NASA, James-Webb, Hubble… Are space photos faked? We explain to you

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No, the Earth is not flat, and no, the images of human beings’ first steps on the Moon were not filmed in a NASA center. On the other hand, the photographs of planets, nebulae and other space objects that we regularly see are (a little) faked. That’s true.

More it’s for a good causeand what’s more, it’s not exactly a trick. “Let’s say instead that it’s improved,” summarizes Thomas, administrator of the Twitter account @SpaceScience_, withactu.fr.

To put it simply, telescopes are able to detect things that we human beings are incapable of seeing. Two (and a half) “tricks” should be noted: combinations and wavelength filters. The “half” is the long exposure, which is also not a cheat in the literal sense of the term. We’ll explain it to you.

Long exposure, obligatory for distant objects

Already, when it comes to astronomical photography, long exposure is the basis of the basics. The more distant an object is, the less its light reaches us. “If we look, for example, at a nebula with a conventional telescope, we see it gray. The object is too far away and its light is too diffuse,” notes the astronomy enthusiast.

This is where digital devices that manage to store light come into play. Bulk, they will “freeze” the targeted objectwhich we call the signal, to store the light it emits. We’re talking about long exposure: a photo that is not instantaneous, and requires a little time between the moment the button is pressed and the moment the photo is captured. The objective: for as much light as possible to be recorded by the device.

A bit like when you try to take a photo in the dark with a smartphone: the capture takes a long time: the camera is in long exposure mode, but in simplified mode. It is this operating mode that makes it possible to observe distant objects.

Besides, before you ask: yes, the Earth rotates on its axis, so taking a long exposure photo can be complex. This is why telescopes are equipped with mounts that allow them to compensate for the rotation of the Earth.

Filters and wavelengths

Well, so far, the photo we have is not “faked”. It is just taken with a special procedure. It remains a real photo, but on the other hand, it will not necessarily be successful.

To make an astronomical photo look like something, telescopes see colors that the human eye is unable to perceive. It is a question of colorimetric spectrum and therefore of wavelengths.

The further away the object is, the more infrared you will have to look at to see it. Note that not all telescopes are “tuned” to the same wavelengths.

By the way, it’s not the telescope that captures the image. It is a camera that is associated with the device. The telescope, roughly speaking, is just for seeing.

And that’s not all: they can (and are, as a general rule), equipped with filters, or even filter wheels so they can be changed as needed. A filter is a specific glass that is added, and which only allows certain wavelengths to pass through. They are made on purpose for a task. Some let the wavelength emitted by oxygen pass through, others let hydrogen pass through, etc., etc.

It is thanks to the filters we manage to have as much detail in some photos.

For example, if there is some kind of gas cloud in the field, we will apply such a filter so that the wavelength of this cloud is no longer visible. And like that, we manage to see behind this cloud. We don’t colorize the photos, it’s quite the opposite: we allow certain colors to be visible, not others.

If we compare the photographs taken by the famous Hubble and James-Webb telescopes, we see that they are not not at all the same. And yet, it is the same object that is targeted.

This is quite simply because the wavelengths captured, thanks in particular to the filters, are not the same. Ditto in these photos, taken, in fact, by Thomas:

“It’s the same photo. But afterwards, I retouched it using a palette using software. It gives, for one, a dominant of oxygen (blue/cyan) where on the first, the dominant was of hydrogen (red). »

Combining several photos

So if we summarize: we have a telescope equipped with a camera which takes a long exposure photo, and with the right filter. Well even with that, the photo will still not be perfect like the magnificent photos that we can see every day on the Internet and in astronomical magazines.

“Single photos of a planet or a nebula never happen,” emphasizes Thomas.

This is in particular because of the “noise”a phenomenon well known to photographers, caused by a number of factors. He gives a grainy appearance in some places photographs, but never the same. Also, combining the photos will smooth out this famous noise, eliminate this grain, and bring out the signal.

We also use the combination of images when the zoom of a telescope is too great. “It may seem paradoxical, but sometimes, when the objects we are targeting are huge, the zoom is just too strong. So instead, you have to capture a lot of light. If the object is too large, and the zoom is not suitable, we will take several photos and make a kind of mosaic,” says Thomas, with more than 130,000 subscribers on X (formerly Twitter).

“Cheating”, to ultimately better understand space

So yes, astronomers “cheat” a little. It’s true. But it is not a question not here to invent an alternative reality. Quite the contrary.

These are all methods that make it possible to observe such majestic stellar objects. From the breathtaking pillars of creation to our Sun, all of them, these photos have allowed, and still allow, to understand a little better the way our universe works.

“Our senses are our windows to the world, but they are only tiny portholes overlooking an immense sea that is unknown to us. This reality to which our senses have access is not the whole of what exists,” says physicist Christophe Galfard. We might as well help ourselves as best we can in this case. No ?

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