HOW DOES A NEWTONIAN REFLECTOR TELESCOPE WORK?
Updated: Jul 20
A reflector telescope, also known as a Newtonian reflector, is one of the main types of telescope, along with refractors and catadioptric telescopes. In this post, we will take a closer look at the construction of a reflecting telescope and its main characteristics, which in many ways also apply to other types of telescopes. Since I myself own an 8-inch Newtonian reflector telescope and have performed astronomical observations with it for years, I will try to explain what the most important parameters of this telescope are, what parts it consists of, and what accessories are available for it.
Before we look at how reflectors work, let's clarify how optical telescopes are mainly classified.
The most important part of any telescope is its light-collecting and focusing element or objective. If refractors have a lens as an objective, and catadioptric telescopes have a combination of lenses and mirrors, reflectors have a curved mirror as an objective. Thus, depending on how the light is collected and focused determines the type of optical telescope.
Construction of a Newtonian reflector
The simple diagram below illustrates how a Newtonian reflector works. The tube that houses the telescope's optics is called an optical tube assembly (OTA). At the rear end of the OTA is the most important part of the Newtonian reflector - a parabolic or spherical concave mirror. The primary mirror collects the light from the object and focuses it onto the secondary mirror located at the front of the OTA.
The secondary mirror is a plane mirror placed at an angle of 45 degrees, which reflects and focuses the light coming from the primary mirror. As we can see, the focal point of the primary mirror is further away from the location of the secondary mirror. The focal point is formed in the focuser of the OTA tube, which is located on the side of the tube. The focuser holds the eyepiece and can be moved along the optical axis to obtain a sharp image.
The most important feature is the aperture of the reflector
The aperture is the diameter of the primary mirror of a reflector telescope (given in millimeters or inches). Obviously, the larger the surface area of the primary mirror, the greater its ability to gather light, and the brighter and the more detailed the image we see. The light-gathering power of a telescope's primary mirror is directly proportional to its surface area. If we compare how many times the surface area of one mirror is larger than the surface area of another mirror, we can see how much the difference in their ability to collect light is.
If we compare, for example, 200 and 127 apertures, we see that the 200 mm reflector has approximately 2.5 times the light-gathering capacity of the 127 mm reflector.
Aperture is also related to the telescope's maximum useful magnification. This is equal to approximately twice the diameter of the mirror in millimeters or 50 times the diameter of the mirror in inches. For example, the maximum useful magnification of my 200 mm (8 inches) telescope is about 2x200=400 times. This doesn't mean that I can't get higher magnification with my telescope using the appropriate eyepieces, but as a result, distant objects such as galaxies and nebulae appear dimmer. Magnifying beyond a reasonable limit does not mean that the mirror somehow collects more light. At higher magnification, the light is spread over a larger area in the eye, and the image appears larger but very dim.
Magnifying the image with the eyepiece
There are two ways to change the magnification - either change the focal length of the telescope or the focal length of the eyepiece. Since the focal length of the reflector is fixed, we use eyepieces with different focal lengths to obtain different magnifications. Finding the magnification (M) is quite simple, for this, you have to divide the focal length of the telescope (fo) by the focal length of the eyepiece (fe). For example, if we have a reflector with a focal length of 1000 mm and an eyepiece with a focal length of 21 mm, we get a magnification of 1000/21=48 times.
Eyepieces with a shorter focal length (5-10 mm) are more suitable for observing the moon and planets, while eyepieces with a longer focal length (20-30 mm) are more suitable for observing deep space objects, such as star clusters and galaxies. Reflector telescopes usually come with a Barlow lens. The Barlow lens extends the focal length of the reflector, but it can always be removed if desired. My Skywatcher 200p telescope came with a 2X Barlow lens, meaning that instead of 1000mm, the focal length of the primary mirror could be extended to 2000mm if desired. Barlow lens gave the telescope more flexibility as I could also use longer focal length eyepieces to get decent magnification.
What is the focal ratio of a reflector telescope?
The focal ratio of a telescope is the ratio between the focal length and the diameter of the mirror. For example, a telescope with a focal length of 1000 mm and a mirror diameter of 200 mm has a focal ratio of 1000/200=5 (usually written as f/5). Telescopes with a higher focal ratio number are called slow, and telescopes with a lower focal number are called fast. Slower telescopes with a focal ratio of f/9 and above are more suitable for observing planets, the moon, and binary stars. They deliver more magnification with the same eyepiece and have a narrower field of view than fast telescopes, which are well-suited for observing deep-sky objects, such as globular clusters, open clusters, and galaxies.
The above image somewhat illustrates this difference in the field of view, using the Stellarium software. It shows the Hercules globular cluster through two telescopes of the same 200 mm aperture. Both telescopes use the same 14 mm eyepiece, but the focal ratios are f/5 and f/10. Since the focal length of the first telescope is 1000 mm, the magnification with the same eyepiece is also smaller, and the field of view is wider than the second telescope with a focal length of 2000 mm. Although these telescopes have different focal ratios, their mirrors are the same diameter and collect the same amount of light. This means that with the same eyepiece and with the same reflector diameter, the total brightness of the object in an f/10 telescope is the same as in an f/5 telescope. To see a brighter and more detailed image, a telescope with a larger aperture diameter is needed.
Why do reflector telescopes need collimation?
The telescope can lose the correct alignment between the mirrors as it is moved and set up. To get the best possible images with your reflector telescope, it should be regularly collimated. Collimating means aligning the telescope's mirrors so that the light is properly focused. The easiest way to do this is to use a laser collimator, which goes into the telescope focuser tube. By adjusting the screws of the secondary mirror holder, it should be ensured that the laser spot falls in the center of the primary mirror. After that, the primary mirror is aligned using the screws at the back of the telescope tube so that the laser beam is reflected straight back to the center of the laser collimator target. Collimation is also essential if you planning to connect the camera to a telescope to photograph celestial objects.
Astrophotography with a Newtonian reflector
Astrophotography can be an expensive hobby that requires patience and a lot of learning. This usually requires some extra equipment for the telescope, but at the beginner level is doable on a relatively low budget. My Skywatcher 200p reflector telescope came with the original equatorial mount, which is well-suited for astrophotography, so I experimented with it a few years ago. Below are some of the pictures I took back in 2016 and 2017. The first image shows the Orion Nebula, the second image shows the Triangulum Galaxy, and the third image shows the Hercules globular cluster.
My equatorial mount was not motorized and for that, I had to buy and install a small motor so that the telescope could track the rotation of the starry sky. Before taking pictures, the mount must also be polar aligned so that one of its axes is parallel to the earth's axis of rotation. In that way, you can accurately track an object on a rotating celestial sphere. The camera that I used was a Canon EOS 450D, DSLR camera which was attached to the eyepiece holder with the appropriate adapter. Photographing deep-sky objects requires multiple long-exposure shots. Images that are in RAW format contain a lot of data and are stacked together and edited to produce the final image.
Newtonian reflector in a nutshell
First made in 1668 by Sir Isaac Newton, reflector telescopes have become one of the most widely used types of telescopes. Its relatively simple construction and cost of manufacture make it accessible to many astronomy enthusiasts. The price of a telescope and also its capability is mainly determined by the diameter of its main component - the primary mirror. In the case of a Newtonian reflector, it is generally possible to get the largest possible aperture for a fixed price.
If the diameter of the primary mirror is already 10" (250mm) or larger, the telescope is usually mounted on a Dobsonian mount. Dobsonian mounts are more suitable for visual observation than astrophotography, as the specifics of their construction do not allow smooth tracking of celestial objects. As you can see in the top picture, these mounts are computerized. A computerized mount is convenient because it automatically performs star alignment and tracks the observed object. The database also contains thousands of deep sky objects that the telescope can quickly find.
Many different accessories are available for the Newtonian reflector, from eyepieces and filters to astrophoto cameras. How large a telescope is, with which kind of mount, and with what kind of accessories, depends above all on what are your intentions, and how often you are using your telescope.