James Webb Space Telescope Locked Onto Guide Star for High Accuracy Mirror Alignment

James Webb Space Telescope Primary Mirror Alignment

James Webb Space Telescope Primary Mirror Alignment. Credit: NASA’s Goddard Space Flight Center

After starting the mirror alignment with Webb’s first detection of starlight in the Near-Infrared Camera (NIRCam), the telescope team is hard at work on the next steps for commissioning the telescope. To make more progress, the team needs to use another instrument, the Fine Guidance Sensor, to lock onto a guide star and keep the telescope pointed to high accuracy. We have asked René Doyon and Nathalie Ouellette of the Université de Montréal to explain how Webb uses its Canadian instrument in this process.

“After being powered on January 28, 2022, and undergoing successful aliveness and functional tests, Webb’s Fine Guidance Sensor (FGS) has now successfully performed its very first guiding operation! Together with the Near-Infrared Imager and Slitless Spectrograph (NIRISS), the FGS is one of Canada’s contributions to the mission.

“To ensure Webb stays locked on its celestial targets, the FGS measures the exact position of a guide star in its field of view 16 times per second and sends adjustments to the telescope’s fine steering mirror about three times per second. In addition to its speed, the FGS also needs to be incredibly precise. The degree of precision with which it can detect changes in the pointing to a celestial object is the equivalent of a person in New York City being able to see the eye motion of someone blinking at the Canadian border 500 kilometers (311 miles) away!

“Webb’s 18 primary mirror segments are not yet aligned, so each star appears as 18 duplicate images. On February 13, FGS successfully locked onto and tracked one of these star images for the first time. The FGS team was thrilled to see this ‘closed loop guiding’ working! From now on, most of the alignment process of the telescope mirrors will take place with FGS guiding, while NIRCam images provide the diagnostic information for mirror adjustments.”

René Doyon, principal investigator for FGS/NIRISS, Université de Montréal; and Nathalie Ouellette, Webb outreach scientist, Université de Montréal

Written by:

  • Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center
  • And Alexandra Lockwood, project scientist for Webb science communications, Space Telescope Science Institute

5 Comments on "James Webb Space Telescope Locked Onto Guide Star for High Accuracy Mirror Alignment"

  1. very good!Some THoughts for cconsideration
    1 visited an opthaaligist ReCently?
    2. How does he decide how to fix you up with Eyelasses or Contact Lenses?
    3 First he asks you to Read ADCD Chart on the wall from a certain distance…. Alphabets of of Different Sizes.
    4. Then he puts som eye drops in your eyes andasks you to wait for some time, then you asked to place your chin on a device eye front of device and peer through it. As you look through the contraption , first with one eye and then the next eye he keeps asking about the clarity of the Image you are observing.
    5. At the same time he observes carefully the expanded retina (thanks to the liquid added to your eye) of your two human eyes one at a time to ensure Retina Health.
    6. The contraption spits out a small peice of paper to tell him the additional lens you need to get back 20/20 vision. This is what the Opthomoligist writes on his prescription, for the Optician to prepare and give you a additional set of lenses/eyeglass for restoration of 20/20 vision.
    7. The Diagnosis may be tat you are short sighted or have difficulty in focusing on objects some distance away. Negative or Positive power lenses may need to be made to get you to 20/20 vision.
    8. On planet earth they have deviced Progressive Lenses , which decides and adjst to the environment , lighting and other existing conditions so you can get to 20/20 vision and view near and far objects with great clarity!!

    I am confident that the brilliant Scientists have some optholagists and opticians on the team which has designed these telescopes and additional lenses and eye glasses / contact lenses ,as Progressive Glasses for capturing the image from near and distant objects in the universe.

    Views exxpressed are personal and not binding on anyone!

    • Torbjörn Larsson | February 19, 2022 at 9:17 pm | Reply

      Modern telescopes uses mirrors, not lenses, to minimize diffraction problems.

      While the overall alignment procedure may be somewhat analogous to analyze the optical properties of eyes, the guidance system is more like your own eye tracking.

  2. Apologies fr spelling mistakes. Paost was written and sent without reviewing the same.
    An etra set of eyes would have been helpful in minimising errors and mistakes!

  3. Did it again.

    Won’t it be a better world if the Blind could see!

  4. The first star test image revealed that of course the 18 segments were not aligned, totally normal. But each of the individual star images were also horribly distorted, in different ways! Probably most of this will resolve when the segments are aligned, but I wonder if each beryllium segment can be tweaked to account for distortions induced by the launching and space travel?

Leave a comment

Email address is optional. If provided, your email will not be published or shared.