![]() ![]() In principle you also see diffraction from extended sources, but as John Doty comments the pattern will be convolved with the surface brightness of the source, smearing out the spikes. Almost all the other sources in this image are galaxies. If you exposed for long enough, you'd also see a diffraction pattern from the fainter point sources. ![]() The brighter a source is, the brighter the spikes will be. You can see the alignment here on this "selfie" (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 space):īelow you see the resulting diffraction pattern from various spiders:ĭiffraction is seen for point sources, which will typically mean stars, because they are bright. But the upper boom is vertical in the images, and thus gives the fainter, horizontal spikes. The two lower arms are actually aligned with the hexagonal pattern (on purpose, I presume), so the diffraction caused by those fall on top of four of the six spikes. This is the reason for the six brightest spikes.Īdditionally, Webb's secondary mirror is mounted on three arms. The edges of the outermost mirrors hence all follow three different directions, which are aligned at 60° to each other. James Webb's mirror is made up of 18 hexagonal segments. A round mirror, such as Hubble's, results in concentric rings, but its secondary mirror is mounted on a plus-shaped spider, and hence point sources observed with Hubble will have plus-shaped spikes, as seen below: In general, an edge will result in spikes perpendicular to that edge. the secondary mirror and the arms holding it (called "struts" or "spider"). The diffraction pattern depends on the shape of the mirror, as well as on anything that is in front of the mirror, i.e. Earth-based telescopes will usually not have this problem, since the atmospheric seeing will dominate, but James Webb is in space, and so is "diffraction-limited". This sets a fundamental limit to the telescope's resolution, given by the size of its primary mirror. Row.imshow(lp, vmin=-6, cmap='afmhot')Īny telescope will have diffraction of the light due to the edges between mirror and non-mirror. Log_powers = # log powerįig, axes = plt.subplots(len(log_powers), 2)įor img, lp, row in zip(imgs, log_powers, axes): # w = np.hanning(s0) * np.hanning(s1) # windowing not necessary in this caseįts = # img = gaussian_filter(img, sigma=1, mode='mirror', order=0) doesn't change conclusion The spotted pattern within the arms of the stars in some images below will smear out once it is averaged over wavelength (smearing the power spectrum by scaling radially)Īnd here's the script: import numpy as npįrom scipy.ndimage import gaussian_filterįnames = 'rIUME.png', 'modified_1.png', 'modified_2.png', 'big_hex.png' How much of the six-pointed star's power is from the "spider web" of the internal gaps between elements versus the mirror's external jagged edge versus just a big giant hexagonal hole? It's difficult to say without a more careful analysis with a full model. The other two that should appear at at +/- 30° are hidden under the sixfold star pattern of the mirror's "hexagonal theme". The horizontal spike at 0° is the diffraction pattern of the vertical element of the spike, and the light/dark banding in it (characteristic of slit diffraction) is nicely reproduced. ![]() I took the Fourier transform of the monochrome image illustrating JWST's clear aperture from answer and we can instantly see similarities. The three diffraction spikes they will produce will be perpendicular to them, but also spaced every 30° degrees rather than every 60°. The secondary mirror is supported by a spider with elements at 60°, 90 and 120°. The devil is in the details, since the pattern will change depending on how wide of a range of wavelengths is being passed, which will tend to smear out some aspects of the power spectrum. This is exactly what you would see from diffraction by the "spider web" of the dark edges that separate the 18 hexagonal subunits of the primary.īut it's also exactly what you would see from a single giant hexagonal aperture. A quick check by pasting the image into PowerPoint and rotating a line shows that the spikes have threefold symmetry they're at -30°, 30° and 90°. ![]()
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