How do people take photos of galaxies with 50 hours of exposure? How is it possible to aim at the same spot while earth is doing more that 2 rotations in that time?


How do people take photos of galaxies with 50 hours of exposure? How is it possible to aim at the same spot while earth is doing more that 2 rotations in that time?

In: Technology

Hi 🙂

The stated hours are the combined time of multiple exposures, often during multiple nights.

Via software you can then “stack” multiple images, increasing details, reducing noise.

Combined exposure during multiple nights as others have pointed out and also rotating the telescope appropriately as the celestial sphere turns, keeping it fixed on the one point.

An equipment called German Equatorial Mount compensates the earth’s rotation by rotating at the same rate as the earth. The exposures are usually minutes of single exposures stacked together using a stacking software.

Keep in mind that the movement of the Earth is very very slow. 15° an hour is as fast as it turns. So you can make a very large object move very steadily in the direction opposite the movement of the Earth it basically stays pointing at exactly the same part in the sky.

The farther away something is the fewer of its photons you receive an hour or a minute or a second. So they’ll very large telescopes collect individual photons over a long period of time.

when you photograph something here on Earth like your cat you are also collecting individual photons but there are so many of them that you get them all at once.

For ground-based telescopes there’s a lot of extra work to do because it’s not just motion but clouds and the waivering of the air due to thermal changes and stuff like that.

so all photography and indeed all sensing of light and radiation is based on the cumulative effect of individual photon events.

Making images of Small distant objects, or large distant objects that appear small to us because they’re so far away, involved collecting enough photons to make a picture. The delays and the complicated math and advanced photography and just plain keeping things aimed is an ongoing process of improvement.

We’re literally talking clock motors moving entire well-balanced telescopes. Like wound up clock motors powered by springs back in the day.

telescopes are extremely well balanced and extremely precision manufactured and many of the pictures we take today we’re not possible before the advent of computers because the finest grain corrections couldn’t be done mechanically.

One of the side things in this whole issue is like how we no longer have to use as many x-rays to x-ray your body. With precision electronic receptors instead of film it takes way fewer x-ray photons to map and display the structures of the body.

Same thing for space.

And for all that I make it sounds super subtle, it’s all just basic trigonometry. You can use the light from several galaxies that are many many light years away and aren’t the galaxy you’re looking at, to make sure that you’re aimed at the galaxy you are looking at simply because those multiple photons coming in from those multiple angles formed triangles compared to the thing you actually care about.

The thing about math and science is it’s the same basic techniques used again and again to create more complex and more subtle techniques essentially by comparing and contrasting what you have and what you want to know.

Iv done astrophotography and what we do is take allot of photos over the course of a few nights and then use software to add all the light from the images together. So when you get 50 hours for one photo you may have taken 50 1 hour photos or 100 30 minute photos the time depends on your equipment, light pollution and personal preference I would normally expose for 20mins at that point where I live light pollution became a big issue.

To keep aiming at one spot is normally controlled by two things the mount and a second camera.

There are two main types of mounts equatorial mounts and altazimuth mounts. Most astrophotography is done with a equatorial mount. The equatorial mount rotates the telescope around it, the altazimuth mount moves up down left right. I’ll focus on the equatorial mount as it’s the relevant one.
The mount will normally rotate the telescope and camera on it’s axis and you need to set that axis up with the earth’s axis, now in theory if the mount rotates once a day it should always point at the same thing. In practice this is no accurate enough to really track and object in the sky more that a few hours so that’s where the second camera comes in.

The second camera will be looking at a star in the area that you are taking photos. That second camera then takes a photo every 10-60 seconds it will then compare that image to the last one it took. When the two images are compared it looks at the light around the star and try to determine if the star is moving in the frame of the image if it is then the software comparing the two images will send a command to the mount to adjust and recenter the star.

The reason the mounts can’t keep track is because there’s is always slight errors in how well we can match the axis of the mount with the axis of the earth’s and also the gears that help rotate the mount also have little bumps in them.