Telescope Imaging Systems

sombrero galaxy visible.jpg

Have you ever seen an image like this?

Okay, maybe you have…online. What with the spread of the internet these days, I’m guessing that at one point you have seen something like this on a page of image search results.

That’s the thing, though. You’ve seen this incredible phenomenon on a computer screen. But have you ever seen it through a telescope?

Don’t worry—if you haven’t had an opportunity to look through a telescope, you’re not missing out. You’re not going to see the Sombrero Galaxy above in all its photographed glory just from looking through the eyepiece of a telescope.

So…how do we get an image like this, then?

Imagining systems have improved tenfold in recent years. Just as phones advanced from household phones to flip phones to those with “QWERTY” keyboards to iPhones and other smartphones, astronomy technology has evolved incredibly fast.

In Galileo Galilei’s time, telescopes could barely produce grainy little images that would hardly impress astronomers today. But these new innovations were incredible in the eyes of those classical astronomers.

Likewise, astronomy started out with what we call a photographic plate.

Negative Image (photographic plate)

A friend of mine from my astronomy club was so kind as to provide me with an image of a photographic plate. These devices are all but obsolete and are hard to find pictures of. I’m just glad someone had the foresight to photograph my friend at work here.

You might notice something odd about this image. From the looks of it, the stars are black and the sky is white.

What on Earth—um, I mean, what the heck?

This is what’s called a negative. If a “positive” image is one with a black sky and bright stars, then a negative is its opposite. Though we don’t actually call “positive” images positives.

Here’s an example of a negative image.

NGC 891 Negative

This is the barred spiral galaxy NGC 891. Notice the way that the shape of the image matches in both the original image and the negative. You can even see where certain distinctive features of the negative match up with particularly bright stars.

Photographic plates could record faint objects in long time exposures, which basically means that astronomers aimed a camera at an object for a long time to let the camera gather up much more light than you would normally see.

That’s what makes photography such a powerful innovation in astronomy.

Our eyes can’t take long exposures. Your brain processes what you see in a telescope millisecond by millisecond—it doesn’t collect and record the image during the entire time you’re looking through the eyepiece.

Photographic plates could do that. They could get a better image than you could ever see by looking through a telescope just by collecting and holding onto all the light that’s continuously pouring in through your telescope.

And it got even better—they could be stored. We can’t exactly store the image our brain relays to our eyes. We see it while we’re looking through the eyepiece, but then it’s just a memory. Photographic plates meant images could be analyzed later.

But they’ve given way to what we call the charge-coupled device (or CCD)…

Charge-coupled Device

These guys are tiny. Kind of makes sense for the flow of technology, doesn’t it? I mean, just look how the phone evolved…

CCDs are about the size of a postage stamp, and they’re not even the smallest unit of technology we’re talking about. They’re made of about a million microscopic light detectors that record all the light that comes streaming through a telescope.

When we talk about CCDs, we’re talking about advanced astronomy tech. But like many technological innovations that started in astronomy, CCDs have made their way down the line to video and digital cameras. Those ones are just a bit less sensitive.

CCDs used in astronomy are very sensitive, and therefore very expensive. But they’re worth it—they can completely replace photographic plates. We have no need for negatives anymore.

Instead, we’ve taken a dive into false color.

Wait a second. False color? How can color be false?

It can be if your image is of something other than visible light.

electromagnetic spectrum

As you may know from my previous posts, visible light is just a tiny, tiny piece of a vast spectrum of electromagnetic radiation. And astronomers have found ways to record images in wavelengths of radiation besides just the visible ones.

Remember the Sombrero Galaxy up above? Here it is in false color.

sombrero galaxy visible
sombrero false color.jpg

The reds and blues here don’t really mean that the galaxy is red and blue—the real colors are shown above. This is an image taken in the infrared, the wavelength just a bit longer than visible light.

Here, more blue colors mean shorter wavelengths of infrared radiation, and more red colors mean longer wavelengths. And in plain English, this image tells us that the center of the galaxy is hotter than the outside.

This makes sense, when you think about the formation of galaxies, conservation of angular momentum, and a lot of other outer space physics stuff…but we won’t be going into that for a long time yet.

There’s no way a photographic plate could produce a false-color image. They always deal in negatives. And there’s something else a CCD has over a photographic plate—it can detect both bright and faint objects in a single exposure.

horsehead CCD.jpg

Here’s the Horsehead Nebula in all its photographic glory.

The nebula itself is that grayish blur in the middle that vaguely resembles a horse’s head. It’s a very faint object when compared to the stars in the image, which are all much brighter.

This image would have been impossible for a photographic plate, which can’t handle different light levels that need different exposures.

It’s kind of like what happens when you try to photograph an eclipse of the moon, when the moon crosses through the Earth’s shadow…


Notice how you can’t see any detail on the half of the moon yet to be eclipsed?

That’s because there’s a heck of a lot more light shining on that half.

And when you take a long enough exposure to get detail on the darker half, the brighter half basically blinds your telescope.

There is very likely some kind of low-tech CCD in the camera used to take this image. But a photographic plate wouldn’t just have trouble taking this image—it would be impossible.

Advanced CCDs in telescopes used for astronomical research are pretty good at taking shots with both bright and faint objects. They can’t take different exposures in the same shot, but at least they can do the job—photographic plates can’t. Period.

CCDs and false-color images are useful in astronomy. But there’s one more bit of astronomy tech that’s especially important for research—the spectrograph—and I’ll cover that tomorrow.

Questions? Or just want to talk?

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