Telescope Mirror
Written by Tammy Plotner
What is a telescope mirror? Who was the first to make a telescope mirror? How are they shaped and why? What is a telescope mirror commonly made of and what makes it work? Can you make one yourself or is it better to buy one? If you have questions like these, then follow along as we take a closer look at the telescope mirror…
What Is A Telescope Mirror?
At the base of a reflector telescope is a primary mirror that's used to gather and focus light. All celestial objects, including the planets, moon and Sun exert light rays that reach Earth in a parallel fashion. Because these light rays are parallel to each other, the reflector telescope's mirror has a parabolic shape to focus the light rays back to a single point on the secondary telescope mirror.
The secondary telescope mirror is flat and reflects the light to a focal plane at the side of the top of the telescope tube in the simple Newtonian reflector design.
The First Telescope Mirror
It is believed the first telescope mirror was created by Niccolò Zucchi in 1616. However, the techniques at the time were crude and the telescope mirror wasn't a perfect parabola. In 1663, James Gregory designed a reflector telescope using two concave mirrors, but it was until 1668 and Sir Isaac Newton that the art of creating a practical telescope mirror became popular.
How Is A Telescope Mirror Shaped?
The reason most telescope mirrors are shaped like a parabola is simple. Any incoming rays (be it light, radio or even sound) that are parallel can be collected and refocused at a single point. The parabola is a simple concept that dates back into the 10th century and you'll find the design in many applications.
Amphitheatres use partial parabolas to amplify sound… Lighthouses use parabolas to focus beams of light outwards… Satellite dishes collect radio signals… Even the ancients knew you could collect the Sun's rays with a parabola and use it to ignite a fire! The parabolic shaped collects and concentrates all parallel rays into a single point of focus.
Some telescope styles incorporate elliptical mirrors. In very broad terms, it focuses light along a major and minor axis. This is often combined with a hyperbolic mirror as well, since it focuses light in a three-dimensional cone. A few telescopes use a convex mirror – or diverging mirror. This is a curved mirror in which the reflective surface curves toward the light source and form a virtual image "inside" the telescope mirror. A collimated (parallel) beam of light diverges (spreads out) after reflection from a convex mirror and offers a wide, compressed field of view.
What Are Telescope Mirrors Made Of?
While glass seems like the most simple answer – it isn't quite that simple! Most telescope mirrors are made from Pyrex sheet glass both annealed and un-annealed. While it is possible to use ordinary plate glass, you must remember optical quality and that a mirror must be very thick (in most circumstances) to avoid flexure. Many early large aperture and professional telescope mirrors were created from fused quartz.
Today such new materials such as low expansion crystallized glass-ceramic and even liquid parabolas formed from mercury can be in the telescope mirror's future. Single, solid mirrors have given way to ones created in segments and better techniques have allowed thinner mirrors to become more commonplace in larger telescopes.
What makes the telescope mirror work is the reflective coating. The mirror blank is placed in a vacuum chamber and reflective materials deposited on the rear surface. However, more modern techniques involve ion chambers that can apply the reflective film at the glass substrate. Aluminum, quartz, silver… And many other "secret" ingredients go into the coating process and many manufacturers have their own patented formulae for enhanced telescope mirror coatings.
Can You Make Your Own Telescope Mirror?
The answer is yes. There are many optical glass companies which sell mirror glass blanks, including a plethora of websites, books, instructional videos and mirror grinding apparatus. If you are interested in creating your own telescope mirror, the best way to start is to begin with research. It is a painstaking process, but one that offers its own rewards.
Check your local community observatory and local astronomy clubs. Many of these also offer telescope making workshops and you just may find a mentor to help you along the way to designing and creating your own telescope mirror. If you are not interested in doing the mirror yourself, almost all reputable telescope supply companies offer pre-polished and coated telescope mirrors ready made to create your own telescope.
Filed under: Astronomy
