A spectrum of a star is composed of a number of 'lines' which can be either emission or absorption lines. The continuum emission is a product of the blackbody radiation at specific frequencies, caused by electrons in atoms dropping down into lower energy levels. They can also be caused by molecular transitions to lower energy levels. This sort of line appears brighter compared to the region of the spectrum around it. Absorption lines cause dark features in the continuum emission where the radiation is removed. This is caused by atoms (or molecules) absorbing radiation, and moving to a higher energy state. This process causes the lines to look darker when compared to the region of the spectrum around them.
Stars come in a wide range of sizes and temperatures. The hottest stars in the sky have temperatures in excess of 40,000 K, whereas the coolest stars that we can detect optically have temperatures on the order of 2,000-3,000 K. The appearance of the spectrum of a star is very strongly dependent on its temperature. For example, the very hottest stars (called O-type stars) show absorption lines due to ionised helium and a few other ionised elements. On the other hand, the coolest stars (M-type) show lines produced by molecules.
Refracting Telescopes use a lens to gather light and bring the image to the eyepiece. These are the type of instruments that most people think of when they think of a telescope. They are fairly maintenance free and generally provide superb images of the moon, planets, star clusters and general sky gazing. They tend to be smaller in aperture than other types so they are not as good for viewing fainter sky objects such as galaxies and nebulae.
The refractor telescope uses a lens to gather and focus light. The first telescopes built were refractors. Most small telescopes sold in gift shops are refractors.
Advantages
- Refractor telescopes are rugged. After the initial alignment, their optical system is more resistant to misalignment than the reflector telescopes.
- The glass surface inside the tube is sealed from the atmosphere so it rarely needs cleaning.
- Since the tube is closed off from the outside, air currents and effects due to changing temperatures are eliminated. This means that the images are steadier and sharper than those from a reflector telescope of the same size.
Though excellent refractors are still made, the disadvantages of the refractor telescope have blocked the construction of very large refractors for use in astronomical research.
Disadvantages
1. All refracting telescopes suffer from an effect called chromatic aberration (colour deviation or distortion) that produces a rainbow of colours around the image. Because of the wave nature of light, the longer wavelength light (redder colours) is bent less than the shorter wavelength light (bluer colours) as it passes through the lens. This is used in prisms to produce rainbows, but can it ruin an image!
There a couple of ways to reduce chromatic aberration. One way uses multiple compensating lenses to counteract chromatic aberration. The other way uses a very long objective focal length (distance between the focus and the objective) to minimize the effect. This is why the early refracting telescopes were made very long.
2. How well the light passes through the lens varies with the wavelength of the light. Ultraviolet light does not pass through the lens at all.
3. How well the light passes through decreases as the thickness of the lens increases.
4. It is difficult to make a glass lens with no imperfections inside the lens and with a perfect curvature on both sides of the lens.
5. The objective lens can be supported only at the ends. The glass lens will sag under its own weight.
Other uses for refractive telescopes are for some types of binoculars and telescopic rifle sights are the most common uses than telescopes for star-gazing.
Resources:
Staff at Norman Lockyer Observatory
Pictures: Kensington Telescope and its dome are my own.
Birds eye photo of Observatory from
Graph Image from
