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?

Question

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

Answer 1

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.