To interact as a single mirror, the telescopes 18 primary mirror sections require to match each other to a portion of a wavelength of light– roughly 50 nanometers. To put this in point of view, if the Webb main mirror were the size of the United States, each sector would be the size of Texas, and the group would require to line the height of those Texas-sized sectors up with each other to a precision of about 1.5 inches.
Section Image Identification.
Section Alignment.
Image Stacking.
Coarse Phasing.
Great Phasing.
Telescope Alignment Over Instrument Fields of View.
Repeat Alignment for Final Correction.
Scott Acton and Chanda Walker of Ball Aerospace, together with Lee Feinberg of NASA Goddard, stroll through the fundamental actions listed below:.
” With implementation of the mirror sectors now complete, and the instruments turned on, the team has started the many actions required to prepare and adjust the telescope to do its task. The telescope commissioning procedure will take a lot longer than previous space telescopes since Webbs primary mirror consists of 18 individual mirror sectors that need to work together as a single high-precision optical surface. The steps in the commissioning procedure consist of:.
This animation shows the course light will follow as it hits the primary James Webb Space Telescope (JWST) mirror, and is shown to the secondary, and then in through the aft optics assembly where the fine and tertiary steering mirrors are. The light is then reflected and split and directed to the science instruments by pick-off mirrors. JWST is a three-mirror anastigmat telescope. Credit: NASA, ESA, and G. Bacon (STScI).
Today, the three-month procedure of aligning the telescope began– and over the last day, Webb team members saw the very first photons of starlight that took a trip through the whole telescope and were spotted by the Near Infrared Camera (NIRCam) instrument. This milestone marks the very first of lots of actions to capture images that are at first unfocused and use them to gradually fine-tune the telescope. This is the very start of the process, but up until now the initial results match expectations and simulations.
A team of engineers and scientists from Ball Aerospace, Space Telescope Science Institute, and NASAs Goddard Space Flight Center will now use data taken with NIRCam to progressively line up the telescope. The team established and showed the algorithms utilizing a 1/6th scale model telescope testbed. They have simulated and practiced the procedure lot of times and are now prepared to do this with Webb. The procedure will happen in seven phases over the next three months, culminating in a fully lined up telescope ready for instrument commissioning. The images taken by Webb during this duration will not be “pretty” images like the brand-new views of the universe Webb will reveal later this summer season. They strictly satisfy of preparing the telescope for science.
1. Sector Image Identification.
We need to line up the telescope relative to the spacecraft. At first, the position of the spacecraft from the star trackers does not match the position of each of the mirror segments.
We are pointing the telescope at an intense, isolated star (HD 84406) to record a series of images that are then sewn together to form an image of that part of the sky. Depending on the starting positions of the mirrors, it may take several iterations to locate all 18 segments in one image.
Simulated example of a possible initial deployment showing 18 sector images. Credit: NASA.
One by one, we will move the 18 mirror sectors to determine which sector creates which sector image. After matching the mirror segments to their particular images, we can tilt the mirrors to bring all the images near a typical point for more analysis. We call this arrangement an “image array.”.
2. Section Alignment.
After we have the image variety, we can perform Segment Alignment, which fixes the majority of the large positioning errors of the mirror sectors.
We start by defocusing the sector images by moving the secondary mirror somewhat. Mathematical analysis, called Phase Retrieval, is applied to the defocused images to determine the precise placing mistakes of the sections. Adjustments of the sectors then result in 18 well-corrected “telescopes.” The segments still do not work together as a single mirror.
( Left) Before: Simulated preliminary selection of images. (Right) After: Simulated variety of 18 remedied segments. Credit: NASA.
3. Image Stacking.
To put all of the light in a single place, each section image should be stacked on top of one another. In the Image Stacking step, we move the specific segment images so that they fall precisely at the center of the field to produce one merged image. This procedure prepares the telescope for Coarse Phasing.
The stacking is performed sequentially in 3 groups (B-segments, a-segments, and c-segments).
” With release of the mirror sectors now complete, and the instruments turned on, the team has begun the various actions required to prepare and calibrate the telescope to do its task. The telescope commissioning process will take much longer than previous space telescopes since Webbs primary mirror consists of 18 private mirror sections that require to work together as a single high-precision optical surface. One by one, we will move the 18 mirror sectors to determine which section produces which segment image. After matching the mirror sectors to their respective images, we can tilt the mirrors to bring all the images near a typical point for additional analysis. The analysis (bottom) shows the errors associated with each telescope sector.
Simulation of image stacking. Panel: Initial image mosaic. 2nd panel: A-segments stacked. Third panel: A- and B-segments stacked. 4th panel: A-, B-, and C-segments stacked. Credit: NASA.
4. Coarse Phasing.
Image Stacking puts all the light in one place on the detector, the sections are still acting as 18 small telescopes rather than one huge one. The segments require to be associated each other with a precision smaller sized than the wavelength of the light.
Carried out three times throughout the commissioning process, Coarse Phasing measures and fixes the vertical displacement (piston distinction) of the mirror sectors. Utilizing a technology understood as Dispersed Fringe Sensing, we utilize NIRCam to catch light spectra from 20 different pairings of mirror sectors. The spectrum will look like a barber pole pattern with a slope (or angle) determined by the piston distinction of the 2 sectors in the pairing.
In this simulation, the “Barber pole” patterns are produced by the Disperse Fringe Sensor showing a large piston mistake (top) or a small piston error (bottom). Credit: NASA.
5. Fine Phasing.
Fine Phasing is likewise performed 3 times, straight after each round of Coarse Phasing, and after that consistently throughout Webbs life-span. These operations procedure and fix the staying positioning mistakes using the very same defocusing approach used throughout Segment Alignment. Nevertheless, instead of using the secondary mirror, we utilize unique optical aspects inside the science instrument which present varying quantities of defocus for each image (-8, -4, +4, and +8 waves of defocus).
A simulation of the defocused images used in Fine Phasing. The images (top) reveal defocus presented to a practically aligned telescope. The analysis (bottom) suggests the mistakes connected with each telescope segment. Sections with extremely bright or dark colors require larger corrections. Credit: NASA.
6. Telescope Alignment Over Instrument Fields of View.
After Fine Phasing, the telescope will be well aligned at one place in the NIRCam field of view. Now we need to extend the alignment to the remainder of the instruments.
In this phase of the commissioning process, we make measurements at numerous places, or field points, throughout each of the science instruments, as shown listed below. More variation in strength shows bigger mistakes at that field point. An algorithm calculates the last corrections needed to accomplish a well-aligned telescope across all science instruments.
Simulated analysis of the Field of View correction. Credit: NASA.
7. Iterate Alignment for Final Correction.
After applying the Field of View correction, the crucial thing delegated address is the removal of any small, residual positioning errors in the primary mirror sections. We measure and make corrections utilizing the Fine Phasing procedure. We will do a last check of the image quality throughout each of the science instruments; when this is verified, the wavefront sensing and controls procedure will be complete.
As we go through the 7 steps, we might find that we require to repeat earlier actions. The procedure is modular and versatile to permit for iteration. After approximately three months of aligning the telescope, we will be prepared to continue to commissioning the instruments.”.
Written by Scott Acton, Webb lead wavefront noticing and control scientist, Ball Aerospace; Chanda Walker, Webb wavefront sensing and control researcher, Ball Aerospace; and Lee Feinberg, Webb optical telescope component manager, NASAs Goddard Space Flight Center.