Based on this, given that light enters the HST, which is a tube, not a very long one, but a tube, and then light hits reflectors at the back....
... that any light moving slow enough wrt the telescope would hit the sides and never make it to the sensors.
That means... according to the hypothesis... a space telescope that doesn't have the tube, and has reflectors or lenses front and center, wouldn't filter out the slowest moving photons.
[qimg]https://www.nasa.gov/images/content/296207main_JWST_diagram_HI.jpg[/qimg]
Heyo!
If JWST picks up light with z's way beyond what we thought possible, you might have explained the reason why.
For optical systems using lenses and/or mirrors, the tube does nothing critical. (It often holds the optical elements in place, and it's good for keeping bright light that's off to the side from adding spurious stray signals like lens flares. So it can be pretty important, but it doesn't determine the key optical qualities of the system, such as field of view or resolution.) With lenses and parabolic mirrors, it's all about incidence angles. That's why your eye can do what an insect's compound eye can do, but more efficiently.
Do you know what an imaging system actually does? If you don't I'm not surprised; most explanations out there either get bogged down in complex terminology and math, or are too simple, either way seeming to miss the basic point.
Suppose the Starship Enterprise were hovering a few thousands kilometers from the Webb telescope. Assuming the telescope is pointed away from the sun, every visible part of the Enterprise is reflecting sunlight in every direction back toward the telescope.
Consider, for instance, the thingie at the tip of the prong in the middle of the dish at the front. Light from the sun is being reflected off the thingie toward the telescope's primary mirror. Photons from the thingie are striking every part of the mirror. But because those photons are coming from so far away, even though they're hitting the mirror in all different places, they're all hitting it at nearly the same angle relative to the orientation (and motion, if there's relative motion) of the mirror.
If all those photons, after bouncing off the mirror and the secondary mirror and whatever additional optical elements are in their path, end up on the same spot on the image sensor (or on a frame of photographic film or whatever), then two important things are true: the Enterprise (or at least the thingie) must be within the telescope's field of view, and the Enterprise is
in focus. If those photons end up on different places on the image sensor (or film frame, etc.) then the image is blurred. If they don't end up on the sensor (or film frame, etc.) at all, then the Enterprise is out of the telescope's field of view. Or cloaked.
That's what a telescope, a lensed eye, a camera etc. does. It sorts the incident photons onto positions on the image plane based on their incidence angle. (Within a range of angles, aka the field of view.) Photons coming from the direction of the upper left corner of the field of view (even if they contact the primary lens in the center or in the center of the bottom edge) should end up contributing to the qualities of only the upper left corner of the video screen (or developed photo print, your retinal image*, etc.) A focused image is a successful sort operation.
For high resolution optics such as the Webb, the motion of the instrument relative to the incident photons affects that all-important angle of incidence, and so must be compensated for, especially for a long exposure.** That compensation cannot work if the incident photons have varying speeds.
*Actually optical images often get "sorted" in reverse, which is why the image on your retina or on your camera's sensor is said to be upside down. It's easy for further processing of the image to undo the reversal where necessary.
**Note that in a short exposure during which the motion of the telescope remains roughly constant, light from a more distant object having a different speed wouldn't blur the image of the more distant object, but it would alter its position in the frame relative to nearer objects. That position would then appear inconsistent when comparing images taken from different times and places.
