[HN Gopher] James Webb Telescope pictures didn't begin as images
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James Webb Telescope pictures didn't begin as images
Author : tontonius
Score : 45 points
Date : 2022-10-17 08:36 UTC (1 days ago)
(HTM) web link (www.thestar.com)
(TXT) w3m dump (www.thestar.com)
| j_crick wrote:
| How am I supposed to read this if it's behind a paywall?
| ipqk wrote:
| Pay them money to get over the wall?
| bguebert wrote:
| I could read it after turning off javascript. Maybe try that.
| Am4TIfIsER0ppos wrote:
| lol the best kind of paywall is one that doesn't appear if
| you prevent RCE
| geuis wrote:
| Odd I didn't hit one
|
| https://archive.ph/ZijsH
| throwaway742 wrote:
| https://gitlab.com/magnolia1234/bypass-paywalls-firefox-clea...
| bigbillheck wrote:
| Money can be exchanged for goods and services.
| geuis wrote:
| https://archive.ph/ZijsH
| seanalltogether wrote:
| Quick question if anyone knows. One of the examples there is
| showing the "Linear" images compared to the "Stretched" images.
| I'm assuming that stretched means 0-255 RGB greyscale. But what
| are the ranges of "Linear" and why is it so dark? Are those
| floating point values of 0.0 - 1.0? Are they 12.0-18.0 like is
| shown in the Rosolowsky dataset?
| Cerium wrote:
| I think the answer is that the raw data has too much dynamic
| range. The stars are so much brighter than anything else that a
| naive linear scaling from the native depth to 8 bit results in
| all the shadows getting washed out and only the highlights
| showing. Instead, the "stretched" seems to be compressing the
| highlights to allow the shadow data to become brighter.
| jcims wrote:
| N00b astrophotographer here. I can't see the images but this
| sounds correct. (Edit: paste the two images into an image
| editor and look at the histogram. You'll see it)
| mturmon wrote:
| Probably true, and analogous to gamma correction
| (https://en.wikipedia.org/wiki/Gamma_correction) although
| they don't specifically say whether the range-compressing
| transformation that they are using is a power law.
| cconstantine wrote:
| Amateur astrophotographer here. What I'm going to talk about is
| true for my rig. The JWST is astronomically a better telescope
| than what I have, but the same basic principles apply.
|
| The cameras used here are more than 8 bit cameras, so there has
| to be some way to map the higher bit-depth color channels to 8
| bits for publishing. The term for the pixel values coming off
| the camera is ADU. For an 8 bit camera, the ADU range is 0-255.
| For 16bit cameras (like what mine outputs) is 0-65536. That's
| not really what stretching is about though.
|
| A lot of time, the signal for the nebula in an image might be
| in the 1k-2k range (for a 16bit camera), and the stars will be
| in the 30k to 65k range. If you were to compress the pixel
| values to an 8 bit range linearly (ie, 0 adu = 0 pixel, 65536
| adu = 255) you're missing out on a ton of detail in the 1k-2k
| range of the nebula. If you were to say 'ok, let's have 1k adu
| = 0 in the final image, and 2k adu = 255', then you might be
| able to see some of the detail, but a lot of the frame will be
| clipped to white which is kind of awful. That would be a linear
| remapping of ADU to pixel values.
|
| The solution is to use a power rule (aka, apply an exponent to
| the ADU, aka create a non-linear stretch). (EDIT: The specific
| math is probably wrong here) That way you can compress the high
| adu values where large differences in ADU aren't very
| interesting, and stretch the low-adu values that have all the
| visually interesting signal. In the software this is done via a
| histogram tool that has three sliders; one to set the zero
| point, one to set the max point, and a middle one to set the
| curve.
|
| It's kinda like a gamma correction.
| intrasight wrote:
| No digital "images" begin as images.
| Maursault wrote:
| Not so. In digital photography, _they all do._ The headline is
| entirely wrong. JWST 's mirrors are in fact reflecting _an
| image_ onto its _image_ sensors which use tiny light-sensitive
| diodes to convert light into electrical charges which are then
| translated into digital information and recorded as pixels. But
| the entire thing would be pointless _if there wasn 't an image
| to begin with._ The fact is we can never see anything, we only
| see an image. But since this is part of how seeing is defined,
| it is taken for granted that the thing itself is not pressing
| on our retinas, only a reflection of light, aka an image, is.
| nomel wrote:
| Actually, unless you're doing something exotic, like using
| encoded aperture or light field stuffs, most digital images do
| begin as a _real image_ [1] focused on a sensor.
|
| 1. https://en.wikipedia.org/wiki/Real_image
| rwmj wrote:
| https://archive.ph/ZijsH
| supernova87a wrote:
| I have friends/former colleagues who work on these pipelines, and
| I can tell you that it's not a stretch to say that there are
| dozens if not hundreds of people whose entire working lives are
| about characterizing the sensors, noise, electronics, so that
| after images are taken, they can be processed well /
| automatically / with high precision.
|
| (and after all, if these instruments/telescope were 30 years and
| $10B in the works, you would hope there's a fairly well developed
| function to make the data as useful as it can be)
|
| The goal is to get the "true" physical measurement of the light
| that arrives at the telescope. After those photons arrive, the
| measurements get contaminated by everything to do with the
| hardware, sensors, electronics, processing artifacts, and there's
| a whole organization that exists to study and remove these
| effects to get that true signal out.
|
| Every filter, sensor, system has been studied for thousands of
| person-hours and there are libraries on libraries of files to
| calibrate/correct the image that gets taken. How do you add up
| exposures that are shifted by sub-pixel movements to effectively
| increase the resolution of the image? How to identify when
| certain periodic things happen to the telescope and add patterns
| of noise that you want to remove? What is the pattern that a
| single point of light should expect to be spread out into after
| traveling through the mirror/telescope/instrument/sensor system,
| and how do you use that to improve the image quality? (the 6
| pointed star you see)
|
| Most fascinating to me is when someone discovers or imagines that
| some natural phenomenon that you thought was a discovery, turns
| out to be a really subtle effect of the noise in the instrument?
| (ADC readout noise / spike that subtly correlates with a high
| value having passed by during readout of a previous pixel? which
| makes your supernova discovery actually a fluke? I'm trying to
| recall the paper discovering that the pixel value on one chip of
| an instrument was related to the _bitwise_ encoding of the
| readout on a neighboring chip 's pixel...)
|
| Then there's even a whole industry of how to archive data, make
| it useful to the field, across telescopes, across projects, and
| over time.
|
| Lots of science and work here over decades.
| kurthr wrote:
| Once bytedance put their filters on, they'll be a so much more
| sexy!
|
| More seriously, I think so many people are only familiar with
| using image processing techniques to make things look
| subjectively "better", that they find it harder to believe that
| scientists don't do the same things to their research images.
| That is a bit corrosive to society, but real today.
| xani__ wrote:
| chubs wrote:
| Apologies if off-topic, but does anyone have a good source for
| downloading the JWST images in good quality? They are inspiring.
| anon_123g987 wrote:
| https://webbtelescope.org/resource-gallery/images
| awinter-py wrote:
| ugh I mean yes, and yes when published these images should always
| say how they were colorized / how the spectrum was compressed
|
| but that title
|
| this is like saying 'if someone pumped raw H264 into your optic
| nerve you would dance like the guy in the avicii levels video'
|
| like yes, we all know that's how that video got made, but nobody
| does that to their eye
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(page generated 2022-10-18 23:01 UTC)