The Reflecting Telescope
My first recollection of a telescope as a child was the standard refractor that you look through at one end and point the other end at the night sky. For many this is the type of telescope that pops into their head when they think about a telescopes. The interesting thing is that hardly any of these sorts of telescopes are the type that you’ll find on the mountain tops of Chile or in Hawaii. On telescope forums there are lengthy debates about the virtues of apochromats versus achromats versus super achromats while the quite achiever of the telescope world, the reflector, just keeps on collecting photons from deep space. I’m not saying one type is better than the other, both have their special place in astronomy and the refractor has been the mainstay of amateur astronomy for centuries — but the newcomer, since the late 1800s, is the reflector.
The reflector has become very popular in amateur astronomy in recent years mainly owing to the work of John Dobson who designed a cheap and sturdy mount that suddenly made a large telescope affordable, the Dobsonian. Many people also grind their own mirrors for these telescopes which is a huge task and takes quite a bit of patience and skill.
In this article we’ll take a reflector apart and see what’s inside as well as see how they work.
Basically they are a big bucket for collecting light. Newton recognised the value in using a mirror instead of using a lens to to examine the night sky back in 1672, and the design now carries his name. The basic Newtonian relfector that he developed is pretty much unchanged in its fundamental construction though there are many modern variations and the design did spawn a whole family of reflecting telescopes that improved on the original design.
The basic Newtonian has four parts, the tube, the main mirror, the secondary mirror and the focuser — that’s all. These telescopes are the monsters of the telescope world, you can get a lot of aperture for your money (how fat the tube is and therefore how much light the telescope can collect). Basically the more light you can collect the fainter the stuff you can see, like distant galaxies, details in nebulae and rich star fields.
The mirror is a parabolic shape that brings the light that it reflects to a point that is in inside the focuser — the secondary mirror is used to direct the light out of the tube into the focuser.
The primary mirror is a large piece of glass held in place at the bottom of the tube by a mirror cell. The glass has a highly reflective material on the top surface and, depending how much you pay, some coatings to protect the mirror.
The mirror is held in place by three locking screws and three collimation screws (more for the larger telescopes). These screws allow you to adjust the angle that the mirror is pointing down the tube. If it’s not perfectly pointing at the centre of the secondary mirror then the telescope will not be working to its optimum. This is one of the challenges with this type of telescope, they often need readjusting to keep the mirrors lined up.
As you might expect, primary mirrors get dirty after a while, but they can get quite dirty before they become bad enough to require cleaning. Cleaning the primary is not for the faint hearted and it’s not something you want to do very often as you need to be very careful as the coatings and mirror surface are easily scratched and are very expensive to resurface. One of my reflectors I’ve had for over ten years and I’ve only cleaned the primary mirror once, very carefully.
The secondary mirror is held in place by four pieces of metal called spider vanes, these are the cause of diffraction spikes that you often see in picture of the night sky, the four little lines that look like a cross on top of the stars. The secondary mirror is flat and is placed at a 45 degree angle to direct the light from the primary mirror into the focuser.
Just like the primary mirror the secondary mirror has to be lined up with the primary and the focuser — or else the light will not be directed through the eyepiece and you won’t get to see what you want to look at. Usually the secondary mirror position is held by a central screw and three adjustment screws that control the angle. I’ve found on my reflectors that I hardly ever have to readjust these, its normally the position of the primary mirror that requires the most attention.
Like the primary mirror the secondary does get a bit dirty after a while and care must be taken if you decide to clean it — or else you might have to buy a new one. The good news is that because secondary mirrors are flat they don’t cost too much to replace — I’ve had to replace one and it was still a couple of hundred of dollars.
The focuser is the bit that you look through and, unlike the refractor, it’s near the top of the telescope at the same end that the secondary mirror is. The focuser holds the eyepiece and it’s the eyepiece that determines the magnification of what you are looking at (along with the focal length of the telescope itself). The cheaper reflectors will have a rack and pinion focuser whereas the dearer ones will have a Crayford focuser. The rack and pinion has a long row of teeth on the focuser tube and the Crayford has a smooth tube. The other thing to watch is that the cheaper reflectors will have plastic components on the focuser rather than metal.
Reflectors don’t suffer from chromatic aberration that some refracters have, which is the colour fringing you can get. Reflectors do have their own aberrations and the main one being coma, which makes stars around the edge of what you are looking at look a bit pinched.
That’s the basics of the reflecting telescope, the Newtonian version.