APO/ED Refractors
| Watch in a short video the difference between the two telescopes. Apochromatic ED glass are very expensive, but they could give you clear images without color false color.. | |
Helpful information on using the commonly available and inexpensive 60 mm Refractor telescope. | |
A refracting telescope is the most common kind of telescope. Discover the difference between refracting and reflecting telescopes in this free video on home astronomy from a telescope salesperson. |
General features of Apochromatic telescopes
An apochromatic telescope features an apochromatic lens that has a better correction for chromatic aberration than achromat lens.
This type of telescope provides a better quality image than the achromat telescope. This becomes very noticeable when observing very bright celestial objects, such as Moon and Venus.
Observing the skies with Apochromatic telescope
Like all refractors, apochromat is a great telescope for observing stars and planets.
It is also a great telescope for beginners in astrophotography.
Apochromatic telescope VS Achromatic telescope
Apochromatic telescopes are more expensive than achromatic telescopes. The main difference between apochromat and achromat is in their ability to correct light so that different colours lie in the image plane together.
Apochromat lens corrects light so that three colours lie in the image plane together,
while achromat lens corrects only for two.
The apochromatic telescope can become very pricey if the user wishes to have a larger aperture.
Apochromat lens (APO) in Apochromatic telescope
The achromat lens brings only blue and red light to the center on the same plane. The apochromat lens, on the other hand, corrects all wavelengths to focus on the same point (red, blue, and green).
An apochromat lens consists of three different types of glass:
two crown glasses,
and one flint glass.
The
glasses are cemented together or have an air-gap between them, or a
combination of both. Because of a higher number of glass elements, other
defects such as coma and spherical aberration also have better correction.
APO vs ED
ED refers to a range of glasses with varying levels of Fluorite content. Nearly all apochromatic refractors use one or more elements of ED glass so both terms correctly apply to many telescopes. Some use ED glass but fail to achieve apochromatic performance - to those, only the ED term applies.
The refractor telescope uses a lens to gather and focus light. The first telescopes built were refractors. The small telescopes sold in department stores are refractors, as well as, those used for rifle scopes.
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
- All refractors suffer from an effect called chromatic
aberration (``color deviation or distortion'') that
produces a rainbow of colors around the image. Because of the wave nature
of light,
the longer wavelength light (redder colors) is bent less than the shorter
wavelength
light (bluer colors) as it passes through the lens. This is used in prisms
to produce
pretty 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. - 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.
- How well the light passes through decreases as the thickness of the lens increases.
- 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.
- The objective lens can be supported only at the ends. The glass lens will sag under its own weight.
Refractors are best for
-
Observers who want the best possible image quality
