Ken Hough's Website
Most optical devices exhibit optical distortions of various kinds. This section describes the most significant of these distortions and what can be done to minimise them.|
Simple lenses exhibit CHROMATIC ABERRATION. This results in white light being split into it's component colours, as happens with a prism. In the case of a lens this causes the focal length of the lens for red light to be different from that for blue light, making it impossible to accurately focus the entire spectrum of visible light at a common plane of focus. The result is that images will not appear to be sharply focused. Also infra-red light and ultra-violet light (to which cameras can be quite sensitive) will be even further out of focus.
By building compound lenses using elements made from glasses of different refractive indices (for example lead glass and flint glass), it is possible to significantly reduce chromatic aberration. Indeed many telescopes are made using just two such elements and by keeping the aperture number to F10 or greater (ie small diameter lens with long focus), chromatic aberration can be reduced to levels that are acceptable for visual use. This is the basis for the classic ACHROMATIC refractor telescope which when used with suitable infra-red and ultra-violet filters can produce sensibly sharp recorded images.
Recently there has been a trend towards the manufacture of fairly cheap short focus achromatic telescopes. These telescopes can exhibit quite serious chromatic aberration and are not recommended for photographic work.
Chromatic aberrations can be greatly reduced by the use of elements made of special low dispersion glasses. This makes it possible to produce short focus telescopes using only two elements while still achieving low levels of chromatic aberration. These telescopes are sometimes mistakenly referred to as being APOCHROMATIC, or perhaps more accurately as SEMI-APOCHROMATIC (see APOCHROMATIC below). An example of this kind of telescope is the William Optics Megrez 72 which costs several times more than a comparable short focus achromatic telescope.
The use of three or more elements made of various kinds of glass, makes it is possible to practically eliminate chromatic aberrations across the visible spectrum. Telescopes built in this way are referred to as being APOCHROMATIC and are considered to be the best type of refractor (ie lens based) telescope for astrophotography. They are also VERY expensive.
Although true APOCHROMATIC telescopes should be the best, in practice some of the SEMI-APO telescopes can perform as close seconds. Unless a large budget is available, most folks should go for a good SEMI-APO telescope.
Chromatic aberration occurs in refractor (ie lens based) telescopes. Newtonian REFLECTOR (ie concave mirror) telescopes do not exhibit chromatic aberration. Compound telescopes such as Schmidt Cassigrain and Maksutov also produce no significant amounts of chromatic aberrations.
Even if chromatic aberration is absent, most simple lens based (ie refractor) telescopes will exhibit some degree of SPHERICAL ABERRATION. Although an image has been brought to a sharp focus at the axis of the telescope, sharpness will deteriorate away from the axis. This is because the position of optimum focus is of a spherical form rather than a flat plane.
Instead of stars near to the edge of the field of view appearing as sharp points of light, they will appear to be somewhat out of focus. This might not be a problem when using the telescope for direct viewing, but might spoil a photograph.
Optical correction devices known as FIELD FLATTENERS are available to correct for spherical aberrations, but these must be chosen to match the telescope being used. Results can be excellent. However, the amount of spherical distortion might be low enough to ignore.
According to Wiki-pedia:
"Coma occurs in telescopes [that use] parabolic mirrors. Unlike a spherical mirror, a bundle of parallel rays parallel to the optical axis will be perfectly focused to a point (the mirror is free of spherical aberration), no matter where they strike the mirror. However, this is only true if the rays are parallel to the axis of the parabola. When the incoming rays strike the mirror at an angle, individual rays are not reflected to the same point. When looking at a point that is not perfectly aligned with the optical axis, some of the incoming light from that point will strike the mirror at an angle. This results in an image that is not in the centre of the field looking wedge-shaped. The further off-axis (or the greater the angle subtended by the point with the optical axis), the worse this effect is. This causes stars to appear to have a cometary coma, hence the name."
This may sound complicated and might seem to infer that reflector telescopes are inferior. Not so! Just as field flatteners can be used with refractor telescopes, COMA CORRECTORS can be used with reflector telescopes. Results can be very good indeed with sharp images produced across the entire field of view.
When used with a Baader Multi-Purpose Coma Corrector, my own 250mm diameter/1200mm focal length (ie F4.8) Newtonian telescope can produce very sharp images across the entire field of view of a 35mm camera. Being able to use such a "fast" telescope for photography has advantages in terms of short exposure periods.