Why aren't stars visible in space photographs?
One of the questions constantly appearing in the subject of the “ Cosmos ” reddit is: “Why are there no stars in the photo?” Usually these are photos from the Apollo missions on the moon or from Earth satellites, but sometimes these are pictures of Jupiter or the Moon. Recently, many photos of the Falcon Heavy Starman have slipped here.
I kept telling myself that it would be necessary to write an explanation for non-professionals, but I always did not have enough time. And then, finally, they brought me - one comment with a question asked for the millionth time, finally convinced me to do it. And now, when someone asks about it, I can just give him a link here.
So there it is. An explanation of why stars are not visible in many space photographs - from the photographer’s point of view.
Basics: cameras and exposure numbers
In the camera, light passes through the lenses and hits the sensor, or, in ancient times, onto the film. The sensor has millions of small photocells that collect particles of light, photons. If you simplify things a bit, then each of the photocells corresponds to a pixel in the final image, and the brightness of this pixel is determined by the number of photons collected. In the final photo, the dark areas correspond to those places in which fewer photons collided with the sensor, and the light ones correspond to those where there were more photons. You can imagine them as a bunch of buckets collecting photons - a bucket that collects more photons will have a lighter shade in the final image.
The amount of light entering the sensor is measured in exposure numbers, each subsequent of which doubles or halves the amount of light. Intuitively, this can be imagined as an excerpt. Leaving the shutter open for a period twice as long as you collect twice as much light in each bucket. The following image shows what this means. Each step is approximately equal to one additional exposure number. The shutter speed is shown below.
Doubling the shutter's opening time changes it from 1/500 to 1/250 seconds. Another doubling gives 1/125 seconds. This is the exponent squared. An exposition of 1/125 is a stone's throw from 1/500, but collects four times as much light. Another doubling, before exposure to 1/60 (these are approximate numbers) means three steps, but eight times more light. It turns out that in the image from left to right, an increase in the incidence of light is performed, equal to four exposure steps - that is, the right photo received 2 4 , that is, 16 times more light than the left.
In addition to exposure, the camera has two more ways to change the amount of light entering the photomatrix - change the lens aperture or ISO. Aperture - the size of the hole through which light passes.
Do not pay attention to the numbers, just keep in mind that the larger the aperture, the more light passes through it. ISO measures the camera’s sensitivity to light, and acts in much the same way as exposure numbers - ISO 200 is twice as sensitive as ISO 100, and ISO 400 is twice as sensitive as ISO 200.
In the image showing different exposures, on the rightmost photo you can see that in bright areas - the sky and clouds - it is almost impossible to distinguish details, they look just like a white spot. The amount of brightness that the camera sensors are capable of perceiving is limited, and the largest brightness in the photo looks like white. As soon as the photocell reaches this exposure level, an increase in the number of photons arriving at it will not give an increase in brightness. If you imagine photocells in the form of buckets, then when the bucket is full, an attempt to add additional photons to it will not make it more complete. When the brightness of the scene takes the photocells beyond this limit, the result is large white flashes without any details - this is exactly what is shown in the photo above.
In this photograph of astronauts working on the ISS, you can see the light. On the spacesuit and tool box, the astronaut, with his back turned to the camera, has large areas of pure white color, and you can also see them on the brightest parts of the ISS at the top of the photo.
On the other hand, photocells have a lower limit of light recognition. Photocells that do not catch a sufficient number of photons will be represented in black pixels in the photo. Reducing the amount of light to values below this limit will not make the pixel darker, it is already as dark as possible. You cannot get a more empty bucket than an absolutely empty one.
Sections of the image that are darker than this limit will look like black spots with no details.
In this photograph of the third stage and the Saturn-5 lunar module, you can see many shadow areas.
The brightness distance between the darkest black and the brightest white is called the dynamic range. It indicates the range of brightness in which the camera can capture image details. Everything below this range will be black in the photo, and everything above white.
In modern digital cameras, the dynamic range is measured by 10-15 exposure steps. You can find a list of dynamic ranges of the highest quality digital cameras. Exposure steps are indicated in the list as Evs [exposure value]. The film has approximately the same dynamic range.
Since the dynamic range changes as a power of two, the difference in light intensity between the black and white pixels of a camera with 15 exposure steps will be 2 15 , or 32 768. Another way to designate this dynamic range is 32 768: 1, which means that the top the limit for capturing parts before exposure is 32,768 times greater than the lower limit at which the photocell does not work.
Photo in the daylight
One important thing to understand about photographs of the moon and planets, including the Earth, is that they are illuminated by daylight and show the daylight side of the object. In other words, the object is illuminated by the sun.
This photograph of the Earth shows the daytime side of the Earth, turned toward the sun.
This photo is from the Apollo 15 landing site - a daytime photo. You could decide that this is a night photo, because the sky is dark, and this is the moon, which is visible at night - but the photo was taken on the side of the moon facing the sun, and the brightness there is the same as on Earth during the day.
This is a daytime photograph of Jupiter. She is not nightly. The sky is dark and Jupiter can be seen in the night sky, but this photo shows the day side of the planet, turned towards the Sun. The same is true for the recent launch of SpaceX Tesla - the car was lit by the sun.
Comparison of daytime and starlight photographs
Now that we have all the necessary knowledge, we will begin to understand how to compare pictures of the Earth and the Moon in daylight with pictures of stars at night. First, let's see what settings were used during the Apollo missions and other photographs of astronomical objects in daylight and ground photographs. Then we look at the settings used when shooting stars. Finally, we will enter various settings in the calculator, and see how many exposure steps are between photos of stars with the correct exposure and photos from Apollo and other daytime photographs of astronomical objects.
If we find that the difference in exposure levels exceeds 15, this will mean that a camera that records things in space like the day side of the moon, the Earth or other planets, or objects in daylight like Tesla, will not be able to make images of stars . We also recall that 15 is the maximum difference between the brightest and darkest shades in the camera, so the functional number of exposure steps between the subject and the darkest parts will be less, since the exposure with the average brightness rather than the maximum is usually selected for the subject. In the photo of the Earth above the planet is not at the upper end of the brightness scale, so the distance between the brightness of the Earth and the lower edge of the dynamic range will be not 15 steps, but something like 7,
But to simplify the calculations, we will just use 15 steps as a reference figure - if a correctly selected exposure for stars will be more than 15 steps away from a correctly selected exposure for the Earth in daylight, or the Moon, or any other planet, then we we can be sure that no stars will appear on these daytime shots.
We are looking for real exposure settings - starlight
As examples of star shots, I selected three images from our subreddit. For each of them, the photographer indicated the exposure settings.
Bioluminescence in Malibu and the Milky Way; shutter speed: 13 seconds, aperture: f / 1.8, ISO: 4000
Milky Way before dawn over the Atlantic; shutter speed: 25 seconds, aperture: f / 3.5, ISO: 2500
Milky Way over the yacht; shutter speed: 13 seconds, aperture: f / 4.0, ISO: 6400
We are looking for real exposure settings - daylight
In photography, there is such a practical rule called “Sunny 16” ( F / 16 rule ), which states that in order to choose the correct exposure for a photograph in sunlight, you need to set the aperture to f / 16, and the shutter speed to the opposite of ISO; A photograph taken with ISO 100 should use a shutter speed of 1/100 of a second. We will take this rule as the first reference point for the appropriate settings for daytime photos: ISO 100, f / 16 and shutter speed 1/100.
The second reference point will be the lunar images of Apollo. A photograph of some photographic equipment shows the real settings used for photographs taken on the surface of the moon. Take a look at the film reel on the left. ASA is the film equivalent of ISO, so we have ISO 160. The shutter speed is set to 1/250 s. The instruction prescribes shooting with an aperture from f / 5.6 to f / 11. Since the average value is f / 8, we use it as a reference. The difference between f / 5.6 and f / 11 is only two steps, so this is not so important.
The final reference will be this moon shot taken by astronaut Paolo Nespoli. Camera settings are listed on the original page : ISO 400, f / 6.3 and 1/500.
|Sunny 16||1/100||f / 16||100|
|Apollo||1/250||f / 8||160|
|Paolo Nespoli||1/500||f / 6.3||400|
Bring it all together
So how many steps are there between daytime shots and star photographs? There are several online calculators that take exposure, but I used this one . To carry out the calculations, we enter the settings of two photos, and it gives out the difference in exposure in steps. Here is a table of the differences between daytime and starry photos. Remember: the magic number is 15. Anything greater than 15 clearly indicates too large a dynamic range, and any attempt to obtain an image of both objects will result in either flare or shading.
|Difference between shots in steps||Sunny 16||Apollo||Paolo Nespoli|
|Milky Way and Malibu||22||20.67||19.67|
|Milky Way over the Atlantic||20.33||19||18|
|Stars above a yacht||20.33||19||18|
Here is the answer: the difference in brightness between photographs of daytime objects, for example, the surface of the Moon, a view of the Earth and other planets, and photographs of stars is too large to be captured on one image. The dynamic range of 20 exposure levels is beyond the capabilities of our cameras, so stars are not visible in photographs of objects illuminated by daylight in space. There are photographs showing dim stars and objects illuminated by daylight, such as the Earth or the day side of the moon. The result is very dark. Here are some examples: