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A Ramsden eyepiece is the simplest example of an eyepiece for a high quality optical instrument, containing two identical plano-convex lenses. The Ramsden eyepiece is one of several eyepiece options when a single simple magnifying lens is inadequate due to aberrations. It is particularly common in instruments which have a scale or micrometer wires.
If you want to know more about the use and history of the Ramsden eyepiece, then this article can help. Read on to find out how a Ramsden eyepiece works, the pros and cons of using this Eyepiece a few other related queries about Ramsden eyepieces
How Does A Ramsden Eyepiece Work?
Table of Contents
Eyepieces are a fundamental part of complex optical instruments, magnifying the real images formed within the optical system. Eyepiece design impacts magnification, field flatness, image brightness and aberration.
Aberrations are properties of optical systems that cause deviation of light rays passing through lenses, blurring or or distorting the image due to light spreading out rather than focusing to a point.
In an ideal optical system, every point on an object focuses to a sharp point on the image. Different types of eyepieces (huygens, plossl, aspheric etc.)can minimize or eliminate different types of aberration.
The Ramsden eyepiece consists of two lenses made of identical material and possessing equal focal length. The lenses sit with their convex faces pointing towards one another other, at a separation distance of around two thirds of the focal length of the lenses.
A Ramsden eyepiece reduces chromatic aberration, by spacing the lens pair in a way that minimizes dependence on the index of refraction. It is more effective in tackling longitudinal achromatism than lateral.
In standard use of a Ramsden eyepiece, the objective lens forms the primary image in front of the field lens. Micrometer wires or a scale can be placed at this point for measurement purposes.
Who Created The Ramsden Eyepiece?
The Ramsden eyepiece for use in telescopes and microscopes was invented in the late eighteenth century by Jesse Ramsden, a renowned British mathematician and optical instrument maker, who was a Fellow of both the Royal Society and the Royal Society of Edinburgh.
Ramsden was a pioneer in the early invention and production of precision instruments, also designing highly accurate sextants and theodolites (instruments for measuring angles in navigation / astronomy, surveying).
The accuracy of his instruments was based on a novel “dividing engine” device of his own invention that allowed him to designate and mark scales with greater accuracy than could be done by hand.
The Pros Of A Ramsden Eyepiece
The Ramsden eyepiece is not the only eyepiece available for use with telescopes, microscopes and other optical devices. Alternative eyepiece options include Huygens, Kellner and Nagler eyepieces.
Price
Ramsden eyepieces are cheap in comparison to some modern more advanced eyepiece options (e.g. the Nagler eyepiece).
Size
Compared to the Nagler eyepiece and several others, Ramsden eyepieces are comparatively small and light, making them a good choice for amateur devices used in a domestic setting. The weight and price of some other eyepieces can make them impractical outside a professional setting.
Reduces some distortions
A Ramsden eyepiece is relatively effective for its low price point, removing an aberration known as coma and reducing spherical aberration and distortion.
Good with monochromatic light source devices
Ramsden eyepieces only have a small chromatic aberration and are a good option for use with polarimeters or other instruments which use near-monochromatic light sources.
The Cons Of A Ramsden Eyepiece
While the Ramsden eyepiece was high tech in its day and an improvement on earlier eyepieces in several respects, it is not a sophisticated lens system by modern standards.
Narrow field of view
The field of view with a Ramsden eyepiece might typically only be 35 degrees. This can be a major disadvantage for astronomy enthusiasts hoping for a broad view of the night sky through their telescope.
Short eye relief
Eye relief distance is an important concept when using an optical device, particularly for glasses wearers and shooters. Eye relief is the distance between eye and eyepiece at the point where you can first see the maximum field of view.
Those who wear glasses are generally further from the eyepiece and would need a longer eye relief to see the entire field of view.
While offering better relief than the Huygens eyepiece, Ramsden eyepieces still have comparatively short eye relief and may be less useful or comfortable for users who rely on glasses.
Visible dust
In a Ramsden eyepiece optical system any dust on either surface of the field lens is highly visible and liable to irritate the observer. While the appearance of dust may be minimized when using a telescope to look up at a dark night sky, it would be more noticeable if the telescope were used on land areas during daylight rather than the sky at night.
Aberrations
Ramsden eyepieces are not good for tackling lateral achromatism. Some other aberrations are only minimized, not eliminated.
Why Is A Ramsden Eyepiece Used In A Spectrometer?
Studying the spectrum of light emitted by celestial bodies can give us useful information about their distance, movement and nature. A spectrometer device allows us to capture and analyze this data.
The spectrum of a star, planet, black hole etc.. can be thought of as a kind of chart or a graph showing the intensity of light emitted by that body over a range of energies.
Ramsden eyepieces may be used in a spectrometer for ease of measurement. This is because the primary image is formed in front of the field lens in a Ramsden eyepiece. Cross-wires and/or a measurement scale can be readily placed at this point inside the eyepiece.
The Ramsden eyepiece also reduces chromatic aberration more than some other eyepiece options, making it a good choice for a spectrometer where viewing light color distinctions is important.
Summary
The Ramsden eyepiece is a classic optical instrument part, consisting of two convex lenses in a simple arrangement. While not an advanced eye-piece by modern standards, the Ramsden eyepiece is simple, small and cheap, making it a good choice for reliable entry-level telescopes, microscopes and other devices.
Ramsden eyepieces can readily be used with a scale of measurement or micrometer and is often found in spectrometers.
In regards to other great eyepieces for a telescope you main own, check this article out!
References
https://digitalcommons.unl.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1067&context=physicsrudd
https://spie.org/publications/pm92_1121_ramsden_eyepiece?SSO=1
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/eyepiece.html
https://www.sciencedirect.com/topics/physics-and-astronomy/eyepieces
http://salfordphysics.com/gsmcdonald/pp/PPLATOResources/h-flap/p6_4t.pdf
