Optical Elements of the Schmidt-Cassegrain Telescope
A telescope is a group of optical elements that collects light and focuses it for observation by an eyepiece or some other imaging device. There are two types of optical elements: mirrors and lenses. Mirrors reflect light and lenses refract, or bend light. The Schmidt-Cassegrain telescope uses both mirrors and lenses. The diagram below shows a cross-section of a Schmidt-Cassegrain. In this telescope, light first passes through the corrector lens, and then reflects off the primary mirror. Finally, it reflects off the secondary mirror and comes to a focus at the focal plane.
Optical Coatings
The purpose of a telescope is to collect as much light as possible. The amount of light collected affects the brightness of the resulting image. Unfortunately, there are sources of light loss at each optical surface, and within each lens. Fortunately, we can design optical coatings and choose lens materials that minimize the amount of light lost to these sources.
Optical coatings are very thin layers of material that are applied to the glass in a process called 'vacuum deposition'. The physical properties and thickness of each layer in the coating, as well as their orientation with each other and the glass to which they are applied, determine how well they will do their job.
Since the function of a mirror is to collect light by way of reflection, we use highly reflective metallic coatings on these optical elements. A mirror without coatings reflects about 4% of the light that hits its surface. A mirror coated with standard Aluminum coatings reflects about 86 - 88%, and a mirror coated with StarBright XLT reflects 95%.
Light traveling through a lens is a little more complicated. In this case, light is lost to both reflection and absorption. When light first strikes an uncoated lens, about 4% is reflected back and never has the chance to make it through. Some of the remaining 96% will be absorbed on its way through the glass, and then the second lens surface reflects another 4%. To minimize unwanted reflection, dielectric materials are used in pairs of alternating high and low refractive index. A good anti-reflection (A/R) coating for telescope lenses is one that will deliver very low, very 'flat' reflectance across the entire visible spectrum.
Although A/R coatings can dramatically reduce
the amount of light lost to reflection, no optical coating
can reduce the amount of light lost to absorption within the
glass. To reduce this source of light loss, it is important
to choose a glass that absorbs as little light as possible.
For many A/R coating applications, it is standard to measure the reflection of the coated surface and to ignore the amount of light that is being absorbed by the glass. But for a telescope lens, stating how well an A/R coating suppresses reflection without also revealing how much light is lost to absorption within the glass can be quite misleading. For this application, actual transmission, which accounts for light lost to both sources, should be measured directly. You can learn more about how we did these measurements in the section titled Our Measurements.
Telescope System Transmission
System transmission is the percentage of light that arrives
at the focal plane compared to the light that enters the
telescope, and is calculated by taking the product of the
corrector lens transmission, the primary mirror reflectance,
and the secondary mirror reflectance. Here is an example;
if the corrector lens transmits 92% of the light, and the
primary and secondary each reflect 89% of the light, then:
Total System Transmission = .92 * .89 * .89
= .73 (73%) |
