What's brought me out of hiding today is a very cool new account on Twitter. Because the Hubble Space Telescope kind of belongs to the American public, it has started live tweeting where it's looking, what tools it's using to do that looking, and who told it to look there. So you get stuff like this:
The picture is not what Hubble was looking at right then but an image pulled from the Sloan Digital Sky Survey. Hubble can't usefully beam images directly to us, because everything Hubble (and all other telescopes) looks at has to be processed. This notion makes people grumble, because they want to see the raw, unmanipulated data in its purest form rather than rely on whatever artistic license NASA has exercised.I am looking at the extra-solar planetary system HAT-P-12 for Dr. Drake Deming using Wide Field Camera 3! pic.twitter.com/fVTseA7sQk— Hubble Live (@Hubble_Live) August 31, 2016
But raw images in astronomy (and raw data more generally in science) simply aren't useful. In fact, they don't even exist, because any contact with an instrument inevitably distorts the data. The purpose of processing images, then, is to remove the imprint of the instrument on the image and hopefully recover what's actually there.
The coolest part about Hubble_Live is that it tweets out this process, too. There are many ways astronomers attempt to extract the true signal from the data collected, but I want to talk about three of the big ones I've learned about and which Hubble employs. These are:
I am taking a DARK calibration for Joanna Taylor using Space Telescope Imaging Spectrograph! https://t.co/eTwSFLITr1— Hubble Live (@Hubble_Live) September 1, 2016
I am taking a BIAS calibration for Joanna Taylor using Space Telescope Imaging Spectrograph! https://t.co/eTwSFLITr1— Hubble Live (@Hubble_Live) September 1, 2016
Hubble performs these calibrations in order to figure out how it's interfering with the pictures it's taking. To see what these calibrations do, I want to show you some data my classmates and I took with a much smaller, terrestrial telescope last fall. We were looking at the Ring Nebula, which Hubble has an obnoxiously gorgeous picture of here for reference:I am taking a DARK-EARTH calibration for Dr. Peter McCullough using Wide Field Camera 3! https://t.co/YZ9eYO8IKe— Hubble Live (@Hubble_Live) August 31, 2016
|NASA, ESA, and the Hubble Heritage (STScI / AURA)- ESA / Hubble Collaboration|
At each pixel, there's the electric equivalent of a little bucket that collects electrons and converts them into a voltage that can be measured and manipulated digitally by a computer. Ideally, the way the CCD counts electrons is by having them knocked into the pixel bucket by incoming photons. But there are other sources of electrons, too. If you don't take them into account, you end up with an image that doesn't correspond to what you were looking at. So here's the raw data of the Ring Nebula taken by our telescope:
|Ignore the numbers.|
I'm being a little disingenuous here, though. The Ring Nebula is in this data, but because it is very faint compared to some of the pixels in the image, it's not apparent. I can turn up the contrast by bounding the brightness levels you're allowed to see, and then the nebula does appear.
|Color photography is so overrated.|
One electron source is the instrument itself, which because it is not at a temperature of absolute zero consists of vibrating molecules that can occasionally knock an electron into the pixel bucket. This is called the "dark current," because it shows up even when the telescope isn't looking at anything. The warmer your telescope, the larger the dark current will be, which means that weak signals can be lost in the noise of the telescope's heat. You can minimize that heat and detect faint signals by keeping your telescope cold (like, say, by putting it in space).
To determine the dark current, Hubble does a dark calibration, which essentially amounts to taking a picture of the same exposure length as your actual picture but with the lens cap on. That way the only electrons detected will be those coming from the heat of the instrument. Once you know what this average amount of heat is, you can subtract it from the electron counts of your image. Here is an image of the dark frame from our observations:
|Think TV static.|
Another source of electrons is the electronic components of the CCD. To operate properly, a CCD requires a certain voltage to be coursing through it constantly. For Hubble, this is the BIAS calibration, because you can think of the CCD voltage as being a bias introduced into the electronics in order to produce usable data. Telescopes acquire a bias frame by taking a zero-second exposure that doesn't let in dark current electrons or photoelectrons. Hubble does this separately from taking its dark calibration, but in certain situations you can also simply assume that your dark current includes the bias electrons. In that case, subtracting out the dark frame gets rid of the bias electrons, too. That was the case for the data we collected. If you look at what's left over after this subtraction, this is the image you get:
|The Thumbprint Nebula (I've zoomed in a bit here.)|
Finally (for our scenraio), the individual pixels in the CCD might have varying levels of light sensitivity. Since we want each photon to count equally, we have to adjust for these effects. Balancing the variable sensitivity is known as flat fielding, and you produce a flat field by shining a light of uniform intensity across the CCD. When you do this, the CCD should record the same number of electrons (more or less) at each pixel. If some regions of the CCD are too bright or too dim, you know this corresponds to unequal sensitivity. To remove the effects of this sensitivity, you divide your image by the (normalized) flat field, so that the brightness at each pixel is adjusted by a factor proportional to its sensitivity.
In space, unfortunately, it's difficult to shine a uniformly bright light on Hubble. You might think the Sun would work, but the Sun is way too bright and even a very short exposure would saturate Hubble’s sensors. Saturation causes electrons to bleed over into neighboring pixels and gives you electron counts that are not proportional to the number of photons detected. Instead, Hubble takes flat fields by looking at the Earth, which (with a lot of processing, aided by the fact that the Earth moves beneath Hubble very quickly, blurring any image it takes) can reproduce a flat field. So the DARK-EARTH calibration is Hubble's way of adjusting for the varying sensitivity of its equipment.
On Earth, flat fields are usually produced by shining a light on the dome of your observatory and having the telescope look at that, or looking at a small region of the dark sky before any stars become visible. Here's the flat field we produced:
|I think the telescope has floaters.|
|Possibly the Eye of Sauron (More zooming done.)|
None of these, of course, look like the beautiful pictures we see from Hubble or APOD. There are two reasons for this. One, our telescope simply doesn't have the resolution (or other exquisite features) that Hubble does, so there's a limit to how nice a picture it can take. The second reason, however, is that pretty pictures are created to be pretty, not for doing science. As I said above, this is just a representation of the data, but there are other representations.
In fact, one purpose of this lab was to determine the three dimensional structure of the nebula. That is, is it a donut or a shell? A picture alone can be deceiving. But other methods of interpreting the data might be more useful. So here's another representation, plotting the brightness of the nebula along a particular axis in different wavelengths of light:
|Graphs are the best, you guys.|
Nevertheless, what's astronomy without cool pictures? In addition to looking at the nebula with a clear filter, we also used filters that passed only red light from glowing hydrogen and blue/green light from doubly-ionized oxygen. When you clean up that data, assign a color to each filter, and plot them on top of each other, you get this:
|Insert riff on Beyoncé lyrics here.|