 Greetings and welcome to the Introduction to Astronomy. In this lesson we are going to look at astronomical detectors, or methods that we use to detect the light that comes from space. Now there's a number of different astronomical detectors that have been used over the years and we're going to look at a few of those over the course of this lecture. So the very first one that would have been used would be the eye. So what was the various first thing that we used was the human eye. That was how we had to observe what we saw. This goes back to prior to the telescopic times, so what we could see before telescopes, and it also applied for the first couple of centuries the telescopes were used. What people would do would be to glance through the telescope and then draw their image. And here we see some sketches from made by Galileo of the moon. And you can tell that they would be rather subjective. They're not going to be exactly the same each time because of atmospheric conditions and even just personal perspective what you're happening to see at that time. So these were some of the earliest ones that were used. They're not very accurate and they were different than what was being able to... They would show things differently by the person who happened to be doing the observations. So five people could observe the moon and five people would see slightly different things. You would expect that you'd see the basic patterns the same but the details would be a little bit different depending on each person's eye. So these were then replaced by the photographic plate. So if we look next at photographic plates, again, as compared to the eye they were much less subjective because they were a permanent record. So the photographic plate is essentially a glass plate that had a photographic emulsion on it. So these were much better than the human eye. They were significantly better because they were a very objective measure. There was nothing that was left to the interpretation of the person doing the drawing. So people could study the same object and would then get essentially the same image and be able to study that. Now they were inefficient because you only recorded about one percent of the light that was striking the photographic plate. So here we see an example of a photographic plate. Note that it is a negative and in fact most astronomical objects and detectors will get you a negative image. For a couple reasons, one primarily that it's a lot easier to see a faint object that's dark on a bright surface than it is the other way around. A faint white dot on a black surface is hard to see whereas a faint black dot on a white surface is much easier to see. But they were inefficient. Only one percent of the light was recorded. So you did not were not able to see as faint as you otherwise would be with a more efficient detector. It is also difficult to store the plates. They had to be kept under climate controlled conditions because heat or cold could damage the plates and then ruin the images. It was also hard to share the data. These are glass plates so if you wanted to share them with an astronomer across the country you would have to send the plate there for them to be able to observe to study it. Very difficult and costly to be able to do that. Now of course we have digitized a lot of the photographic plates so that they can be shared much easier than they were in the past. But these were used primarily from the time of the development of photography to not all that many decades ago that they were still the primary source that was used for taking astronomical photographs. So a photographic plate would have been the primary one that would have been used. Now more recently we use what is called the charge coupled device and you're familiar with these. These are used in digital photography but the CCD itself or charge coupled device is the same type of thing that's in a cell phone or a digital camera. They are significantly more sensitive than the previous detectors such as the photographic plate. So you see here it is just a little chip and you have a light sensitive material in the center and then you have all of the little encoding here to be able to read out the different rows of pixels. So each section here was divided into a number of pixels and then you could read out how many photons had hit each section of that and then use that to reconstruct an image and that's essentially what's done when you take a picture with a phone or a digital camera. The light falls on the CCD and then you read that out very quickly and the amount of different light, the amount of light that it struck each little pixel will then give you a brightness for each of those and you can reconstruct that into an image. So let's look a little bit more detail about how this works. So essentially in a CCD you have light strikes that surface that we looked at and produces electrons that are then stored in that chip and once you're done with your image and you read it out then you can count the number of electrons. This is much more efficient. Remember that we had about one percent for a plate, photographic plate, we can get 60 to 70 percent of the light being recorded with a CCD. So it is significantly more efficient, meaning that exposure times can be much less. We don't need to leave things exposed as long. So we can cut exposure times by 60 to 70 times and still be able to see the same amount of detail. So something that you wanted to do for an hour on a photographic plate may be able to be done in a minute or two on a CCD. So significantly more efficient than the photographic plates and again a permanent record unlike the eye. So we did have a permanent record of these. Now at the end of the exposure you essentially use those little devices in there to read out the electrons and that gives a representation of the brightness of each pixel within that. You can imagine that divided into millions and even billions of pixels. So you can have, you know, megapixels being a million pixels a large CCD can have hundreds of those. So you can have a lot of them very easily and it's also all of course digital. So it can be very easily analyzed. In fact, you can see the images immediately after taking the image. You can immediately bring it up on a computer screen and find out whether your image came out the way you wanted it to or if there was an error or a problem that you need to retake that image. You still have the chance to do that. With a photographic plate you would have to go through the developing process and do that and that would be at the end of the night. So after you're done observing then you can take your photographic plates, develop them and see if they came out. But by the time you see them it's now too late to be able to make any corrections. So these you can actually see things in real time and they can be shared immediately. You can send the digital file to a colleague across the country or across the world to be able to share that information or to be able to compare what has been found there. So those are some of the ways that we do images and CCDs are the primary one now. The other astronomical instrument that we want to look at here is the spectroscope. Now spectra are very important because these are the ways we split up light into its component colors. They are the way that we can figure out things like the speeds, how fast things are moving or what things are made up of or what their temperatures are out in space because we can only see that light from them. So we use what was called a spectroscope to split the light into its component colors and wavelengths. So what happens here is the light comes through a slit and then goes through a prism or we call a diffraction grating and that is bent by different amounts. So the blue light will end up over here and the red light will end up over here. The blue light being bent more. The red light being bent less. It's going to spread the light out into the colors of the spectrum. So different wavelengths being bent by different amounts. We can then put a detector here which could have been a photographic plate long ago or would be a CCD now and that can be analyzed. This is very important because it can tell us about the motion of an object, what it's made up of, what its temperature is, and much more. This is really how we learn a lot of the details. We can see pictures of objects and photographic plates and CCDs can give us very nice pictures, but the spectroscope is really where we learn about the details of those astronomical objects. So let's finish up here with a summary of what we've gone over today. We have astronomical detectors that are used to record images, make a permanent record of things that are seen through the telescope. We had early ones using the eye and just drawing objects which of course were very subjective. The more modern ones like the photographic plate and the CCD were much less subjective. CCD, Earth Charge Couple Device is the primary one that is used now and it's a way to record images and spectra. And the idea is that now these digital images can be very easily and quickly shared with astronomers around the world. So if you need to share something with a colleague who is not right with you, you can immediately do that and get feedback and maybe be able to make adjustments for future observations. So that concludes our lecture on astronomical instrumentation. We'll be back again next time for another topic in astronomy. So until then, have a great day everyone and I will see you in class.