Date: December 4, 2010
Title: The High End World of Reflector Telescope Optics
Podcaster: Edgardo Molina
Organization: Pleiades. Research and Astronomical Studies A.C. www.pleiades.org.mx (web site soon to be presented also in English)
Description: Today we are talking about the reflector telescopes. Telescopes that are based on Isaac Newton’s idea of using a parabolic mirror with a highly reflective coating on the curved surface to concentrate light in the optical path of an eyepiece. These telescopes are simpler to manufacture than refractors, and although refractor were highly positioned in my last podcast, the monster telescopes nowadays worldwide are reflectors. So for this and other interesting reasons, let’s start with the reflector telescope theory before the refractor world get jealous!
Bio: Edgardo Molina. B.S. in Mechanical Engineering from the Anahuac University in Mexico City. Post graduate studies in IT Engineering and a Masters Degree in IT Engineering. Working for IPTEL, an IT firm delivering solutions to enterprises since 1998. Space exploration enthusiast who participated in several Mexican space related activities. Licensed amateur radio operator with call sign XE1XUS. Amateur astronomer since childhood and actual founder and president of the Pleiades. Research and Astronomical Studies A.C. in Mexico City, Mexico. Avid visual observer and astrophotography fan. Public reach through education in exact sciences, engineering and astronomy. Lectures and teaching in several universities since 1993.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Wayne Robertson, who encourages you to join him in supporting this great podcast.
Transcript:
The High End World of Reflector Telescope Optics
Hi, This is Edgardo Molina, from Pleiades, Research and Astronomical Studies in Mexico City, Mexico. I have the pleasure of being your host again for this episode of the 365 Days of Astronomy podcast.
My apologies for the low tone sexy voice, today I am suffering from a terrible cold. Hope I do not pass the virus along the audience!
Today we are talking about the reflector telescopes. Telescopes that are based in Isaac Newton’s idea of using a parabolic mirror with a highly reflective coating on the curved surface to concentrate light in the optical path of an eyepiece. These telescopes are simpler to manufacture than refractors, and although refractor were highly positioned in my last podcast, the monster telescopes nowadays worldwide are reflectors. So for this and other interesting reasons, let’s start with the reflector telescope theory before the refractor world get jealous!
A reflecting telescope (also called a reflector) is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a “catoptric” telescope.
Optical theory
A curved primary mirror is the reflector telescope’s basic optical element that creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or a secondary mirror may be added near the focus to modify the optical characteristics and/or redirect the light to film, digital sensors, or an eyepiece for visual observation.
The primary mirror in most modern telescopes is composed of a solid glass cylinder whose front surface has been ground to a spherical or parabolic shape. A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective first surface mirror.
Optical aberrations
Reflecting telescopes, just like any other optical system, do not produce “perfect” images. The need to image objects at distances up to infinity, view them at different wavelengths of light, along with requirement to have some way to view the image the primary mirror produces, means there is always some compromise in a reflecting telescope’s optical design.
Because the primary mirror focuses light to a common point in front of its own reflecting surface almost all reflecting telescope designs have a secondary mirror, film holder, or detector near that focal point partially obstructing the light from reaching the primary mirror. Not only does this cause some reduction in the amount of light the system collects, it also causes a loss in contrast in the image due to diffraction effects of the obstruction as well as diffraction spikes caused by most secondary support structures.
The use of mirrors avoids chromatic aberration but they produce other types of aberrations. A simple spherical mirror cannot bring light from a distant object to a common focus since the reflection of light rays striking the mirror near its edge do not converge with those that reflect from nearer the center of the mirror, a defect called spherical aberration. To avoid this problem most reflecting telescopes use parabolic shaped mirrors, a shape that can focus all the light to a common focus. Parabolic mirrors work well with objects near the center of the image they produce, (light traveling parallel to the mirror’s optical axis), but towards the edge of that same field of view they suffer from off axis aberrations:
‚ñ™ Coma – an aberration where point sources (stars) at the center of the image are focused to a point but typically appears as “comet-like” radial smudges that get worse towards the edges of the image.
‚ñ™ Field curvature – The best image plane is in general curved, which may not correspond to the detector’s shape and leads to a focus error across the field. It is sometimes corrected by a field flattening lens.
‚ñ™ Astigmatism – an azimuthal variation of focus around the aperture causing point source images off-axis to appear elliptical. Astigmatism is not usually a problem in a narrow field of view, but in a wide field image it gets rapidly worse and varies quadratically with field angle.
‚ñ™ Distortion – Distortion does not affect image quality (sharpness) but does affect object shapes. It is sometimes corrected by image processing.
There are reflecting telescope designs that use modified mirror surfaces (such as the Ritchey–Chrétien telescope) or some form of correcting lens (such as catadioptric telescopes) that correct some of these aberrations.
Use in astronomical work
Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this:
‚ñ™ In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
‚ñ™ Light of different wavelengths travels through a medium other than vacuum at different speeds. This causes chromatic aberration in uncorrected lenses and creating an aberration-free large lens is a costly process. A mirror can eliminate this problem entirely.
‚ñ™ Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or catadioptric.
‚ñ™ There are structural problems involved in manufacturing and manipulating large-aperture lenses. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter. In contrast, a mirror can be supported by the whole side opposite its reflecting face, allowing for reflecting telescope designs that can overcome gravitational sag. The largest reflector designs currently exceed 10 meters in diameter.
Reflecting telescope designs
Gregorian
Light path in a Gregorian telescope.
The Gregorian telescope, described by James Gregory in his 1663 book Optica Promota, employs a concave secondary mirror that reflects the image back through a hole in the primary mirror. This produces an upright image, useful for terrestrial observations. Some small spotting scopes are still built this way. There are several large modern telescopes that use a Gregorian configuration such as the Vatican Advanced Technology Telescope, the Magellan telescopes, the Large Binocular Telescope, and the Giant Magellan Telescope.
Newtonian
Light path in a Newtonian telescope.
The Newtonian telescope was the first successful reflecting telescope, completed by Isaac Newton in 1668. It usually has a paraboloid primary mirror but at focal ratios of f/8 or longer a spherical primary mirror can be sufficient for high visual resolution. A flat secondary mirror reflects the light to a focal plane at the side of the top of the telescope tube. It is one of the simplest and least expensive designs for a given size of primary, and is popular with amateur telescope makers as a home-build project.
The Cassegrain design and its variations
Light path in a Cassegrain telescope.
The Cassegrain telescope (sometimes called the “Classic Cassegrain”) was first published in an 1672 design attributed to Laurent Cassegrain. It has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Folding and diverging effect of the secondary creates a telescope with a long focal length while having a short tube length.
Ritchey–Chrétien
The Ritchey–Chrétien telescope, invented by George Willis Ritchey and Henri Chrétien in the early 1910s, is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of coma and spherical aberration at a nearly flat focal plane if the primary and secondary curvature are properly figured, making it well suited for wide field and photographic observations. Almost every professional reflector telescope in the world is of the Ritchey–Chrétien design.
Dall–Kirkham
The Dall‚ÄìKirkham Cassegrain telescope’s design was created by Horace Dall in 1928 and took on the name in an article published in Scientific American in 1930 following discussion between amateur astronomer Allan Kirkham and Albert G. Ingalls, the magazine editor at the time. It uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to grind than a classic Cassegrain or Ritchey‚ÄìChr√©tien system, it does not correct for off-axis coma and field curvature so the image degrades quickly off-axis. Because this is less noticeable at longer focal ratios, Dall‚ÄìKirkhams are seldom faster than f/15.
There is of course no need to mention who is who in the High End Reflector Telescope Optics because these are major leagues. Now in a down to earth comment. One can get the best refractor optics for advanced amateur use from the following manufacturers, some that also are the creme de la creme for refractive optics for the amateur telescope market.
Takahashi: Epsilon and Mewlon series. Hyperbolic and Dall-Kirkham optics
RC Optical: Ritchey-Chretien optics
Astro-Physics: Maksutov and Riccardi-Honders optics
Plane Wave Technologies: Dall-Kirkham optics
Just to mention a few. So, if one has the fever to look through the best reflector optics available for the down to earth people, it is advisable to stay single and not compromise the family economy when buying any of the telescopes mentioned before. Of course if you really have a big desire for one, get a second job or better yet, convince your boss that your income is not up to your hobby expectations.
Next time you look through a high end reflector telescope, make friends among the owners, you never know what they are going to dispose out of the window!
For the 365 Days of Astronomy Podcast, this is Edgardo Molina, signing off and wishing you clear skies throughout this winter season. Thank you for listening.
End of podcast:
365 Days of Astronomy
=====================
The 365 Days of Astronomy Podcast is produced by the Astrosphere New Media Association. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.