Webb is Almost Ready. There’s One Last Thing To Do

The James Webb Space Telescope is now in the final phase of commissioning as it readies for science observations. Of the more than 1,000 milestones the observatory has needed to reach since launch to become fully operational, the team said today they are down to about two hundred activities to go. But those 200 are all part of the final phase of commissioning the instruments.

“I call it the home stretch,” said Michael McElwain, Webb observatory project scientist in a media briefing on May 9. “There are 17 scientific modes we need to bring online in the next two months, and we need to demonstrate the telescope’s operational capabilities before we are ready to turn the science instruments loose on the Universe.”

Currently, the team said they are digging into the details of the science instruments to complete the commissioning of each. In the next two months they will be making detailed measurements of the performance of the science instruments before routine science operations begin in the summer.

Klaus Pontoppidan, Webb project scientist from the Space Telescope Science Institute said he anticipates that the first official images, called early release observations (ERO) will be made public in mid-July, with a more precise date coming later.

“The early release observations will be a package of spectacular color images and spectra to demonstrate that Webb is fully operational,” said Pontoppidan. “It will be an opportunity to celebrate the beginning of Webb science and to showcase all of Webb’s capabilities.”

Pontoppidan added although JWST is an infrared observatory, the ERO images will be color images, where the infrared colors will be translated into visible colors that humans can see.

The mirrors are now fully aligned and the observatory near its final cryogenic temperatures. The four science instruments have been powered on, and include the Near-Infrared Camera (NIRCam), Near-Infrared Spectrometer (NIRSpec), Near-Infrared Imager and Slitless Spectrometer (NIRISS), and the Mid-Infrared Instrument (MIRI), as well as the Fine Guidance Sensor (FGS), which provides high-precision pointing information as input to the telescope’s attitude control systems.

The last set of commissioning activities include items for each instrument as well as for mechanisms for the whole observatory, wrote Scott Friedman, lead commissioning scientist for Webb, in a blog post.

“We have operated [each instrument’s] mechanisms and detectors, including filter wheels, grating wheels, and the NIRSpec microshutter assembly,” said Friedman. “The Webb optics team used images of isolated stars taken with each of the instruments to align the primary and secondary mirrors of the observatory. But we have more work to do before Webb is fully ready to embark on the ambitious science observations that will reveal the secrets of the universe.”

This includes an extensive set of calibrations and characterizations of the instruments using a wide variety of astronomical source to test out each instrument’s capabilities and strengths.

“We will measure the instruments’ throughput – how much of the light that enters the telescope reaches the detectors and is recorded,” Friedman said. “There is always some loss with each reflection by the mirrors of the telescope and within each instrument, and no detector records every photon that arrives. We will measure this throughput at multiple wavelengths of light by observing standard stars whose light emission is known from data obtained with other observatories combined with theoretical calculations.”

They will also perform astrometric calibration of each instrument by observing a small patch of sky in a nearby galaxy, the Large Magellanic Cloud, and comparing the images to previous observations by other observatories, including the Hubble Space Telescope. You can see more comparisons of the first views of the instruments to other observatories here.

The team will also test out Webb’s abilities to do ‘time series observations,’ “where one takes a very long series of exposures to track what is happening during an exoplanet transit,” said Marcia Rieke, principal investigator for Webb’s Near-Infrared Camera at the May 9 briefing. This is where an exoplanet passes in front of its host star, and JWST can measure the dimming of the star’s light when planet goes in front. “We’ve already proven we can upload the right commanding and time sequence to catch those transits,” Reike said.

They also need to test JWST’s moving target tracking. This is for observing objects in our own solar system objects, such as moons around other planets, asteroids and ice-covered Kuiper Belt Objects. “Since these objects are always moving, we have to adjust the pointing of JWST to keep the object centered in the field of view,” Reike explained. “This requires coordination between the instrument, the fine guidance system and the controls that point the telescope. More to come but we are learning how to do all these exquisitely useful and complicated techniques of taking data. We’re finding out what the telescope can do.”

And so far, the telescope is exceeding even the most optimistic predictions of what JWST will be able to see and do. The scientists said the sharper views they are now seeing hint at even more possibilities for the types of science the new observatory will be able to perform.