Telescopes- Where to start?

One of the first things most people will think about when they hear ‘astronomy’ is telescopes. Of course, you do not need a telescope to get started in astronomy and you can get a lot out of just looking up with a star chart or a pair of binoculars; a telescope will always open up the opportunity to explore the cosmos further from your back garden.

For backyard astronomy, we use an optical telescope -an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects, rather than looking at the radiowaves or infrared radiation.

Originally invented in the Netherlands in the 17th century with glass lenses, the first telescopes were refractors, which use specially designed lenses to focus the light into an image. Most refractors contain two lenses:

• The larger is called the objective lens, which focuses incoming light inside the tube

• The smaller is called the eyepiece lens, which focuses the light for viewing

Nowadays, amateur astronomers have access to reflector telescopes too, which were invented only a few decades after the refractor - these use mirrors to reflect and focus light at various angles within the optical tube, extending the overall path of light.

A reflector consists of a primary mirror located at the opposite end of the telescope from the aperture (where light enters) and a smaller secondary mirror near where the reflected light converges. This secondary mirror reflects the light toward the eyepiece.

So, reflector or refractor?

Reflector Pros:

• Generally cheaper than refractors as manufacturing large mirrors is usually more affordable than manufacturing large lenses

• Very easy to use

• Large aperture for observing deep sky objects

• Can capture large amounts of light

• No chromatic aberration (colour fringing due to different wavelengths of light being reflected at different angles by the same lens)

• Better option for deep space objects (eg. galaxies/nebulae)

Reflector Cons:

• Mirrors require collimations (aligning the mirrors so that all the optical elements in the telescope are aligned with one another)

• Mirrors will need cleaning every few years

• Open tube can open up the possibility for the equipment to get dusty and humid

• Usually quite heavy and large to maneuver

Refractor Pros:

• Tend to be smaller in width and easier to move

• Low image distortion

• Images are the right side up (Reflectors will have everything upside down) - making it easier for begginers to find objects in the sky

• Closed system allows for less maintenance and cleaning

• Better for astrophotography

• Lenses do not require collimation -near permanent optical alignment

• Higher revolving power (ability to separate the images of two objects, which are close together) per inch of aperture

• No interruption on the light path

Refractor Cons:

• Chromatic aberration (but this can be fixed using a apochromatic telescope - which use 3 air-spaced ED (Extra-low Dispersion) lenses to bring the light of 3 different frequencies to a common focus)

• Smaller Apertures as large apertures are very expensive

• Less light collected

• The objective lens has to be supported at the edges and can potentially sag under their own weight

• To achieve a large focal length, the telescope has to be long

• In the glass, light is absorbed and scattered

• The large lenses are difficult to make so are more prone to imperfections. 

As with any equipment, there are many different types of each and every design has different capabilities:

For reflectors, there are several popular designs:

• Newtonian

The simplest type of reflector; a parabolic (satellite dish shaped) mirror that reflects the light back to a secondary mirror at a 45º angle, which directs it to an eyepiece or camera. These are excellent for beginners and the most popular type of reflectors. They have short focal ratios -focal length divided by aperture - (which can make the telescope more expensive or not work so well at higher magnification), and can have large apertures. Newtonians require regular collimation and are quite heavy.

• Dobsonian

The Dobsonian is a Newtonian telescope mounted on a modified alt-azimuth mount (a two-axis mount for supporting and rotating the telescope about two perpendicular axes – one vertical and the other horizontal), making it relatively easy to use. This mount lets it rotate freely and removes the need for a tripod. They are great for visual astronomy and can have large apertures, though they are not well-suited to deep-sky astrophotography and are large and heavy.

• Richey-Chretién (RC)

A variation of the Cassegrain (which has a combination of a primary concave mirror and a secondary convex mirror) design, the RC telescope uses hyperbolic instead of parabolic mirrors. This eliminates comatic aberration, which can make stars appear distorted with a tail (coma) like a comet, almost entirely, though they suffer from astigmatism (or “wings” on either side of bright stars). This can be helped by using a Barlow lens though. They are great for deep-sky astrophotography but are more expensive and not well-suited to planetary work due to how the large central obstruction decreases contrast.

Different types of refractor telescopes:

• Achromatic telescope: This telescope is designed to fix the major drawback of refracting telescopes: chromatic aberration. This type was first invented by an English barrister named Chester Moore Hall in 1733. They are particularly good for observing bright objects such as the moon, planets and resolving things like double stars, but many astronomers who image deep sky and other objects use very high quality apochromatic refractors, due to their superior optics. The achromatic lens is used in this type to bring two different wavelengths into the focus points in the same plane. The two wavelengths are red and blue

• Apochromatic telescope: The apochromatic lens is used in this type to bring three different wavelengths into the focus points in the same plane. The three wavelengths are red, blue, and green.

  Although this article is a whistlestop tour of the different types and their variations of telescopes, keep an eye out for later articles on how to use a telescope, magnification and more!

  Thanks so much for reading and if you have any questions comment them below or tweet @WeeklySpaceman and @EveCarruthers_ on X. We would also love to include your stories, images and ideas in special Down the Telescope issues so make sure to share them with us!