How to Purchase a Telescope.
1. Introduction
Before introducing ourself in the difficult and somewhat challenging purpose of this page, i think it is better to give some simple information on the features that define the performance of a telescope and of the different types of telescope commercially available.
2. Definitions
The most important parameters in a telescope are the following:
Aperture: this is the diameter of the primary mirror or of the objective lens. The aperture of a telescope is the single most important feature in the choice of a telescope (if we do not count the price). This is because the aperture determines the light gathering capacity of the telescope, that is how much light from distant and faint objects the telescope is able to focus in single point.
Focal Length: the focal lenght is a geometric parameter which is determined by the distance between the objective lens, or the mirror group in a reflector telescope, and the focal point of the system. This parameter is proportional to the curvature of the refracting or reflecting surface and to the magnification capacity of the telescope.
Focal Ratio: the focal ratio is the ratio between the focal length and the aperture. Therefore for a telescope with 10" aperture and 100" focal length the focal ratio is 10. In this case it is usually indicated as f/10. The focal ratio has no importance for the visual performance of the telescope but it plays a fundamental role in astrophotography as it is proportional to the exposure necessary for a deep-sky picture. Basically the higher the focal ration the longer the exposure required to have the same brightness on a film when the same object is imaged with the same aperture. The higher focal ratio telescopes are then defined as "slow" and the lower focal ratio telescopes are defined as "fast".
Resolving Power: this is the minimum angular dimension a telescope can resolve in optimal atmosphere conditions. This limit depends on teh aperture and can be determined dividing 4.56 by the telescope aperture. For a 10" telescope is 4.56/10=0.456 arcsecond.
Exit Pupil: is the diameter of the light beam at the output of the eyepiece. The size can be detrmined dividing the focal length of the eyepiece by the focal ratio of the telescope.
Light Gathering: this is related to the aperture of the telescope. The eye gathering is the ratio between the square of the aperture in millimeter and 7mm (eye aperture when dark adapted).
Limiting Magnitude: this is the magnitude of the faintest star visible in optimal conditions with the telescope. It is directly proportional to the aperture.
Chromatic Aberration: This is a common defect in refractor telescopes (see below for description). The problem arises form the fact that in the real world there is no such a lens able to bend teh light with a constant refraction index versus the wavelength. Basically that means that the different colors constituting the light beam are not bent to the same focal point. This causes the blue halo around brighter stars because the blue component is less bent than the other ones by the lens. The chromatic aberration depends on the lens curvature and then on the focal length of the telescope. The more the curvature the stronger the aberration. That is why older refractor telescopes were as long as possible and thus very slow.
Spherical Aberration: this is typical in reflector telescopes (see below for description). The spherical aberration is due to the non perfect parabolic (or hyperbolic) curvature of the telescope mirrors. Parabolic curves are the only ones able to focus light beam in a single point called focus. If some deviation from this curvature were generated during the manufacturing of the mirrors the telscope would suffer from spherical aberration. This was the cause of the malfunctioning of the Hubble telescope after the launch.
Coma: this aberration comes from the fact that the mirror in reflector telescope is curve. This curvature causes the image at the edge of the mirror to be elongated making the stars similar to comet. It comes without saying that the stronger the curvature teh stronger is the telescope coma aberration. Since the curvature of a mirror is related to the focal length and then to the focal ratio, faster telescopes suffer more from this aberration than slower telescopes.
3. Telescope Design
There are three main kind of telescopes commercially available today. Before entering in the discussion of what is the best telescope for you it is better to understand how any of the different designs operates in order to understand its features and limits.
Refractor. The refractor operates by bending the light to a common focal point by means of a curve glass surface (objective lens) place at the input of a tube. The performance of this design are strongly influenced by the quality of the glass in the objective lens. Based on the material and the coating on the glass one can define two main classes fo refractors: achromatic and apochromatic.
The first class makes reference to a family of refractor whose objective lens is made in such a way that almost all the components of the light beam are bent to the same focal point.
The second family consists of all those refractor telescopes whose lenses are capable to bend the light to a single focal point with no component exception. These telescopes are able to provide star images like point with no color haloes around them.