Large eyes to look the heavens

Large Eyes to look the heavens

Debi Prasad Choudhary

Los Angeles, 02/05/2014

God gifted divine vision to his friend and disciple in a battlefield to enable him see his “cosmic self”. While listening to this episode in the story of Mahabharat from my granny, I wished such a vision to see the cosmos. Twenty years later, the sight of Saturn, Jupiter and Orion Nebula through a moderate telescope, reminded me of the feelings of God’s friend if he had really seen these objects all at once!! Telescopes are large eyes to look at the tiny light glows in the sky situated far away.

Human eye is a sophisticate camera with a complex lens of about 8 mm size. In broad daylight, we use only a small part about 2mm of this this lens through which light enters to make an image inside the eye. As we enter a low light area the iris fully opens up for the light to enter through entire 8 mm lens. Usually it takes some time for this to happen, so we cannot clearly see in the first few moments of entering the dark from bright illuminated area. The 8 mm lens collect enough light to see the stars of about 6th magnitude and can distinguish two stars separate by about 1 arc minute. The range of color that our eye can see is limited to the colors of rainbow.

Light is electromagnetic wave because the electric field and the magnetic field vibrate in planes perpendicular to the direction of its propagation. The distance between the maximum vibrations in same direction is called wavelength. The two extreme color of rainbow are violet and red have wavelengths 400 nanometer and 700 nanometer respectively.  The electromagnetic waves of long wavelength are of lower energy compared to shorter wavelength. There are electromagnetic waves beyond this visible range. The electromagnetic waves of wavelength lower then 400 nanometer are x-ray waves that are used in dentist office, UV waves that cause skin problem if sunscreen is not used while lying in beach. The waves with larger than 700 nanometer wavelength are Infrared that we use for pain reliever, radio and microwave that are used for communication via TV, radio stations and mobile phone technology. All these electromagnetic waves are exactly same as the light we see with our necked eye, but have different wavelength. The stars and galaxies in the sky produce light in a wide range from very short to very long wavelengths.

Light propagates in the form of waves that prevent the formation of sharp shadow of knife-edges. Infect, if you closely look at the shadow of a sharp object like a knife-edge, you will find dark and bright bands that are produced due to diffraction. The same effect limits us distinguishing two pennies kept side by side beyond about 225 ft. In order to distinguish them, we need to increase the size of our eye (make it bigger than about 8 mm), which is not possible. The other alternate is to use a telescope that has a bigger lens.

The wave nature of light has another consequence. As the light wave front from a distant star passes through the earth’s atmosphere, different parts move at different speed through packets of air with different density. By the time it reaches our eye the light wave is distorted from its original shape before entering the earth. Since the air pockets with different density move, we see the stars twinkle. The planets do not twinkle because they are situated relatively nearer and send many light waves slightly separated in space. These days, we can repair the distortion using modern image stabilizer technology somewhat similar to the one available in many cameras (adaptive optics) and prevent the star from twinkling.

Astronomers build telescopes and detectors to look at them in all wavelengths in minute detail. About 400 years ago, Galileo used the telescope to discover dark spots on the sun and moons of Jupiter that made great impact in modern science. While working as a professor in Padua University in Italy, he used to buy lenses in neighboring town Venice and select the best ones after examining them by looking at the stars. The stars would look like a point of bright light if the lens were perfect. But, since the images in blue, yellow and red color are made at different distances from the lens, they might still look blurred.

The bigger lenses are used to make telescopes to see faint stars by collecting more light. Large lenses are supported only at the periphery, so they become bent due to their own weight making it impossible to make them larger than about one meter size. This difficulty can be overcome by using reflecting mirrors of spherical shape, since they can be supported from the back. Mirror based telescopes produce images of color at same distance from the reflecting surface. But, the light reflected from the outer part of large spherical mirrors are focused at different distance that also results in blurred images. So, large reflecting telescopes are made with mirrors that are not spherical but parabolic.

The thickness of the mirrors should be about 1/6 of the diameter in order to prevent flexure leading to the distorting the parabola shape. This requirement creates problem in making mirrors for the large telescopes of sizes greater than 3 to 4 meter as they become too heavy. Making segmented mirrors and using an active support overcomes this problem. The segmented mirrors use smaller parts of a parabolic mirror held by active supports that maintain the parabolic shape. The active supports continuously tilt to adjust the orientation of mirrors to keep them in a parabolic form. The surface of the mirrors or the lens that produce the images of astronomical objects must be smooth relative to the wavelength of the light.

Since the earth rotates around north-south axis, special mounts are built to align them along local latitude and track the stars by moving at a rate of 360 degree in 24 hours. There are other sophisticate mounts and tracking systems for large telescopes. Building the enclosure or telescope dome and the location of the observatory is also an important part of these astronomical facilities. The observatories are built at high altitudes – on the top of mountains, that are dark and have clear sky for most of the year. For installing very large telescopes, like the 30-meter telescope being made by Caltech and University of California, extensive site survey is undertaken. Additional complexity arises for telescopes built for observing the sun because it heats the surroundings. The solar observatories are built surrounded by large body of water or in very high altitudes where the atmosphere is stable in presence of the sun.

A large part of light in ultraviolet, X-ray, gamma ray and Infrared is blocked by the atmosphere and do not reach the earth. Satellite based space born telescopes are built to observe the stars and galaxies in these wavelengths. These great instruments include Hubble Space Telescope, Chandra X-ray telescope and Spritzer telescope for IR light. 

The light in radio wavelength similar to FM radio or TV communication is long so the telescope surface need not be very smooth. At the same time large mirrors are needed for achieving the images of higher resolution. Many separate telescopes are used to collect light and combined to get an image from an effectively large telescope. These are called interferometers, a technique that is also used for optical wavelengths.

Like, we get lots of information about an object from the amount of light it emits and its color, telescopes serve the same purpose for astronomical sources. The information about the nature of the object is embedded in the amount of light it emits or reflects, the color or wavelength of light and amount of light wave in different orientation, which is also known as the state of polarization. Instruments are used at the focal plane of the telescopes to examine these properties of light from the astronomical sources.

Modern big eyes enable us to see the objects very faint in the sky and learn about them in all colors that are visible to our eye and beyond. Often these faint objects are situated far away, such that light from them takes many many years to reach us. This means we are able to see a star that may not be there at this time, but what it was when the light left from it. Today, we have built such large telescopes that we can see the objects that were born just after the creation of the universe. Some times, I think the fantastic imaginations of the ancient stories are taking real shape.