Deep-sky astrophotography

Deep-sky photography involves capturing images of nebulae, galaxies and star clusters, which are too small and distant to see with normal lenses – so you normally require a telescope. However, you should be able to obtain usable images of larger targets like the Pleiades cluster or the Magellanic Clouds with a 300-500mm lens. You’ll also need a mount that lets you attach the camera body and it must be compatible with the camera’s lens mount.

You’ll need a telescope and a sturdy tripod to photograph distant objects like star clusters and galaxies. The telescope shown here is an enthusiast-level refractor, which has a spotting scope mounted on its barrel to help you find small objects in the sky. (Source: Camera House.)

Telescope types

Telescope buyers can choose between refractors and reflectors. Refractors resemble a long telephoto lens with a large front element that collects the light and two or more internal elements that focus it on the eyepiece at the end of the barrel.

Modern refractors can be either achromatic or apochromatic, with both types designed to reduce chromatic aberration (CA). Achromatic lenses correct the light for two colours (typically red and blue), while apochromatic lenses merge all three wavelengths into a single image plane. The residual colour error is usually much less than that of an achromatic lens.

A basic 70mm refractor telescope with a lightweight tripod and panning handle control for pointing it at the sky. This telescope is suitable for viewing the Moon, planets, star clusters and comets but too limited for viewing deep sky objects. (Source: Camera House.)

Reflectors are built like mirror lenses, with a primary mirror at the base and a secondary mirror positioned adjacent to the telescope’s eyepiece. The main mirror captures the light and directs it towards the secondary mirror where it’s focused by the eyepiece’s lens.

The table below outlines the main differences between refractors and reflectors.

	Refractor	Reflector
Size and weight	Normally much longer and heavier	Relatively shorter and lighter but with a larger barrel diameter
Price	Normally higher	Usually more affordable
Optical complexity	Complex, like a telephoto lens	Much simpler
Light-collecting capability	Relatively poor	Much better
Chromatic aberration	Can be susceptible	Immune
Susceptibility to atmospheric contaminants	Low since barrel is sealed	Open tubes are susceptible to dust and humidity
Optical performance	Superior sharpness and contrast	Optical quality may be below expectations

High-quality refractors are good for general astrophotography and planetary/lunar observing as well as observing binary stars. But the available fields of view are relatively wide compared to other types of telescopes.

This illustration shows a Newtonian reflector telescope with a smartphone adapter that allows an app on the phone to analyse the night sky and calculate its position in real time. The telescope’s 130mm diameter main mirror has enough light gathering ability to view distant nebulae, galaxies and star clusters. (Source: Camera House.)

Reflector telescopes normally have shorter, fatter barrels with greater light-gathering capabilities. Most popular models use catadioptric designs that use both lenses and mirrors, making them compact and transportable.

Many have sealed tubes to keep out dirt and dust and only require occasional collimation. Collimation keeps the mirrors in the telescope aligned with the light path. The process is described in detail here on the Sky & Telescope website.

A large reflector telescope set up for photographing the night sky, with a smaller telescope beside it. Note the use of red lights to preserve the photographer’s dark adaption. (© Andrew Murrell.)

Reflector telescopes come in four main types:

Newtonian reflectors are based on a 17th century design by Isaac Newton and use a large curved mirror to reflect light back to a secondary mirror or optical flat that directs the light to the eyepiece near the top of the barrel to form an image.

Dobsonian telescopes are altazimuth-mounted Newtonian reflectors optimised for observing faint, deep-sky objects such as nebulae and galaxies. Efficient light gatherers, they are compact and competitively priced.

A ‘tabletop’ Dobsonian reflector like this one is ideal for families with its simple, portable design and 76 mm reflector optical tube. Affordably priced it has a wide field-of-view and is simple to use for panning the Milky Way and exploring star clusters and larger nebulae. (Source: Camera House.)

Schmidt-Cassegrain reflectors use a spherical primary mirror and convex secondary mirror to correct for spherical aberration. This design is popular for consumer telescopes because it combines easy-to-manufacture spherical optical surfaces to create an instrument with a long focal length at a low cost for its magnification.

Maksutov (commonly shortened to ‘Mak’) reflectors combine a spherical mirror with a weakly negative meniscus lens in a design where the negative lens is usually full diameter and placed at the entrance pupil of the telescope to correct the opposite errors in the spherical primary mirror.

Telescope choices

The design of your telescope determines its size, magnification, focal length and light-gathering ability – and also the types of subjects for which it is best suited. The best type of telescope for you depends on your intended uses.

The Jewel Box star cluster is a popular subject for astrophotographers because it’s located in the Southern Cross and looks like a hazy star when viewed with the naked eye. It’s bright and quite colourful and contains just over 100 stars. (Source: Camera House.)

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This article by Margaret Brown is an excerpt from Astrophotography pocket guide – click here to order print or ebook edition.

Pocket guide Partner: Camera House