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Understanding > Fundamental concepts > Observations II

REFRACTORS AND REFLECTORS

Refractors and reflectors (telescopes) are primarily light collectors which will rebuild the image of a part of the sky in their focal plane. Let's review some simple principles of optics.

Refractors and reflectors are optical systems that form in the focal plane, a stigmatic image of an object at infinity, so a fraction of the sky.

Refractors and reflectors give an image of the celestial sphere. This image has the curvature of a sphere in the focal plane. At a given angle on the sky will correspond the distance in millimeters on the image of the focal plane, which leads to define a fundamental parameter of the telescope: the scale of the image. The image of the portion of the celestial sphere will have a radius equal to the focal length of the instrument. Thus a radian angle will have as image a circular arc the length of which will be the focal length of the telescope.

The second main parameter of a instrument is its aperture, that is to say the diameter of the mirror of the telescope (reflector) or the diameter of the lens (refractor). It is associated to the beam of light which is equal to the ratio f/D where f is the focal length and D the aperture. The higher this ratio is, the more light per unit area in the focal plane is large and thus less exposure times will be necessary.

Another important parameter characterizing an instrument is the field available in full light. This field is characterized by an angle on the sky and by millimeters in the focal plane. The wide-field instruments were built especially for that purpose.

The last fundamental parameter of an instrument is its resolving power (or angular resolution), that is to say, the angular size of the smallest measurable object. For example, if two points separated by one second of a degree are the closest points seen through the telescope as two separate points in the focal plane, then the separation power of the instrument is one second of a degree. What limits this resolution? To explain this, it is necessary to call the phenomenon of diffraction: the aperture of the instrument (size of the lens or the mirror) is a kind of screen to the beam of light infinitely wide and serves as entrance aperture. It will diffract the beam and we will get in the focal plane of a different image of the object from which it originates. For a punctual object at infinity and a circular entrance aperture, the image will look like this :

It is noted that to increase the resolving power of an instrument simply increase its diameter. An instrument of 30cm aperture has a resolving power of 0.46 arcsecond, an instrument of 1 meter-aperture, 0.14 arcsecond and an instrument of 8 meters : 0.017 arcseconds, for a wavelength of 0.55 micrometer (visible). This resolution, however, is theoretical because of the atmosphere. Its agitation spread the Airy disk. We characterize this agitation by "seeing" (turbulence) of the sky at the time of observation. The best observational sites located in high mountains reach 0.6 arcsecond at best. Two solutions are possible to increase the angular resolution despite that: l' adaptative optics that compensates for atmospheric agitation or watching outside the Earth's atmosphere through a spatial telescope.

The refractor telescope

The refractor is the oldest astronomical instrument: it has an objective lens forming the image in the focal plane. This lens is formed of two cemented lenses: a convergent lens and a divergent lens in order to optimize, for a defined wavelength, the concentration of light in the focal plane (a single lens forms images for each wavelength in different focal planes since it uses the refraction of light rays in the glass to make the image). Glass lens could not exceed a diameter of one meter for technical reasons (weight). In the case of a refractor, the position of the focal plane relative to the lens depends on the wavelength of the radiation considered. The second lens -divergent-, must be attached to the first to limit this effect.

The reflector telescope

The reflecting telescope is composed of mirrors and uses reflections to make images: it has not the inconvenience of glass lens and the images are all in the same plane. A reflector is said achromatic. The making of a mirror only requires the work of a single glass surface while a refractor requires the work of four surfaces. There are mainly two different types of optical systems for the secondary mirror which reflects the image and changes the focal length of the instrument.

The refractors, during the last century, have been supplanted by reflecting telescopes. The reasons for this are many:
- first, the achromaticity as we discussed above;
- then , in a refractor, light passes through the glass of the lens since in a reflecting telescope, the glass, polished, only serves to support a reflective layer. It is therefore necessary that a glass lens is very homogeneous, which is very difficult to achieve for lenses with a diameter greater than one meter;
- for the same reason that the light passes through the glass of the lens, some wavelengths of the spectrum are stopped by the glass: a refractor is completely blind in the infrared;
- finally , it is not possible to make lenses whose focal length is of the same order of magnitude as the diameter. The refractors are thus always instruments of great length. Thus a telescope must be able, within a very different positions to reach all parts of the sky: mechanical stress on an instrument of great length (especially bending) prohibit a precise and permanent accuracy of the instrument. Instead, reflecting telescopes, more compact, are less distorted during a night of observation. Furthermore , it is possible today to build telescope mirrors which have a focal length of the same size of their diameter. These telescopes are very open, which means that the light beam converging to the focal plane has an important angle. The greater this angle, the greater the amount of light received at the focus is high, therefore, at equivalent exposure time more faint objects will be observable. In astronomy, seeing faint objects often means seeing far...

The mounting of the telescopes

Telescopes are generally heavy instruments made to:
-point a celestial object with a precision of one minute of a degree;
-have a stability such as uncontrolled movements of the telescope must not exceed one-tenth of a second degree;
-follow celestial objects in their diurnal motion, that is to say compensate the apparent motion of celestial bodies due to the rotation of the Earth around its axis.

To be able to point any celestial body point, the telescope must have two degrees of freedom, one about an axis parallel to the axis of rotation of the Earth and the other on either side of the equatorial plane perpendicular to the axis. Thus, the telescope will point naturally in hour angle and declination.

The stability of a telescope will come from a perfect engineering and perfectly balanced around the axes of rotation regardless of the position of the telescope.

Monitoring the apparent diurnal motion of the celestial bodies will be provided by a rotational motion around the north-south axis as the telescope would make a complete revolution in 23h 56m 4s (sidereal rotation of the Earth).

The adoption of the equatorial mounting ensures just this monitoring. Contrarily, for very large modern telescopes, an equatorial mounting cannot be sufficiently stable with respect to the weight of the instrument. It then takes an altazimuth mounting (similar to guns) providing the degree of freedom as a telescope rotatable around the vertical axis and a rotation above the horizontal plane. Monitoring the apparent diurnal motion of the celestial bodies is provided by a computer which continuously performs the conversion of the hour angle and declination into azimuth and height above the horizon.

On trouvera Examples of refracting and reflecting telescopes can be found at the end of the chapter.

The Space Telescope.

Let us now consider the very special case of the space telescope whose exact name is Hubble Space Telescope (HST) named after the famous American astronomer who first realized that the universe is expanding. A telescope in orbit outside the atmosphere is obviously a good response to a number of constraints inherent in astronomical ground based observation(click here for an image of the Space Telescope ).

The most important is the ability to totally and permanently get rid of the presence of the atmosphere. Nothing is ever as perfect as the elimination of the cause of trouble. Here are benefits of this at three levels. No turbulence, thus resolving power equal to the theoretical resolving power. No atmosphere, so no parasite radiation in the infrared. And yet, no atmosphere, so no atmospheric extinction (absorption of a portion of light by gas molecules composing the atmosphere). More, the notion of observation at the meridian has no meaning here and an observation may last as long as necessary.
Second advantage is that the same place you have access to the whole sky at any time of the year. Being weightless also eliminates many secondary problems which we have not discussed, such as the deformation of the metallic structures that limit, too, the performance of very large instruments.
Finally , being also in a perfect vacuum has a number of advantages regarding the problem of oxidation of the components of any kind. The equipment has a life expectancy great .

But there are also serious criticisms.
The first is, of course, the cost of such an instrument. Many people think this will be the only such telescope. But we do not make astronomy with a single telescope.
The lack of flexibility of use: this telescope works on a specific program established in advance. It is indeed essential to minimize the maximum mispointings of the instrument. In space, any misalignment should be corrected using gas the amount of which is necessarily limited. If you can imagine a non- programmed observation of an unexpected phenomenon, this mode should be exceptional.
For observations "at the limit" or requiring decisions "on the job", nothing can replace the presence of an astronomer. In such circumstances, the space telescope is poorly adapted.
Finally, it will never be a telescope to do everything. How many important works not requiring great means of observation would be sacrificed without ground-based telescopes.

So whatever the real and irreplaceable advantages of this telescope, it is not question of abandoning the effort to develop both new equipment, new generations of collectors on the ground. Americans themselves have understood and continue to prepare new instruments for ground-based observatories.

Examples of refracting and reflecting telescopes.

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Credit : J.E. Arlot/L. Vapillon/observatoire de Paris