I've transitioned to a new home at https://jwstlog.blogspot.com/.
Webb In Focus
Webb Image Sharpness Check Highlights - Engineering images of sharply focused stars in the field of view of each instrument demonstrate that the telescope is fully aligned and in focus. For this test, Webb pointed at part of the Large Magellanic Cloud, a small satellite galaxy of the Milky Way, providing a dense field of hundreds of thousands of stars across all the observatory’s sensors. The sizes and positions of the images shown here depict the relative arrangement of each of Webb’s instruments in the telescope’s focal plane, each pointing at a slightly offset part of the sky relative to one another. Webb’s three imaging instruments are NIRCam (images shown here at a wavelength of 2 microns), NIRISS (image shown here at 1.5 microns), and MIRI (shown at 7.7 microns, a longer wavelength revealing emission from interstellar clouds as well as starlight). NIRSpec is a spectrograph rather than imager but can take images, such as the 1.1 micron image shown here, for calibrations and target acquisition. The dark regions visible in parts of the NIRSpec data are due to structures of its microshutter array, which has several hundred thousand controllable shutters that can be opened or shut to select which light is sent into the spectrograph. Lastly, Webb’s Fine Guidance Sensor tracks guide stars to point the observatory accurately and precisely; its two sensors are not generally used for scientific imaging but can take calibration images such as those shown here. This image data is used not just to assess image sharpness but also to precisely measure and calibrate subtle image distortions and alignments between sensors as part of Webb’s overall instrument calibration process. Read more... Credit: NASA/STScI
How awesome is JWST/MIRI? Well, let's compare the latest press release image to that of the WISE all-sky survey at 4.6 microns. This is the closest wavelength image I could find. Spitzer IRAC would have been better (slightly higher resolution and similar wavelength). pic.twitter.com/EXqP57sULt
— Andras Gaspar 🌻🇺🇦 (@AndrasGaspar) April 29, 2022
Not quite enough distant background galaxies for my taste, but #JWST is looking ever more awesome! pic.twitter.com/pyJ8VH4fUo
— gbrammer (@gbrammer) April 29, 2022
Thank you to @gbrammer for pointing out that the Spitzer SAGE survey took IRAC images of this region! I completely missed this. Check out the evolution of Infrared Space Telescopes! pic.twitter.com/BjrbAgk4ND
— Andras Gaspar 🌻🇺🇦 (@AndrasGaspar) April 29, 2022
Actually here's an even better zoomed view: pic.twitter.com/y49MCm52uy
— Karin Sandstrom (@karinsandstrom) April 29, 2022
Since #JWST's MIRI is getting lots of before-and-after love, I thought I'd do the same for the Fine Guidance Sensor: here's one of its two fields in the Large Magellanic Cloud as previously imaged in the near-IR by @eso's VISTA survey telescope.
— Mark McCaughrean (@markmccaughrean) April 30, 2022
1/ pic.twitter.com/G4yfhPWTqQ
Ok, I’ve never done this, but …here is what this image really says about the first ever deployed, segmented, light-weighted (did I mention cryo?) space telescope, so buckle up… Let’s start with sharp images across 20 arc minutes, that’s almost double the field of view of Hubble thanks to adding an elliptical tertiary that not only expands the field but gives us a reimaged pupil in just right place for a fine steering mirror… (Oh, and that fine steering mirror uses cryogenic difference impedance transducers to sense tilt and voice coils to position and that all had to withstand launch, cooldown…but I digress)… and lets us take these 5-12 minute exposures keeping those stars stable with just tiny mirror adjustments every second without having to body point the 22 meter Sunshield and 8 meter secondary structure (a la Hubble …) And those sharp images are thanks to not only diameter but 18 mirrors polished to under 50 nanometers over 25.4 square meters including compensating for over 100 nm rms per segment of gravity distortion, roughly 150 nanometer cryogenic distortion unique to each segment, and the hardest challenge of all …matching the radius of curvature across all 18 mirrors which required figuring out how to test the mirrors radius of curvature in a cryo chamber (there’s vibration and a refractive window in there folks)… attached to 132 actuators that move coarsely in microns, finely in nanometers all at 30-55 Kelvin attached to rad hard cryo multiplexers and junction boxes and all of that attached to a cryo nanometer-stable custom laminate structure including two deployed wings to a deployed tower with deployed harnesses and cooler lines (who does that?) not to mention vibration attenuators and constrained layer 1 hz isolators at the warm to cold interface so our images don’t jitter and we had to place those mirrors on the backplane with a huge robotic arm, laser trackers, laser radar, and install them strain free within 10’s of microns, shake them to g’s, acoustically blast them to db’s with humongous speakers, then the mother of all tests in a large helium shroud with 4 large windmills with cryo photogrammetry cameras, 3 1.5 meter cryo autocollimating flats, a custom multi wavelength interferometer, novel reflective null lens all to check the primary mirror and cryo alignments so that after launch, then 50 large deployments, then 19 mirror deployments we could finally execute the wavefront algorithms we developed 20 years earlier, demonstrated on a scaled testbed (an engineering marvel unto itself but I digress). All only using the main science camera as the wavefront sensor so we can fit in the rocket without a dedicated wavefront sensor and ... finally, go from 18 blurry dots of a bright isolated star to a phased primary mirror aligned better than 1/5000th of a human hair across 6.5 meters with a secondary mirror 7.2 meters away on a deployed tripod positioned in 6 degrees of freedom to microns all to assure it is not just one central star in focus but stars across the field all perfectly aligned and limited optically by the laws of physics. Nothing elliptical or aberrated or blurry and it all worked. That’s what this picture means… -- lee feinberg
Selfies, Alignment Mosaics, Image Arrays, Image Stacking and Fine Phasing - James Webb Space Telescope's First 9 Images Released To The Public
Primary Mirror "selfie" - This “selfie” taken by Webb of its primary mirror was not captured by an externally mounted engineering camera, but with a special lens within its Near Infrared Camera (NIRCam). This special lens is meant for engineering, not science, and allows NIRCam to capture an “inward-looking” image of the primary mirror. This image helps us to check that the telescope is aligned with the science instruments. What you are seeing in this image is the actual primary mirror of Webb as it observes its engineering target, a bright star. All the mirror segments are seeing starlight, but the bright segment is bright because, from NIRCam’s view, the segment is directly aligned with the star. Read more about this image and the other engineering images captured by Webb on our blog. Credit: NASA/STScI
NIRCam Alignment Selfie - This “selfie” was created using a specialized pupil imaging lens inside of the NIRCam instrument that was designed to take images of the primary mirror segments instead of images of the sky. This configuration is not used during scientific operations and is used strictly for engineering and alignment purposes. In this image, all of Webb’s 18 primary mirror segments are shown collecting light from the same star in unison. Read more... Credit: NASA/STScI
Initial Alignment Mosaic and Annotated Initial Alignment Mosaic - This image mosaic was created by pointing the telescope at a bright, isolated star in the constellation Ursa Major known as HD 84406. This star was chosen specifically because it is easily identifiable and not crowded by other stars of similar brightness, which helps to reduce background confusion. Each dot within the mosaic corresponds to a primary mirror segment. These initial results closely match expectations and simulations. The annotated image identified which mirror segment corresponds to which dot. Read more... Credit: NASA
Alignment Image Array and Labeled Alignment Image Array - This early Webb Telescope alignment image, with dots of starlight arranged in a pattern similar to the honeycomb shape of the primary mirror, is called an “image array.” Read more... Credit: NASA/STScI/J. DePasquale
Post-Global Alignment Image - This hexagonal image array captured by the NIRCam instrument shows the progress made during the Segment Alignment phase, further aligning Webb’s 18 primary mirror segments and secondary mirror using precise movements commanded from the ground. Read more... Credit: NASA/STScI
Post-Image Stacking image - During this phase of alignment known as Image Stacking, individual segment images are moved so they fall precisely at the center of the field to produce one unified image instead of 18. In this image, all 18 segments are on top of each other. After future alignment steps, the image will be even sharper. Read more... Credit: NASA/STScI
Telescope Alignment Evaluation Image - While the purpose of this image was to focus on the bright star at the center for alignment evaluation, Webb's optics and NIRCam are so sensitive that the galaxies and stars seen in the background show up. At this stage of Webb’s mirror alignment, known as “fine phasing,” each of the primary mirror segments have been adjusted to produce one unified image of the same star using only the NIRCam instrument. This image of the star, which is called 2MASS J17554042+6551277, uses a red filter to optimize visual contrast. Read more... Credit: NASA/STScI
Webb Telescope’s Deployment (Infographic)
JWST'S JOURNEY TO L2 -- JWST’s journey from launch to the Sun-Earth L2 point will be filled with a steady stream of spacecraft activities, from unfurling the sunshield (starting 3 days after launch) to unfolding the telescope mirror (13 days after launch). Image: AURA / S. Lifson
Subscribe to:
Posts (Atom)