Rieke said.

File photo of engineers assemblng the flight unit of the NIRSpec instrument. Credit: NASA
 
The delivery of the Near-Infrared Spectrograph, or NIRSpec, to Goddard will occur in August 2013, Greenhouse said. NIRSpec's prime contractor is EADS Astrium of Ottobrunn, Germany.

Engineers had to reconstruct the NIRSpec instrument this year after discovering cracks in its optical bench, the backbone of its optics and detectors.

The replacement of the optical bench necessitates a second round of environmental testing to ensure the instrument can stand up to the harsh conditions of space.

The second environmental test on NIRSpec should begin by early 2013, officials said.