Today, we’re going to pretend we are a photon of light, rather than looking at photons of light. This lets us take a break from time traveling for a minute and spend some time talking about what happens to light when it goes through the Time Machine and ends up on the sensor of our camera.
Light is made up of photons. So are radio waves, X-rays, gamma rays, infrared, and ultraviolet. Together, this is known as the electromagnetic spectrum (or EM) and together, it makes up every possible type of electromagnetic radiation that there is, including most of which you can’t see with your eyes.
For most of the past two centuries, photography was designed to show other people what something looked like as if they were there. So the process developed (no pun intended) over time to show the portion of the EM that our eyes respond to. Some people thought it would be neat to see the world in infrared, but that was the main exception.
When scientific photography started, people realized that they could capture images using non-visible EM and shift it chemically (or more recently, with a computer) to the band of EM that we recognize as visible light. These “false color” images are very popular in astroimaging, since a lot of what is sent out as radiation from distant objects is in the X-ray, infrared, ultraviolet, and radio portions of the EM spectrum, not in visible light.
So why is this important? Well, photons are the vehicles that carry the EM information. Let’s consider just visible light for a moment. This is EM that’s traveling in wavelengths of about 380nm to 760nm. Anything with a wavelength shorter than 380nm is in the ultraviolet range (or higher) and anything longer than 760nm is in the infrared (or radio). We don’t see those, so we’ll ignore them for a minute. To get the photon to carry a 400nm wavelength of light, for instance, it has to absorb just enough energy so that it vibrates at 7.5E14 Hertz. That means it vibrates 750,000,000,000,000 times each second to produce a nice purple for you to see.
Okay, so photons carry light, or any EM radiation, by vibrating at ridiculously fast rates. So what? Eventually, a star or a nebula or a quasar or some object in space will radiate a photon towards Earth. If we’re lucky, it will go in the end of our Time Machine and suddenly Astropotamus can see the star through they eyepiece and say “wow, that’s beautiful.” Or better yet, since it’s starting to get cold outside, it can go onto the sensor of the camera connected to the Time Machine.
The sensor is a light sensitive device that detects when a photon of a certain wavelength hits it. Do this a 12 million times, and you have a 12MP digital image. Sensors detect all the light that hit them, but are only sensitive to certain ranges of EM radiation. In the case of our DSLR, that’s visible light. Or is it?
Remember I said that photography mostly developed (again, no pun intended) to provide the ability to show someone else what you saw? You can’t see infrared, so why capture it on film? Same for digital – you can’t see it, so why capture it? Well, the sensor could capture it if it were allowed to capture it, but there is a filter in front of it that prevents it from doing so. Same for ultraviolet. So by design, we chop off the light that gets to the sensor so that it can only see what we can see. This makes for great pictures of the cat but not so great ones of deep space objects that are shining all over the place in non-visible wavelengths of light.
So we can modify our camera to remove that filter and be able to see more of what there is to see. This is generally a warranty-voiding activity and not for the feint of heart. It also makes the camera next to useless for “normal” photography. That’s why Astropotamuses usually use a camera for astroimaging that is dedicated to astroimaging. It’s easier than explaining why everyone looks brown and the sky is orange when you take family portraits.
Later, we’ll discuss how we can take these images and align them, stack them, process them, and combine them to create the beautiful images you see here and from other astroimaging sources.