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

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
Credit: Spitzer: NASA/JPL-Caltech; MIRI: NASA/ESA/CSA/STScI
Credit: Spitzer: NASA/JPL-Caltech; MIRI: NASA/ESA/CSA/STScI
Credit: Credit: NASA/JPL-Caltech (left), NASA/ESA/CSA/STScI
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