Team Begins Syncing Web Space Telescope – Archyde

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Established in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. It’s vision is “To discover and expand knowledge for the benefit of humanity.”

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This week, the three-month process of aligning the telescope began – and over the last day, Webb team members saw the first photons of starlight that traveled through the entire telescope and were detected by the Near Infrared Camera (NIRCam) instrument. This milestone marks the first of many steps to capture images that are at first unfocused and use them to slowly fine-tune the telescope. This is the very beginning of the process, but so far the initial results match expectations and simulations.

A team of engineers and scientists from Ball Aerospace, Space Telescope Science Institute, and NASA’s Goddard Space Flight Center will now use data taken with NIRCam to progressively align the telescope. The team developed and demonstrated the algorithms using a 1/6th scale model telescope testbed. They have simulated and rehearsed the process many times and are now ready to do this with Webb. The process will take place in seven phases over the next three months, culminating in a fully aligned telescope ready for instrument commissioning. The images taken by Webb during this period will not be “pretty” images like the new views of the universe Webb will unveil later this summer. They strictly serve the purpose of preparing the telescope for science.

To work together as a single mirror, the telescope’s 18 primary mirror segments need to match each other to a fraction of a wavelength of light – approximately 50 nanometers. To put this in perspective, if the Webb primary mirror were the size of the United States, each segment would be the size of Texas, and the team would need to line the height of those Texas-sized segments up with each other to an James Webb Space Telescope Laser-Focused Sight

Ball Aerospace’s Scott Acton and Chanda Walker, along with NASA Goddard’s Lee Feinberg, walked through the following basic steps:

“Now that the deployment of the mirror segment is complete and the instrument is powered up, the team has begun many of the steps needed to prepare and calibrate the telescope for its work. The telescope’s commissioning process will take longer than previous space telescopes because Webb’s primary mirror consists of 18 individual mirror segments that must work together as a high-precision optical surface. The steps in the commissioning process include:

  1. Segmentbildidentifikation
  2. Segmentausrichtung
  3. Image stacking
  4. Rough phasing
  5. smooth phase
  6. Telescope alignment via instrument field of view
  7. Iterate alignment for final correction

1. Identify image segments

First we need to align the telescope relative to the spacecraft. The spaceship is able to perform very precise aiming movements with the help of a “Star Tracker”. Think of the star tracker as the GPS for the spaceship. Initially, the position of the Star Tracker spacecraft did not match the position of the individual mirror segments.

We aimed the telescope at a bright, isolated star (HD 84406) to take a series of images, which were then combined into one image of that part of the sky. But remember, we don’t just have a mirror to look at this star; We have 18 mirrors, each initially tilted to a different part of the sky. As a result, we will actually capture 18 slightly shifted copies of the star – each uniquely blurred and distorted. We call this copy of the initial star a “segment picture”. In fact, depending on the initial position of the mirror, it may take several iterations to find all 18 segments in a single image.

We will shift the 18 mirror segments one by one to determine which segment creates the image of which segment. After matching the mirror segments to their respective images, we can tilt the mirror to bring all images closer to the same point for further analysis. We refer to this arrangement as an “image array”.

2. Segmentkoordination

Once we have a series of images, we can perform segment alignment, which corrects for most of the large mirror segment positioning errors.

We start by defocusing the segment image by slightly moving the secondary mirror. A mathematical analysis called phase capture is applied to blurred images to determine the exact segment position error. Segmentation adjustments then resulted in 18 well-corrected “telescopes”. However, the segments still do not work together as a single mirror.

Simulation of web segment alignment

(Left) Before: Simulation of the initial image layout. (Right) After: A simulated array of 18 corrected segments. Photo credit: NASA

3. Image stacking

In order to place all the light in one place, all image segments must be stacked on top of each other. In the image stacking step, we shift the individual segment images so that they fall exactly in the middle of the plane to create a single unified image. This process prepares the telescope for coarse phasing.

Stacking was performed sequentially in three groups (A segment, B segment and C segment).

Web-Mirror-SegmentWeb-Mirror-Segment

Web Image Stacking-SimulationWeb Image Stacking-Simulation

Image stacking simulation. First Panel: Initial Image Mosaic. Second Panel: Stacked A-Segments. Third panel: Segments A and B are stacked. Fourth panel: Segments A-, B- and C are stacked. Photo credit: NASA

4. Rough phasing

Although Image Stacking puts all the light in one spot on the detector, the segments still function as 18 small telescopes rather than one large telescope. The segments must align with each other with a precision less than the light wavelength.

Coarse Phased is performed three times during the commissioning process and measures and corrects for the vertical displacement (piston difference) of the mirror segment. Using a technology known as Dispersed Fringe Sensing, we used the NIRCam to capture the light spectrum from 20 separate pairs of mirror segments. The spectrum differs much like a barber’s wand pattern, with the tilt (or angle) determined by the pistons of the two segments in the pair.

Webb-Peripheriesensor-SimulationWebb-Peripheriesensor-Simulation

In this simulation, the “Barber Pole” pattern is generated by the Disperse Fringe Sensor, which indicates a large butt error (top) or a small butt error (bottom). Photo credit: NASA

5. Subtle phases

The subtle stages are also performed three times, immediately after each round of the gross stages and then periodically throughout Webb’s life. This process measures and corrects any remaining alignment errors using the same blurring method used during segment alignment. However, instead of a secondary mirror, we use a special optical element in the science instrument that introduces a different amount of blur for each image (blur waves -8, -4, +4, and +8).

Good web phasing simulationGood web phasing simulation

A simulation of the defocused image used in Fine Phasing. The image (above) shows the blur introduced into the telescope, which is nearly parallel. The analysis (below) shows the errors associated with each segment of the telescope. Segments with very light or dark colors require more correction. Photo credit: NASA

6. Align the telescope to the instrument’s field of view

After fine phasing, the telescope is well aligned at a point in the NIRCam’s field of view. Now we need to extend the alignment to other instruments.

At this stage of the commissioning process, we take measurements at multiple locations, or field points, on each scientific instrument, as shown below. A larger intensity variation indicates a larger error at that field point. The algorithm calculates the final correction needed to achieve a telescope that is aligned across all scientific instruments.

Webb's field of view correction simulationWebb's field of view correction simulation

Field of View Correction Simulation Analysis. Photo credit: NASA

7. Iterate alignment for final correction

After applying the field-of-view correction, the elimination of small residual positioning errors in the primary mirror segment remains. We measure and correct with the Fine Phasing method. We perform a final image quality check in each scientific instrument; Once this is verified, the wavefront detection and control process will be complete.

As we go through the seven steps, we may find that we also need to repeat the previous steps. The process is flexible and modular to allow for iteration. After about three months of aligning the telescope, we can proceed to commissioning the instrument.”

Webb, written by Scott Acton, is lead scientist for wavefront sensing and control at Ball Aerospace; Chanda Walker, Webb Wavefront Sensing and Control, Ball Aerospace; and Lee Feinberg, Webb Optical Telescope Element Manager, NASA Goddard Space Flight Center.

Reference-www.nach-welt.com

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