8 ms·

Why you can't color calibrate deep space photos

233 points by LorenDB 1y ago

Retr0id 1y ago
The next space mission should be to leave a colour calibration chart on the moon.
- jofer 1y ago
  The moon itself already is one. Moonshots are widely used in calibration, at least for earth observation satellites. The brightness of the full moon at each wavelength at each day of the year is predictable and well-known, so it makes a good target to check your payload against.
- embedded_hiker 1y ago
  They brought a gnomon, with a color chart, on the Apollo missions. They would set it up for many of the pictures of samples. https://airandspace.si.edu/collection-objects/gnomon-lunar-apollo/nasm_A19810890000 https://airandspace.si.edu/collection-objects/gnomon-lunar-a...
- shagie 1y ago
  They also put color calibration charts on Mars rovers. For example https://www.lucideon.com/news/colour-standards-on-mars https://www.lucideon.com/news/colour-standards-on-mars
  - JNRowe 1y ago
    There is even a Damien Hirst¹ haphazardly spread about the surface for that purpose. One of the great gifts Pillinger² had was being able to shake up public interest via pop culture; there was also call sign by Blur for Beagle 2. ¹ https://www.researchgate.net/figure/Spot-Painting-Beagle-2-C-2003-Damien-Hirst-photo-by-Mike-Levers_fig7_266562112 https://www.researchgate.net/figure/Spot-Painting-Beagle-2-C... ² https://en.wikipedia.org/wiki/Colin_Pillinger https://en.wikipedia.org/wiki/Colin_Pillinger
- pgreenwood 1y ago
  Here's a shot of a color chart on the moon from Apollo 17 (AS17-137-20900): https://tothemoon.im-ldi.com/data_a70/AS17/extra/AS17-137-20900.small.png https://tothemoon.im-ldi.com/data_a70/AS17/extra/AS17-137-20...
  - gowld 1y ago
    Is there a document that explains that color chart? This version shows different shades of colors: https://eol.jsc.nasa.gov/SearchPhotos/photo.pl?mission=AS17&roll=137&frame=20900 https://eol.jsc.nasa.gov/SearchPhotos/photo.pl?mission=AS17&...
    - rtkwe 1y ago
      The first one has been adjusted to bring out the contrast and saturation while the one you link is an original scan. They brought a number of different swatches like that and took a picture of a calibration array at the end to account for the effects of space exposure on the film.
      - pgreenwood 1y ago
        The one I posted is from an original scan.
        rtkwe 1y ago
        Doesn't mean it hasn't been altered before posting to juice the shadows and make it more vibrant. The one from NASA's site looks like an unaltered scan.
    - pgreenwood 1y ago
      I think we are looking at different scans. See: https://tothemoon.im-ldi.com/about https://tothemoon.im-ldi.com/about
kurthr 1y ago
What's the white point? Is it D65? Not when the sun isn't out.
- klysm 1y ago
  I've always been confused by what the white point actually _means_. Since we are dealing with strictly emissive sources here, and not reflected sunlight, does the whitepoint even mean anything?
  - esafak 1y ago
    In a scene lit overwhelmingly by one approximately Planckian light source, the white point is the color of the closest Planckian light source. If the light source is not approximately Planckian, or if multiple illuminants have different temperatures, a white point is not defined.
    - klysm 1y ago
      So in this case there is no sensible white point since there is no illuminant right?
      - esafak 1y ago
        I'm not sure what case we're talking about but if it serves visible light it is an illuminant.
        klysm 1y ago
        Deep space photos - I don’t think there is an illuminant at all
    - queuebert 1y ago
      As an aside, I think you are referring to black body sources, which is described by Planck's law. Stars dgaf about color spaces of computer monitors. They are relatively close to black bodies as thermal emitters, though, at least on main sequence and +/- some spectral lines. We physicists never use the term Planckian for thermal black bodies. That adjective would be used in quantum mechanics, though, for very small things.
bhouston 1y ago
> Many other cameras, particularly those with aggressive UV-IR cut filters, underespond to H-a, resulting in dim and blueish nebula. Often people rip out those filters (astro-modification), but this usually results in the camera overresponding instead. Hmm... astrophotographers do not use cameras with UV-IR cut filters at all. For example, I owned a few of these: https://www.zwoastro.com/product-category/cameras/dso_cooled_cameras/ https://www.zwoastro.com/product-category/cameras/dso_cooled... They also generally do not use sensors that have Bayer filters. This also screws things up. Instead they use monochromatic sensors with narrowband filters (either one band or multiple) over them keyed to specific celestial emissions. The reason for this is that it gets rid of light pollution that is extensive and bumps up the signal to noise for the celestial items, especially the small faint details. Stuff like this: https://telescopescanada.ca/products/zwo-4-piece-31mm-ha-sii-oiii-7nm-filter-set https://telescopescanada.ca/products/zwo-4-piece-31mm-ha-sii... https://telescopescanada.ca/products/zwo-duo-band-filter https://telescopescanada.ca/products/zwo-duo-band-filter Often these are combined with a true color capture (or individual RGBL narrowband) just to get the stars coloured properly. Almost everything you see in high end astrophotography is false color because they map these individual narrowband captures on the monochrome sensors to interesting colours and often spending a lot of time manipulating the individual channels. This is done at the medium to high end using the PixInsight software - including by NASA for the recent James Webb images: https://www.pbs.org/video/new-eye-on-the-universe-zvzqn1/ https://www.pbs.org/video/new-eye-on-the-universe-zvzqn1/ The James Web telescope has a set of 29 narrowband filters for its main sensor: https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam-instrumentation/nircam-filters https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam... Hubble pictures were famously coloured in a particular way that it has a formal name: https://www.astronomymark.com/hubble_palette.htm https://www.astronomymark.com/hubble_palette.htm (My shots: https://app.astrobin.com/u/bhouston#gallery https://app.astrobin.com/u/bhouston#gallery)
- verandaguy 1y ago
  > astrophotographers do not use cameras with UV-IR cut filters at all I'll be pedantic here and say that the author's probably talking to people who use DSLRs with adapter rings for telescopes. I've been interested in doing this for a while (just unable to financially justify it), and I think this is actually something people in this niche do. Then there are things like the Nikon D810A, which remove the UV-IR filter from the factory (but IIRC retain the Bayer filter).
system2 1y ago
Isn't this why they always use the term "artist's impression" when they are colored?
- recipe19 1y ago
  I think that term is reserved mostly for actual artwork (renderings, paintings, etc). Some deep-space astronomy pictures are in completely made-up color, often because they're taken at wavelengths different than visible light and then color-mapped to look pretty. But the point here is even if you're taking images with a regular camera pointed at the sky, it's pretty much impossible to match "reality".
- okanat 1y ago
  There are different reasons for that. Things like black holes are really hard to observe even in other light spectrums. Same for other objects like planets. So the drawings are made in hypothetical expectations based on simulations rather than direct observations. Many observations come from scientific cameras rather than actual visible spectrum cameras discussed in TFA. They are not artist's impression like the first case. They will have a completely different view of the object so any visible-light predictions will have some guessing in it but the final picture will be not 100% you would see.
- nwallin 1y ago
  No. When you see "artist's impression" in a news article about space, what you're looking at is a painting or drawing created from whole cloth by an artist. This article is about how sensors turned signals into images. When you take pictures with a 'normal' camera, we've designed them so that if you take certain steps, the image on your screen looks the same as what it would look like in real life with no camera or monitor. This article is stating that with the cameras and filters they use for telescopes, that same process doesn't really work. We use special filters to measure specific spectral properties about an astronomical object. This gives good scientific information, however, it means that in many cases it's impossible to reconstruct what an astronomical object would really look like if our eyes were more sensitive and we looked at it.
bhickey 1y ago
The tiniest of corrections: Ha is 656.28nm not 565.
- execat 1y ago
  At risk of going off-topic, when I see comments like these, I wonder how the comment author comes up with these corrections (cross-checked, the comment is in fact true) Did you have the number memorized or did you do a fact check on each of the numbers?
  - kragen 1y ago
    I didn't know the number was wrong, but something about the statement seemed very wrong, because the 565nm number is only 10nm away from 555nm, conventionally considered the absolute maximum wavelength of human visual sensitivity (683lm/W). And you can see that in the photopic sensitivity curves in the rest of the article: both red and green cones respond strongly to light all around that wavelength. So it seemed implausible that 565nm would be nearly invisible. But I didn't know whether Ha was actually highly visible or just had a different wavelength. I didn't know 683lm/W either, and I wasn't exactly sure that 555nm was the peak, but I knew it was somewhere in the mid-500s. If I'd been less of a lazy bitch I would have fact-checked that statement to see where the error was.
    - cindyllm 1y ago
      [dead]
    - kragen 1y ago
      I see that there's a [dead] reply by the kind of person who thinks "tryhard" is an insult and has applied it to me. When I compare people I know about who tried hard to the people I know about who didn't try hard, literally every single person I would want to be like is one of the people who tried hard. I'm unable to imagine what it would be like to want to be like the other group. I mean, I don't want to be like Michael Jordan, but I can imagine wanting to be like him, and in part this is because specifically what he's famous for is succeeding at something very difficult that he had to try unbelievably hard at. So I'm delighted to declare myself a tryhard, or at least an aspiring tryhard. Completely by coincidence, when I saw the tryhard comment, I happened to be reading https://www.scattered-thoughts.net/writing/things-unlearned/ https://www.scattered-thoughts.net/writing/things-unlearned/: > People don't really say this [that intelligence trumps expertise] explicitly, but it's conveyed by all the folk tales of the young college dropout prodigies revolutionizing everything they touch. They have some magic juice that makes them good at everything. > If I think that's how the world works, then it's easy to completely fail to learn. Whatever the mainstream is doing is ancient history, whatever they're working on I could do it in a weekend, and there's no point listening to anyone with more than 3 years experience because they're out of touch and lost in the past. > Similarly for programmers who go into other fields expecting to revolutionize everything with the application of software, without needing to spend any time learning about the actual problem or listening to the needs of the people who have been pushing the boulder up the hill for the last half century. > This error dovetails neatly with many of the previous errors above eg [sic] no point learning how existing query planners work if I'm smart enough to arrive at a better answer from a standing start, no point learning to use a debugger if I'm smart enough to find the bug in my head. > But a decade of mistakes later I find that I arrived at more or the less the point that I could have started at if I was willing to believe that the accumulated wisdom of tens of thousands of programmers over half a century was worth paying attention to. > And the older I get, the more I notice that the people who actually make progress are the ones who are keenly aware of the bounds of their own knowledge, are intensely curious about the gaps and are willing to learn from others and from the past. One exemplar of this is Julia Evans, whose blog archives are a clear demonstration of how curiosity and lack of ego is a fast path to expertise.
vFunct 1y ago
You can if you use hyper spectral imaging...
- choonway 1y ago
  probably will come out within the next 5 iphone generations. POC already out... https://pmc.ncbi.nlm.nih.gov/articles/PMC8404918/ https://pmc.ncbi.nlm.nih.gov/articles/PMC8404918/
  - kragen 1y ago
    People have been making production hyperspectral sensors for decades, including hobbyists in garages; we're well beyond the proof-of-concept stage.
- nothacking_ 1y ago
  The problem with hyperspectral imaging is that it ends up throwing away 99.9% of all the light that hits your camera. It's been done for the sun and some very bright nebulae, but really isn't practical for most of the stuff in space.
klysm 1y ago
Recently I've been on a bit of a deep dive regarding human color vision and cameras. This left me with the general impression that RGB bayer filters are vastly over-utilized (mostly due to market share), and are they are usually not great for tasks other than mimicking human vision! For example, if you have a stationary scene, why not put a whole bunch of filters in front of a mono camera and get much more frequency information?
- queuebert 1y ago
  That would trade time and frequency information for spatial information, which is what you want in astronomy, but maybe not for candid family photos.
- jofer 1y ago
  In case you weren't already aware, that last bit basically describes most optical scientific imaging (e.g. satellite imaging or spectroscopy in general).
- nothacking_ 1y ago
  That's common in high end astophotography, and almost exclusively used at professional observatories. However, scientists like filters that are "rectangular", with a flat passband and sharp falloff, very unlike human color vision.
  - rachofsunshine 1y ago
    Assuming the bands are narrow, that should allow approximately true-color images, shouldn't it? Human S cone channel = sum over bands of (intensity in that band) * (human S-cone sensitivity in that channel) and similarly for M and L cone channels, which goes to the integral representing true color in the limit. Are the bands too wide for this to work?
    - nothacking_ 1y ago
      > Are the bands too wide for this to work? For wideband filters used for stars and galaxies, yes. Sometimes the filters are wider then the entire visible spectrum. For narrowband filters used to isolate emission from a particular element, no. If you have just the Oxygen-III signal isolated from everything else, you can composite it as a perfect turquoise color.
jofer 1y ago
These same things apply to satellite images of the Earth as well. Even when you have optical bands that roughly correspond to human eye sensitivity, they're a quite different response pattern. You're also often not working with those wavelength bands in the visualizations you make. Scientific sensors want as "square" a spectral response as possible. That's quite different than human eye response. Getting a realistic RGB visualization from a sensor is very much an artform.
dheera 1y ago
It's worth noting that many NASA images use the "HSO" palette which is false color imagery. In particular the sulfur (S) and hydrogen (H) lines are both red to the human eye, so NASA assigns them to different colors (hydrogen->red, sulfur->green, oxygen->blue) for interpretability.
- Aaargh20318 1y ago
  Amateur astrophotographers, like myself, also often use the HSO palette. The nice thing about deep space objects is that they don't change at a rate that there is any visible difference over timespans relevant to humans. This means that you can do very, very long exposures. It also means that you don't need to capture all colours at once. The linked article talks about using a color camera with a bayer matrix that uses dyed glass to get color data and how the filters don't filter out light in the 800-1000nm range. Lots of amateur astrophotographers don't use color camera's but monochrome cameras in combination with separate filters. These filters are of much higher quality than the dye based filters used on camera sensors. Instead of dyes they use interference filters[1]. These filters do not have the same issue as described in the article. For example the LRGB filter set I use only lets through a 100nm band for each color. Next to that you can use filter that only let through emissions from specific elements, like the HSO filters mentioned (Hydrogen-alpha, Sulphur-II, Oxygen-III). In my case I have filters with a bandwidth of 3nm around the emission line of each of these elements. The bigger issue with making these photo's look like what they 'really look like' is that they really look like nothing at all. The light from these objects is so faint that even with a highly sensitive cooled camera you need many hours of exposure to grab enough data to make a nice picture. What they 'really look like' to the human eye is black. The issue is worse when you add in the narrowband data, because that overlaps with the visible spectrum, but in that case you usually emphasise certain bands to stand out more in the final picture. It's not uncommon to combine the LRGB data with e.g. H-alpha data. In the end the colours you choose are up to the photographer and their goals. Are you just going for a pretty picture or are you trying to visualize scientific data? [1]: https://en.wikipedia.org/wiki/Interference_filter https://en.wikipedia.org/wiki/Interference_filter
  - dheera 1y ago
mystraline 1y ago
The proper color of an image would be a multispectral radiograph similar to a waterfall plot for each point. Each FFT bin would be 100GHz in size, and the range would be over 1000THz. And in a way, that'd what a color sensor is doing at the CCD level too - collapsing and averaging the radio energy its susceptible to a specific color.
- 7373737373 1y ago
  https://en.wikipedia.org/wiki/Hyperspectral_imaging https://en.wikipedia.org/wiki/Hyperspectral_imaging
monkeyelite 1y ago
Disappointing that most space photos are made by mapping an analog input onto a gradient and that this isn’t stated more directly.
- nothacking_ 1y ago
  That's really just photography in general. Mapping an analog input (photocurrent) to a pixel brightness.
  - monkeyelite 1y ago
    That’s technically true - but the goal is to recreate the image for human perception. The nasa images are colored artistically with no correspondence to how it would look if you were to visit.
hliyan 1y ago
I still haven't forgiven whoever made Voyager's first images of Jupiter's moon Io bright red and yellow, and The Saturnian moon Enceladus green.
- ianburrell 1y ago
  Neptune was shown as deep blue for a long time, but it is really a similar color as Uranus, a pale greenish-blue.
cyb_ 1y ago
Having dabbled a bit in astrophotography, I would suggest that color is best used to bring out the structure (and beauty) of the object. Trying to faithfully match the human eye would, unfortunately, cause a lot of that data to be harder to see/understand. This is especially true in narrowband.
- stavros 1y ago
  Plus, what's the point? It's not like anything will change if the object looks a bit more green rather than blue, it makes no difference to the wonder of the universe.
- malfist 1y ago
  The Hubble palette was specifically chosen to make contrast better. So you can actually see different parts of the nebula. Otherwise it's all just slightly different shades of red and IR.
strogonoff 1y ago
It is not just in space where nothing is lit by a uniform light source or with a uniform brightness. This is also true for many casual photos you would take on this planet. Outside of a set of scenarios like “daylight” or “cloudy”, and especially if you shoot with a mix of disparate artificial existing light sources at night, you have a very similar problem. Shooting raw somewhat moves this problem to development stage, but it remains a challenge: balance for one, make the others look weird. Yet (and this is a paradox not present in deep space photography) astoundingly the same scene can look beautiful to the human eye! In the end, it is always a subjective creative job that concerns your interpretation of light and what you want people to see.
- HPsquared 1y ago
  I suppose the human visual system is already adapted to deal with the same problem.
jpizagno 1y ago
As a former astronomer, this was a great post. (The website can use some post-90s styling however :> )
- indy 1y ago
  That aesthetic is how you know you're on a good astronomy site
ekunazanu 1y ago
> Because there’s a lot of overlap between the red and green cones, our brain subtracts some green from red, yielding this spectral response: No, cones do not produce a negative response. The graph shows the intensity of the primaries required to recreate the spectral colour at that wavelength. The negative implies that the primary was added to the spectral colour to match it with itself, instead of adding it with the other primaries. https://en.wikipedia.org/wiki/CIE_1931_color_space#Color_matching https://en.wikipedia.org/wiki/CIE_1931_color_space#Color_mat...
- rf15 1y ago
  > No, cones do not produce a negative response. not what was claimed at all...
  - ekunazanu 1y ago
    >> yielding this spectral response: [graph with negative values] That's what I gathered from spectral response. Usually spectral response in this context refers to the responsivity of the cones. Even when accounting for 'brain subtracting green from red' (which I assume comes from the opponent process theory) the following graph has nothing to do with it. The captions too read 'Yes, this results in red having negative sensitivity @500 nm', implying the red (L) cones have a negative sensitivity to cyans — which, again, is not really the case.
- nomel 1y ago
  > No, cones do not produce a negative response. Yes, they do, after the photoreceptors. Those CIE colorspace curves aren't biology, and shouldn't be interpreted as such. LMS colorspace is the (currently understood) biological colorspace [1], and contains inhibitions, from the opponent process [2] found in the meatware [3]: red-green: L - M blue-yellow: S - (L + M) This contains a nice introduction to biological colorspace [4]. [1] https://en.wikipedia.org/wiki/LMS_color_space https://en.wikipedia.org/wiki/LMS_color_space [2] https://en.wikipedia.org/wiki/Opponent_process https://en.wikipedia.org/wiki/Opponent_process [3] https://en.wikipedia.org/wiki/Lateral_geniculate_nucleus#Color_processing https://en.wikipedia.org/wiki/Lateral_geniculate_nucleus#Col... [4] https://color2.psych.upenn.edu/brainard/papers/Stockman_Brainard_ColorVision.pdf https://color2.psych.upenn.edu/brainard/papers/Stockman_Brai...
echoangle 1y ago
That's not a inherent property of deep space photos though, just of the sensors that are commonly used. There's no physical reason you couldn't build a telescope with a human response curve. It just maybe doesn't make a lot of sense from a science standpoint.
pizzathyme 1y ago
The question that the general public always wants answered is roughly, "If I was floating in a safe glass bubble in outer space looking at this object with my eyes, what would I see?" Does anyone know the answer to this? Would it just be black? Or just a bright white star?
- cconstantine 1y ago
  For the vast majority of things in the sky, they'd see black. This stuff is incredibly dark, and we need hours of exposure time to get enough signal. Even after hours of exposure the raw stacked frame is a black field with some pinpoints of lights. The exception to this is stuff in our own solar system.
- hnuser123456 1y ago
  Well, some of the densest, nearest, most interesting structures are right behind the rest of the Milky Way, which causes up to 30 magnitudes of extinction in visible light, but deeper IR and UV passes through much more easily. I think a better way to describe the issue is that much of the structure of the cosmos is only visible in non-visible wavelengths, so while calibrated, accurate visuals "like you were in a safe glass bubble" is a good category of astrophotography to continue to refine, it's a tiny slice of what's emitting electromagnetic radiation that's worth visualizing. And if cameras can convert invisible colors into visible ones, that's a blessing of a capability.
  - yig 1y ago
    Can we infer what that visible light would look like unobstructed and with appropriate intensity to activate our eyes' cones?
    - hnuser123456 1y ago
      Not necessarily, various objects can look completely different in different wavelengths and it's not really possible to interpolate without taking a picture of each part of the spectrum.
  - 1y ago

233 points by LorenDB 1y ago

Retr0id 1y ago

The next space mission should be to leave a colour calibration chart on the moon.

jofer 1y ago
The moon itself already is one. Moonshots are widely used in calibration, at least for earth observation satellites. The brightness of the full moon at each wavelength at each day of the year is predictable and well-known, so it makes a good target to check your payload against.
embedded_hiker 1y ago
They brought a gnomon, with a color chart, on the Apollo missions. They would set it up for many of the pictures of samples. https://airandspace.si.edu/collection-objects/gnomon-lunar-apollo/nasm_A19810890000 https://airandspace.si.edu/collection-objects/gnomon-lunar-a...
shagie 1y ago
They also put color calibration charts on Mars rovers. For example https://www.lucideon.com/news/colour-standards-on-mars https://www.lucideon.com/news/colour-standards-on-mars
- JNRowe 1y ago
  There is even a Damien Hirst¹ haphazardly spread about the surface for that purpose. One of the great gifts Pillinger² had was being able to shake up public interest via pop culture; there was also call sign by Blur for Beagle 2. ¹ https://www.researchgate.net/figure/Spot-Painting-Beagle-2-C-2003-Damien-Hirst-photo-by-Mike-Levers_fig7_266562112 https://www.researchgate.net/figure/Spot-Painting-Beagle-2-C... ² https://en.wikipedia.org/wiki/Colin_Pillinger https://en.wikipedia.org/wiki/Colin_Pillinger
pgreenwood 1y ago
Here's a shot of a color chart on the moon from Apollo 17 (AS17-137-20900): https://tothemoon.im-ldi.com/data_a70/AS17/extra/AS17-137-20900.small.png https://tothemoon.im-ldi.com/data_a70/AS17/extra/AS17-137-20...
- gowld 1y ago
  Is there a document that explains that color chart? This version shows different shades of colors: https://eol.jsc.nasa.gov/SearchPhotos/photo.pl?mission=AS17&roll=137&frame=20900 https://eol.jsc.nasa.gov/SearchPhotos/photo.pl?mission=AS17&...
  - rtkwe 1y ago
    The first one has been adjusted to bring out the contrast and saturation while the one you link is an original scan. They brought a number of different swatches like that and took a picture of a calibration array at the end to account for the effects of space exposure on the film.
    - pgreenwood 1y ago
      The one I posted is from an original scan.
      - rtkwe 1y ago
        Doesn't mean it hasn't been altered before posting to juice the shadows and make it more vibrant. The one from NASA's site looks like an unaltered scan.
  - pgreenwood 1y ago
    I think we are looking at different scans. See: https://tothemoon.im-ldi.com/about https://tothemoon.im-ldi.com/about

kurthr 1y ago

What's the white point? Is it D65? Not when the sun isn't out.

klysm 1y ago
I've always been confused by what the white point actually _means_. Since we are dealing with strictly emissive sources here, and not reflected sunlight, does the whitepoint even mean anything?
- esafak 1y ago
  In a scene lit overwhelmingly by one approximately Planckian light source, the white point is the color of the closest Planckian light source. If the light source is not approximately Planckian, or if multiple illuminants have different temperatures, a white point is not defined.
  - klysm 1y ago
    So in this case there is no sensible white point since there is no illuminant right?
    - esafak 1y ago
      I'm not sure what case we're talking about but if it serves visible light it is an illuminant.
      - klysm 1y ago
        Deep space photos - I don’t think there is an illuminant at all
  - queuebert 1y ago
    As an aside, I think you are referring to black body sources, which is described by Planck's law. Stars dgaf about color spaces of computer monitors. They are relatively close to black bodies as thermal emitters, though, at least on main sequence and +/- some spectral lines. We physicists never use the term Planckian for thermal black bodies. That adjective would be used in quantum mechanics, though, for very small things.

bhouston 1y ago

> Many other cameras, particularly those with aggressive UV-IR cut filters, underespond to H-a, resulting in dim and blueish nebula. Often people rip out those filters (astro-modification), but this usually results in the camera overresponding instead. Hmm... astrophotographers do not use cameras with UV-IR cut filters at all. For example, I owned a few of these: https://www.zwoastro.com/product-category/cameras/dso_cooled_cameras/ https://www.zwoastro.com/product-category/cameras/dso_cooled... They also generally do not use sensors that have Bayer filters. This also screws things up. Instead they use monochromatic sensors with narrowband filters (either one band or multiple) over them keyed to specific celestial emissions. The reason for this is that it gets rid of light pollution that is extensive and bumps up the signal to noise for the celestial items, especially the small faint details. Stuff like this: https://telescopescanada.ca/products/zwo-4-piece-31mm-ha-sii-oiii-7nm-filter-set https://telescopescanada.ca/products/zwo-4-piece-31mm-ha-sii... https://telescopescanada.ca/products/zwo-duo-band-filter https://telescopescanada.ca/products/zwo-duo-band-filter Often these are combined with a true color capture (or individual RGBL narrowband) just to get the stars coloured properly. Almost everything you see in high end astrophotography is false color because they map these individual narrowband captures on the monochrome sensors to interesting colours and often spending a lot of time manipulating the individual channels. This is done at the medium to high end using the PixInsight software - including by NASA for the recent James Webb images: https://www.pbs.org/video/new-eye-on-the-universe-zvzqn1/ https://www.pbs.org/video/new-eye-on-the-universe-zvzqn1/ The James Web telescope has a set of 29 narrowband filters for its main sensor: https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam-instrumentation/nircam-filters https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam... Hubble pictures were famously coloured in a particular way that it has a formal name: https://www.astronomymark.com/hubble_palette.htm https://www.astronomymark.com/hubble_palette.htm (My shots: https://app.astrobin.com/u/bhouston#gallery https://app.astrobin.com/u/bhouston#gallery)

verandaguy 1y ago
> astrophotographers do not use cameras with UV-IR cut filters at all I'll be pedantic here and say that the author's probably talking to people who use DSLRs with adapter rings for telescopes. I've been interested in doing this for a while (just unable to financially justify it), and I think this is actually something people in this niche do. Then there are things like the Nikon D810A, which remove the UV-IR filter from the factory (but IIRC retain the Bayer filter).

system2 1y ago

Isn't this why they always use the term "artist's impression" when they are colored?

recipe19 1y ago
I think that term is reserved mostly for actual artwork (renderings, paintings, etc). Some deep-space astronomy pictures are in completely made-up color, often because they're taken at wavelengths different than visible light and then color-mapped to look pretty. But the point here is even if you're taking images with a regular camera pointed at the sky, it's pretty much impossible to match "reality".
okanat 1y ago
There are different reasons for that. Things like black holes are really hard to observe even in other light spectrums. Same for other objects like planets. So the drawings are made in hypothetical expectations based on simulations rather than direct observations. Many observations come from scientific cameras rather than actual visible spectrum cameras discussed in TFA. They are not artist's impression like the first case. They will have a completely different view of the object so any visible-light predictions will have some guessing in it but the final picture will be not 100% you would see.
nwallin 1y ago
No. When you see "artist's impression" in a news article about space, what you're looking at is a painting or drawing created from whole cloth by an artist. This article is about how sensors turned signals into images. When you take pictures with a 'normal' camera, we've designed them so that if you take certain steps, the image on your screen looks the same as what it would look like in real life with no camera or monitor. This article is stating that with the cameras and filters they use for telescopes, that same process doesn't really work. We use special filters to measure specific spectral properties about an astronomical object. This gives good scientific information, however, it means that in many cases it's impossible to reconstruct what an astronomical object would really look like if our eyes were more sensitive and we looked at it.

bhickey 1y ago

The tiniest of corrections: Ha is 656.28nm not 565.

execat 1y ago
At risk of going off-topic, when I see comments like these, I wonder how the comment author comes up with these corrections (cross-checked, the comment is in fact true) Did you have the number memorized or did you do a fact check on each of the numbers?
- kragen 1y ago
  I didn't know the number was wrong, but something about the statement seemed very wrong, because the 565nm number is only 10nm away from 555nm, conventionally considered the absolute maximum wavelength of human visual sensitivity (683lm/W). And you can see that in the photopic sensitivity curves in the rest of the article: both red and green cones respond strongly to light all around that wavelength. So it seemed implausible that 565nm would be nearly invisible. But I didn't know whether Ha was actually highly visible or just had a different wavelength. I didn't know 683lm/W either, and I wasn't exactly sure that 555nm was the peak, but I knew it was somewhere in the mid-500s. If I'd been less of a lazy bitch I would have fact-checked that statement to see where the error was.
  - cindyllm 1y ago
    [dead]
  - kragen 1y ago
    I see that there's a [dead] reply by the kind of person who thinks "tryhard" is an insult and has applied it to me. When I compare people I know about who tried hard to the people I know about who didn't try hard, literally every single person I would want to be like is one of the people who tried hard. I'm unable to imagine what it would be like to want to be like the other group. I mean, I don't want to be like Michael Jordan, but I can imagine wanting to be like him, and in part this is because specifically what he's famous for is succeeding at something very difficult that he had to try unbelievably hard at. So I'm delighted to declare myself a tryhard, or at least an aspiring tryhard. Completely by coincidence, when I saw the tryhard comment, I happened to be reading https://www.scattered-thoughts.net/writing/things-unlearned/ https://www.scattered-thoughts.net/writing/things-unlearned/: > People don't really say this [that intelligence trumps expertise] explicitly, but it's conveyed by all the folk tales of the young college dropout prodigies revolutionizing everything they touch. They have some magic juice that makes them good at everything. > If I think that's how the world works, then it's easy to completely fail to learn. Whatever the mainstream is doing is ancient history, whatever they're working on I could do it in a weekend, and there's no point listening to anyone with more than 3 years experience because they're out of touch and lost in the past. > Similarly for programmers who go into other fields expecting to revolutionize everything with the application of software, without needing to spend any time learning about the actual problem or listening to the needs of the people who have been pushing the boulder up the hill for the last half century. > This error dovetails neatly with many of the previous errors above eg [sic] no point learning how existing query planners work if I'm smart enough to arrive at a better answer from a standing start, no point learning to use a debugger if I'm smart enough to find the bug in my head. > But a decade of mistakes later I find that I arrived at more or the less the point that I could have started at if I was willing to believe that the accumulated wisdom of tens of thousands of programmers over half a century was worth paying attention to. > And the older I get, the more I notice that the people who actually make progress are the ones who are keenly aware of the bounds of their own knowledge, are intensely curious about the gaps and are willing to learn from others and from the past. One exemplar of this is Julia Evans, whose blog archives are a clear demonstration of how curiosity and lack of ego is a fast path to expertise.

vFunct 1y ago

You can if you use hyper spectral imaging...

choonway 1y ago
probably will come out within the next 5 iphone generations. POC already out... https://pmc.ncbi.nlm.nih.gov/articles/PMC8404918/ https://pmc.ncbi.nlm.nih.gov/articles/PMC8404918/
- kragen 1y ago
  People have been making production hyperspectral sensors for decades, including hobbyists in garages; we're well beyond the proof-of-concept stage.
nothacking_ 1y ago
The problem with hyperspectral imaging is that it ends up throwing away 99.9% of all the light that hits your camera. It's been done for the sun and some very bright nebulae, but really isn't practical for most of the stuff in space.

klysm 1y ago

Recently I've been on a bit of a deep dive regarding human color vision and cameras. This left me with the general impression that RGB bayer filters are vastly over-utilized (mostly due to market share), and are they are usually not great for tasks other than mimicking human vision! For example, if you have a stationary scene, why not put a whole bunch of filters in front of a mono camera and get much more frequency information?

queuebert 1y ago
That would trade time and frequency information for spatial information, which is what you want in astronomy, but maybe not for candid family photos.
jofer 1y ago
In case you weren't already aware, that last bit basically describes most optical scientific imaging (e.g. satellite imaging or spectroscopy in general).
nothacking_ 1y ago
That's common in high end astophotography, and almost exclusively used at professional observatories. However, scientists like filters that are "rectangular", with a flat passband and sharp falloff, very unlike human color vision.
- rachofsunshine 1y ago
  Assuming the bands are narrow, that should allow approximately true-color images, shouldn't it? Human S cone channel = sum over bands of (intensity in that band) * (human S-cone sensitivity in that channel) and similarly for M and L cone channels, which goes to the integral representing true color in the limit. Are the bands too wide for this to work?
  - nothacking_ 1y ago
    > Are the bands too wide for this to work? For wideband filters used for stars and galaxies, yes. Sometimes the filters are wider then the entire visible spectrum. For narrowband filters used to isolate emission from a particular element, no. If you have just the Oxygen-III signal isolated from everything else, you can composite it as a perfect turquoise color.

jofer 1y ago

These same things apply to satellite images of the Earth as well. Even when you have optical bands that roughly correspond to human eye sensitivity, they're a quite different response pattern. You're also often not working with those wavelength bands in the visualizations you make. Scientific sensors want as "square" a spectral response as possible. That's quite different than human eye response. Getting a realistic RGB visualization from a sensor is very much an artform.

dheera 1y ago

It's worth noting that many NASA images use the "HSO" palette which is false color imagery. In particular the sulfur (S) and hydrogen (H) lines are both red to the human eye, so NASA assigns them to different colors (hydrogen->red, sulfur->green, oxygen->blue) for interpretability.

Aaargh20318 1y ago
Amateur astrophotographers, like myself, also often use the HSO palette. The nice thing about deep space objects is that they don't change at a rate that there is any visible difference over timespans relevant to humans. This means that you can do very, very long exposures. It also means that you don't need to capture all colours at once. The linked article talks about using a color camera with a bayer matrix that uses dyed glass to get color data and how the filters don't filter out light in the 800-1000nm range. Lots of amateur astrophotographers don't use color camera's but monochrome cameras in combination with separate filters. These filters are of much higher quality than the dye based filters used on camera sensors. Instead of dyes they use interference filters[1]. These filters do not have the same issue as described in the article. For example the LRGB filter set I use only lets through a 100nm band for each color. Next to that you can use filter that only let through emissions from specific elements, like the HSO filters mentioned (Hydrogen-alpha, Sulphur-II, Oxygen-III). In my case I have filters with a bandwidth of 3nm around the emission line of each of these elements. The bigger issue with making these photo's look like what they 'really look like' is that they really look like nothing at all. The light from these objects is so faint that even with a highly sensitive cooled camera you need many hours of exposure to grab enough data to make a nice picture. What they 'really look like' to the human eye is black. The issue is worse when you add in the narrowband data, because that overlaps with the visible spectrum, but in that case you usually emphasise certain bands to stand out more in the final picture. It's not uncommon to combine the LRGB data with e.g. H-alpha data. In the end the colours you choose are up to the photographer and their goals. Are you just going for a pretty picture or are you trying to visualize scientific data? [1]: https://en.wikipedia.org/wiki/Interference_filter https://en.wikipedia.org/wiki/Interference_filter
- dheera 1y ago

mystraline 1y ago

The proper color of an image would be a multispectral radiograph similar to a waterfall plot for each point. Each FFT bin would be 100GHz in size, and the range would be over 1000THz. And in a way, that'd what a color sensor is doing at the CCD level too - collapsing and averaging the radio energy its susceptible to a specific color.

7373737373 1y ago
https://en.wikipedia.org/wiki/Hyperspectral_imaging https://en.wikipedia.org/wiki/Hyperspectral_imaging

monkeyelite 1y ago

Disappointing that most space photos are made by mapping an analog input onto a gradient and that this isn’t stated more directly.

nothacking_ 1y ago
That's really just photography in general. Mapping an analog input (photocurrent) to a pixel brightness.
- monkeyelite 1y ago
  That’s technically true - but the goal is to recreate the image for human perception. The nasa images are colored artistically with no correspondence to how it would look if you were to visit.

hliyan 1y ago

I still haven't forgiven whoever made Voyager's first images of Jupiter's moon Io bright red and yellow, and The Saturnian moon Enceladus green.

ianburrell 1y ago
Neptune was shown as deep blue for a long time, but it is really a similar color as Uranus, a pale greenish-blue.

cyb_ 1y ago

Having dabbled a bit in astrophotography, I would suggest that color is best used to bring out the structure (and beauty) of the object. Trying to faithfully match the human eye would, unfortunately, cause a lot of that data to be harder to see/understand. This is especially true in narrowband.

stavros 1y ago
Plus, what's the point? It's not like anything will change if the object looks a bit more green rather than blue, it makes no difference to the wonder of the universe.
malfist 1y ago
The Hubble palette was specifically chosen to make contrast better. So you can actually see different parts of the nebula. Otherwise it's all just slightly different shades of red and IR.

strogonoff 1y ago

It is not just in space where nothing is lit by a uniform light source or with a uniform brightness. This is also true for many casual photos you would take on this planet. Outside of a set of scenarios like “daylight” or “cloudy”, and especially if you shoot with a mix of disparate artificial existing light sources at night, you have a very similar problem. Shooting raw somewhat moves this problem to development stage, but it remains a challenge: balance for one, make the others look weird. Yet (and this is a paradox not present in deep space photography) astoundingly the same scene can look beautiful to the human eye! In the end, it is always a subjective creative job that concerns your interpretation of light and what you want people to see.

HPsquared 1y ago
I suppose the human visual system is already adapted to deal with the same problem.

jpizagno 1y ago

As a former astronomer, this was a great post. (The website can use some post-90s styling however :> )

indy 1y ago
That aesthetic is how you know you're on a good astronomy site

ekunazanu 1y ago

> Because there’s a lot of overlap between the red and green cones, our brain subtracts some green from red, yielding this spectral response: No, cones do not produce a negative response. The graph shows the intensity of the primaries required to recreate the spectral colour at that wavelength. The negative implies that the primary was added to the spectral colour to match it with itself, instead of adding it with the other primaries. https://en.wikipedia.org/wiki/CIE_1931_color_space#Color_matching https://en.wikipedia.org/wiki/CIE_1931_color_space#Color_mat...

rf15 1y ago
> No, cones do not produce a negative response. not what was claimed at all...
- ekunazanu 1y ago
  >> yielding this spectral response: [graph with negative values] That's what I gathered from spectral response. Usually spectral response in this context refers to the responsivity of the cones. Even when accounting for 'brain subtracting green from red' (which I assume comes from the opponent process theory) the following graph has nothing to do with it. The captions too read 'Yes, this results in red having negative sensitivity @500 nm', implying the red (L) cones have a negative sensitivity to cyans — which, again, is not really the case.
nomel 1y ago
> No, cones do not produce a negative response. Yes, they do, after the photoreceptors. Those CIE colorspace curves aren't biology, and shouldn't be interpreted as such. LMS colorspace is the (currently understood) biological colorspace [1], and contains inhibitions, from the opponent process [2] found in the meatware [3]: red-green: L - M blue-yellow: S - (L + M) This contains a nice introduction to biological colorspace [4]. [1] https://en.wikipedia.org/wiki/LMS_color_space https://en.wikipedia.org/wiki/LMS_color_space [2] https://en.wikipedia.org/wiki/Opponent_process https://en.wikipedia.org/wiki/Opponent_process [3] https://en.wikipedia.org/wiki/Lateral_geniculate_nucleus#Color_processing https://en.wikipedia.org/wiki/Lateral_geniculate_nucleus#Col... [4] https://color2.psych.upenn.edu/brainard/papers/Stockman_Brainard_ColorVision.pdf https://color2.psych.upenn.edu/brainard/papers/Stockman_Brai...

echoangle 1y ago

That's not a inherent property of deep space photos though, just of the sensors that are commonly used. There's no physical reason you couldn't build a telescope with a human response curve. It just maybe doesn't make a lot of sense from a science standpoint.

pizzathyme 1y ago

The question that the general public always wants answered is roughly, "If I was floating in a safe glass bubble in outer space looking at this object with my eyes, what would I see?" Does anyone know the answer to this? Would it just be black? Or just a bright white star?

cconstantine 1y ago
For the vast majority of things in the sky, they'd see black. This stuff is incredibly dark, and we need hours of exposure time to get enough signal. Even after hours of exposure the raw stacked frame is a black field with some pinpoints of lights. The exception to this is stuff in our own solar system.
hnuser123456 1y ago
Well, some of the densest, nearest, most interesting structures are right behind the rest of the Milky Way, which causes up to 30 magnitudes of extinction in visible light, but deeper IR and UV passes through much more easily. I think a better way to describe the issue is that much of the structure of the cosmos is only visible in non-visible wavelengths, so while calibrated, accurate visuals "like you were in a safe glass bubble" is a good category of astrophotography to continue to refine, it's a tiny slice of what's emitting electromagnetic radiation that's worth visualizing. And if cameras can convert invisible colors into visible ones, that's a blessing of a capability.
- yig 1y ago
  Can we infer what that visible light would look like unobstructed and with appropriate intensity to activate our eyes' cones?
  - hnuser123456 1y ago
    Not necessarily, various objects can look completely different in different wavelengths and it's not really possible to interpolate without taking a picture of each part of the spectrum.
- 1y ago