Telescopes - Part 0
A telescope. What do you think it is?!
The common perception is that a telescope is a device which is used to look at the stars! Telescopes have mirrors oriented in a a particular fashion which helps us look at stars! Well, this is just part of the story and the tip of an ice berg.
A telescope can be described as any device which has the following parts
- A reflecting surface, to reflect the incoming electromagnetic radiation
- A receiver which can decipher this electromagnetic signal into a digital signal which we can understand and analyze!
So, in the common perception, the reflecting surface is a mirror, one which has been ground to a micrometer finish and one which has been coated to be a (perfect) reflecting surface. And this setup is for observations in the visible part of the electromagnetic (EM) spectrum.
- In the context of this blog post, light will refer to EM radiation of any frequency and not just the visible frequencies i.e when i mean IR light, i mean radiation in the IR frequencies, Radio light means radiation in the Radio wave frequencies. Get it?
- And, if you dont know what the Electromagnetic spectrum is, read the wiki page here.
- We also assume that the wavefronts of light coming from outer space are planar and are parallel to each other, which is a good enough approximation considering the sources are very very far away.
You might be wondering why I keep talking about the electromagnetic(EM) spectrum in general and not just the visible part of it, as If we can look and measure energy in the other parts of the EM spectrum. Umm, well, YES WE CAN. And the reason why many people don’t realize this is that the common optical telescope cannot look at energy and emissions from the other parts!
Why you ask?!
For one, t he reflecting surface only works for visible and IR light. Secondly, the receiver (the human eye, a photographic plate or a CCD) is only sensitive to energies of the visible IR light. So, as you can figure out, the tradeoff leads to you only being able to observe the visible and Near IR light.
So, you might be wondering now what the different kinds of telescopes are and how they differ in design as to be able to observe the different portions of the EM spectrum. As you just saw, the reflecting surface and the receiver play a crucial role in designing a telescope to observe a particular spectrum!
Coming to the relationship between a reflecting surface and the observable frequencies, I hope you guys already know about diffraction and interference, a couple of important concepts which will help you understand the reason better. If you dont know about them, i suggest you read up their wiki pages. If you do know them but only in the context of optical light, understand that the theory can be extrapolated to any frequency, not just optical. So, as mentioned, diffraction will disturb a plane wave in it's path! And for any object whose size is comparable to the frequency of light (& in general EM spectrum) will cause diffraction!
Now, in the case of an optical telescope, if we had surface indentations or bumps of the order of micro meter, there'd be diffraction and the reflecting surface wouldn’t be able to focus the light perfectly! Which is the reason why a quality mirrors are coated in vaccum sealed chambers to avoid contamination! A good mirror will be able to reflect light from the visible to the radio spectrum!
Radio waves have wavelengths of the order of one cm to ten meters. As the theory behind diffraction tells us, an incoming wave will undergo diffraction when it meets an obstacle if the size of the obstacle is comparable to that of the wavelength of light. Based on this, do you realize the fact that a reflecting surface for radio waves need not be continuous i.e a fine aluminium mesh (with gaps in the order of cms), shaped as a parabola will be a perfect reflecting surface for radio waves of wavelength in the order of meters. Dont believe me?! Then google it!
Now that we broadly know about reflecting surfaces, let's look at receivers!
Even in the case of optical telescopes, receivers were a big problem in the 50s. As the only means to record visible data was huge photosensitive plates, it was a very time consuming process to replace the old ones, develop the plates to study the observations made! Fast forward and now we have CCDs which can be used to decipher visible and Near IR light! Antennas are the preferred receivers for the radio and microwave parts of the EM spectrum. Antennas can be built for specific frequencies or frequency independent antennas which have the same gain over a wide frequency band can be constructed! similarly, there are many other kinds of receivers for the other parts of the EM spectrum
So, coming back to ground 0, any emission from outer space, no matter what part of the EM spectrum it is in, can be observed using a telescope! And the brilliant part about this revolution in astronomy is that the design and construction of these different receivers and telescopes require a different skill sets compared that of an astrophysicist who works on the data from these telescopes or an astronomer who uses these equipment to make measurements. This helps get people from varied backgrounds like electronics, metallurgy, computer sciences get interested in astronomy! And well, the more the merrier!
I have to stop now because studies show that people tend to lose interest if the article is too long. And i dont have anymore time now to continue as well. And i think i've given you enough links to go through till next time.
Btw, this is the first of a series of articles on telescopes, different telescopes for different kinds of light, their construction and working, measurements from these telescopes and a bit of theory as well. In the next article, i will elaborate on the different kinds of optical telescopes there are, different receivers there are and the advancements (read awesomeness) in Optical Astronomy.
Until next time...