What is the James Webb Space Telescope doing now, which will become mankind’s new eyes? The James Webb Space Telescope, which will begin its first observations in May or June of this year, has already been launched more than three months ago. On the occasion of Science Month, we summarize the progress from the launch of the James Webb Space Telescope to the present, and summarize the plans for the future.
JWST mirror and fine alignment work begins
As of February 1, 2022, JWST, which has finally completed all the deployment process and has spread its wings and started to show off its wings (shortcut to related article), has only the telescope mirror and micro-alignment work, which is scheduled for about 3 months, and the cooling process of telescopes and instruments. is left behind
Ball Aerospace and a team of engineers and scientists at NASA’s Goddard Space Flight Center will now use data taken with NIRCam to gradually align the telescope. The team developed and demonstrated the algorithm using a model telescope, which is one-sixth the size of the real model, and successfully completed several tests before launching the JWST. The above alignment process is a process to prepare for the test operation, so when the alignment process is completed, the JWST telescope will start the first test operation.
The main mirror of the JWST consists of 18 individual mirror segments (segmetns) for reference. If the main diameter is the size of the continental United States, then each of the 18 segments is about the size of the state of Texas. Because they must work together as a single, high-precision optical surface, the JWST telescope’s preparation for a test drive takes significantly longer than previous space telescopes.
Each segment and location of the main mirror ⓒ JWST/NASA
For a main mirror made up of 18 primary mirror segments to act as a single mirror, all segments must match at a constant wavelength of light (approximately 50 nm). The alignment and test run preparation steps can be divided into a total of 7 steps: Segment Image Identification, Segment Alignment, Image Stacking, Coarse Phasing, and Fine Phasing. , Telescope Alignment Over Instrument Fields of View, and Iterate Alignment for Final Correction.
The seven steps are not necessarily all in order. It follows a modular algorithm designed to be flexible and repeatable, and after about three months of telescope alignment, JWST is finally ready for equipment test operation.
JWST Mirror and Fine Alignment Operation 1st Step – Segment Image Identification
First, it is necessary to align the telescope based on the JWST spacecraft. The JWST spacecraft can perform very accurate pointing motions using “star trackers” called spacecraft GPS, but the position of the spacecraft identified by the first star tracker may not exactly match the position of each of the 18 mirror segments. there is. This is because each mirror is initially tilted towards a different part when deployment is complete after launch.
On February 2nd, the JWST team announced that they had started Segment Image Identification, the first step in mirror alignment, using a NIRCam (near infrared camera) device. The reason NIRCam was used in the initial alignment stage is that the crisis device can operate safely at a higher temperature than other devices and has a wide field of view. When the NIRCam, which is still undergoing cooling, reaches 120K (-153 degrees Celsius), the optics team meticulously moves the 18 main mirror segments, ready to form a single mirror surface. Therefore, the first device to detect photons among all devices in JWST is NIRCam.
Segment image identification process ⓒ JWST/NASA
The JWST team first targeted HD 84406, a bright, isolated star in the constellation Ursa Major, and began identifying light from the same star in each of its 18 primary mirror segments to ensure it was ready to collect light through NIRCam. Using a near-infrared camera (NIRCam) device to detect the first starlight, the JWST team obtained a mosaic of images of 18 randomly constructed starlight dots that align and focus the telescope. For reference, the initial results above were almost in agreement with the simulation results. Therefore, starting with segment image identification, gradual manipulation is performed until the above 18 images become one image.
Segment image identification complete ⓒ JWST/NASA
JWST Mirror and Fine Alignment Step 2 – Segment Alignment
After the segment image identification process is completed, segment alignment should be performed to correct positional errors so that all images can point to one common place through additional analysis. The above operation starts by moving the auxiliary mirror slightly to defocus the segmented image. A mathematical analysis called phase retrieval is used on the defocused images to determine the exact position error of the segment. The above step can be said to be a key step in making the light of all mirrors overlap and work harmoniously.
Segment image alignment completed ⓒ JWST/NASA
According to Dr. René Doyon and Dr. Nathalie Ouellette of the University of Montreal, on February 13, they successfully performed the first microscopic work using the Fine Guidance Sensor (FGS). For reference, the fine workmanship is so accurate that a person in New York City can capture the eye movement of a person blinking at the Canadian border, 500 kilometers away.
Segment image alignment completed ⓒ JWST/NASA
After correcting for segment positioning errors, 18 well-corrected “telescopes” are created, yet the segments do not work together as a single mirror.
JWST Mirror and Fine Alignment Operation 3rd Step – Image Stacking
To bring all the light into one place, each segmented image must be superimposed on each other. In the image stacking step, the individual segment images are moved so that they fall precisely in the center of the field to create one unified image.
Image Stacking Simulation Results ⓒ JWST/NASA
The above operation is performed by a total of three groups (A-segment, B-segment, and C-segment) divided according to distance. That is, after activating a set of six mirrors at a time, it runs until all the points in the starlight overlap each other.
3 groups of segments divided according to distance from the center ⓒ JWST/NASA
On February 25, the JWST team announced that they had successfully performed up to the third stage of a total of 7 stages of mirror alignment, and announced that they had entered the fourth stage, the coarse phase adjustment stage.
Image Stacking Process ⓒ JWST/NASA
Image stacking completed ⓒ JWST/NASA
The cooling of JWST is also progressing steadily.
In February 2022, JWST began the long process of aligning the telescope mirrors, but the cooling process of the JWST telescope is also continuing.
JWST’s cooling has been steadily progressing with NIRCam first. ⓒ JWST/NASA
Huge sunshields protect telescopes, cameras and scientific instruments from direct sunlight and sunlight reflected from the Earth and the Moon. The cold side of the sunshield is passively cooled and begins to radiate heat into outer space. The milliwatt energy passing through the sunshield and the heat generated by the device’s own electronics precisely balance the heat loss. The passive cooling process above started after the sunshield was fully deployed and will continue until the telescope and three near-infrared (NIR) instruments reach steady-state temperatures.
The main mirror cools to less than 50K (-223 degrees Celsius) and NIR instruments are expected to reach around 40K (-233 degrees Celsius). Each segment of the main mirror has a different temperature. As of February 9th, the segments close to the sunshield are slightly warmer. Overall, all parts of the mirror are expected to be cooled by another 10K, and the final temperature is expected to be around 15-20K. On the other hand, the auxiliary mirror (minor mirror) hanging from the end of the support structure is already cold-cooled (~30K).
Temperatures of major and minor components (PMSA) and minor components (SMA) as of February 9 © JWST/NASA
A mid-infrared instrument (MIRI), on the other hand, requires much more cooling. In addition to passive cooling, the MIRI is cooled to temperatures below 7K by closed cycle gaseous helium cryogenic coolers. Unlike some previous cryogenic missions, where they were cooled by evaporating liquid helium and ejecting it into space, MIRI’s coolers recycle helium as the cooler continues to recycle its own coolant. As of February 10, scientific instruments, excluding telescopes, have a temperature of approximately 75K (-198°C) and several additional cooling processes have begun to reach their final operating temperature.
Cooling is essential to achieve the JWST’s objectives, which have been steadily prepared through many years of infrared missions. Observation using infrared rays started in the 1800s, but the observation missions that made a big mark in infrared observation are IRAS (Infrared Astronomy Satellite) and ISO (Infrared Space Observatory). In the above observations, the biggest issue was how much the telescope can be cooled. Usually, liquid helium can be used to keep the inside cool, but there is a disadvantage that the size of the telescope cannot be increased to a certain extent for this purpose. Therefore, JWST chose to recycle helium by continuously releasing heat into space, a technique first used by NASA’s Infrared Space Telescope Spitzer and ESA’s Herschel Telescope.
* Please note that all times are based on Eastern Standard Time.
[이전 편 : 제임스 웹 우주 망원경은 현재 무엇을 하고 있을까? (1) 보러가기]
[이전 편 : 제임스 웹 우주 망원경은 현재 무엇을 하고 있을까? (2) 보러가기]
(2695)
