[iliaznk]

I'm sorry, I haven't read the article, maybe it's explained there, but as far as I understand by 100 hours they mean that the telescope was sitting there for 100 hours exposed open like a camera, right? How come the image is so sharp then? Wouldn't all those galaxies be constantly moving and end up blurred in a photo?

[pryelluw]

From the article:

...In total, the telescope took 342 pictures of the region, each of which was exposed for between 25 and 45 minutes. The images were processed and combined, then colored, and 17 days later, released to the public.

[iliaznk]

Oh, that explains it, thanks a lot!

[munificent]

So there are two things you do to take very long "exposures" like this for astrophotography:

1. You put the telescope on a motorized mount that rotates the telescope exactly counter to the rotation of the Earth (or I guess its orbit in the case of Hubble). This cancels out most of the blurry star trails you get from the Earth's rotation causing the stars to move across your frame.

2. For particularly long exposures, you take a series of separate photos (each of which is probably done using step 1). Then you "stack" those in software by aligning all of the images to maximize their sharpsness. There is software that will do this for you automatically. Stacking helps correct for thermal noise and other imperfections and non-linearities that photon sensors have when collecting for a really long time.

It's a good question.

[ben_w]

Hubble is in space. They just pointed it somewhere and said “don’t move” (well, multiple times and stacked, but the effect is the same). The galaxies themselves are so far away their own motion is way less than a single pixel.

[atonse]

Yes but isn't Hubble moving? Or is it in a fixed position in space?

[ben_w]

There’s no such thing as a fixed position in space.

The motion of Hubble (and basically everything inside the Milky Way) relative to the galaxies in that picture is ~250,000 km/s, or 0.83 c.

At this speed, it would take ~100,000 years for the galaxy to move its own length, and even then the movement would be in a direction which doesn’t cause much blur.

http://www.wolframalpha.com/input/?i=12e9%20light%20years%20...

[atonse]

To be clear I meant is it in a fixed position relative to earth? I know if you’re photographing galaxies, they move so little relative to us that they might as well be stationary. But isn’t Hubble in an orbit?

[ben_w]

Motion on the scale of “orbit” is utterly irrelevant on these scales. Rotation around an internal axis is the only thing which matters, and that’s something space telescopes are designed around.

[iliaznk]

Good point. But still, I think, if it really was exposed for 100 hours the photo wouldn't be as sharp.

[vlasev]

Since it's done by stacking 30-45 min exposures, it would be sharp. All you have to do is align the images well.

[ben_w]

Why do you think that?