“The Optical Train” is a collective term for all the optical components in a camera. Before light can form an image on a camera sensor, it has to be collected and brought under control so that it can form an image on the sensor. Every lens, mirror, filter, and other part that touches the light is a part of the optical train. If using a camera phone or a compact camera, the entire optical train is built into the camera and you never have to think about it. DSLR and other more advanced camera designs give more control, though, by letting you replace lenses, fit filters, and manually adjust the entire system for the finest control over the light. Astrophotographers have the option of making it even more interesting by removing the lens altogether and replacing it with a telescope, and CCD cameras do not accept lenses at all – they can only be used with a telescope. But even if you’re just capturing star trails with a compact camera, or holding your smartphone up to a telescope eyepiece, it’s useful to know a bit about how the optical train works. After all, it is the most fundamental component in your photography rig and its configuration directly affects what sort of images you will be able to capture.
Lenses and Mirrors
The most important part of any telescope is the objective lens or mirror. This is the biggest piece of glass and is the first optical surface that the light touches on its way to forming an image. It has two jobs: collect as much light as possible, and bring it to a very precise focus. This element can either be a large lens, which causes rays of light to bend towards a common point behind the element when they pass through it, or a curved mirror which reflects rays of light at angles that bring them to a common point in front of the mirror. The diameter of the objective element is called its aperture, and the distance at which the image is formed is called the focal length.
Smaller optical systems, such as camera lenses or small telescopes, are usually built around lenses (and are called “Refractors”), while larger systems are based on mirrors (“Reflectors”). Mirrors have the obvious problem of creating an image on the same side of the element as the object you’re looking at – you need to find a way to move that image aside so that you can look at it without your head (or camera) blocking the light. This is usually done with a second mirror, either reflecting the image off to the side, or back down through a hole in the objective. This secondary mirror unavoidably blocks some of the light, but lenses have their own problems.
First off, any lens will split incoming light into its component colours in the same way that a prism does – a problem called “Chromatic Aberration”. Optical designers are able to compensate for this by combining three lenses made from different types of glass in such a way that their chromatic aberrations cancel each other out. But this means that the optician now has to make three separate lenses, all precisely matched to each other, instead of just one. Secondly, lenses have to be made from the highest grade optical glass, which is completely free of bubbles of other impurities, and which is absolutely colourless. And finally, a lens element has two sides, which must not only be ground to a very precise shape, but which must be ground perfectly in line with each other. And finally, glass flexes slightly under its own weight. A large lens will sag as it moves, ruining its ability to focus light accurately.
Mirrors, by contrast, are made with their reflective coatings on the outer surface so that no light passes through the glass at all, so that even quite cheap glass can be used. Mirrors reflect all wavelength of light equally, so they do not suffer chromatic aberrations. And mirrors only have a single optical surface, which not only makes them much easier to manufacture, but allows them to be properly supported on their back sides so that they keep their shape. Small lenses can be mass-produced cheaply and efficiently, which is why small optical systems like binoculars and most camera lenses are exclusively refractors, but the problems increase exponentially with size which is why large amateur telescopes are almost always reflectors, and why workin gobservatories only use reflectors.
The aperture of the objective affects the brightness and the sharpness of the image. Since all the light which reaches the surface of the objective will be brought to a focus to create the image, a larger aperture means more light, which results in a brighter image. A large aperture also reduces diffraction effects, which affects how finely light can be brought to focus: Larger aperture lets you concentrate light to a finer point, which makes for a much higher resolution image (pro-tip: most camera sensors have a much higher resolution than most lenses could ever hope to support! This is why the number of megapixels should be one of the last things you think about when buying a camera). However, a larger aperture is also more sensitive to atmospheric distortions. If the air is unstable, then the view of a star or planet is blurred even before it reaches the telescope. Astronomers describe the impact of this effect as either “Good Seeing” or “Bad Seeing”.
The focal length affects the magnification of the final image: A long focal length magnifies more than a short one. Since the size of the sensor in your camera doesn’t change, a highly magnified image covers a smaller area of the sky than an image which is less magnified, as anybody who has ever played with the zoom feature of a camera knows. But this also means that increasing magnification makes for a dimmer image: Light from a smaller area is being used to create an image on the same sized sensor. The light is more spread out, and so the resulting image is less bright.
So it happens that both aperture and focal length affect the brightness of the image: A bigger aperture lets in more light, and a longer focal length “dilutes” it. Photographers deal with this by discussing the focal ratio (f/ratio) of an optical system. Divide the aperture by the focal length, and you get the f/ratio. My telescope has an aperture of 200mm and a focal length of 2000mm, so its focal ratio is f/10. In photography terms, this is a relatively “slow” system, since the camera will need a fairly long exposure time to properly capture a decent image. If I was to reduce the focal ratio (by switching to a telescope with either a wider aperture or shorter focal length), the image would be much brighter and I could use a much faster exposure setting.
In practice, the optical system can be quite complex. My telescope is fitted with a corrector lens, that reduces the focal ratio from f/10 to f/6.3, giving me a wider field of view and shorter exposure times. Camera lenses can be especially complex, containing seven or more elements and a diaphragm, all of which shift position in response to the various controls so that you can change both the focal length and the aperture, while still finding focus. But regardless of how an optical system is actually built, it can always be seen as a single unit, described by an aperture and a focal length, and this is all you need to provide to another photographer when explaining how you captured your images.
Most astrophotographers optical trains allow accessories to be fitted to modify the incoming light. The most common accessories are additional lenses to change the focal length of the entire system up or down, and filters to only allow certain wavelengths of light to shine through. Filters are usually only used with CCD camera, as these cameras are monochrome, so the only way to get a colour image is by taking three separate images using red, green and blue filters, but the filters can also be used to reveal the hidden structures of an object by only permitting the light emitted by certain elements to pass through. Those beautiful “False Colour” images returned by the Hubble Space Telescope are made in this way.
More advanced photographers might add a spectroscopic grating to split the incoming light into its component colours and reveal the composition of a star, and there are various other filters used for solar observing that reduce the intense heat and light of the Sun to a level that won’t damage the camera.
Putting it together
The optical train is obviously useless without a camera. In the case of a compact digital camera, the optical train is a small module built into the camera body and cannot be removed. Camera phones have even smaller optical trains, forcing them to be even less flexible. CCD cameras need to be fitted to a telescope, and generally end in a standard eyepiece-sized barrel for this reason. Since you’re unlikely to be swapping telescopes casually, care should be taken when buying equipment to ensure that the camera and the telescope complement each other, providing both the field of view and speed that is appropriate for the subjects you’re most interested in imaging.
But if you want flexibility, nothing beats a DSLR. Lenses, while not cheap, are still less expensive than decent telescopes, and can be removed and swapped out in seconds. And with a simple T-ring adaptor, the optical tube of a telescope can be fitted to the camera body as a very big lens, adding to your range.