Optical Instruments s1

ECEN 4616/5616 4/5/2013

Optical Instruments

Some designs from HyperPhysics (Google it): Ramsden Eyepiece

Eyepieces Ramsden Eyepiece Orthoscopic Eyepiece Erfle Eyepiece

In the Ramsden eyepiece, the two lenses are made of the same kind of glass, e.g., spectacle crown. The lenses are of equal focal length and their separation is equal to the focal length. The field lens coincides with the first principal plane, which is convenient for the placing of crosshairs or reticules, but inconvenient in that it highlights any dust on the lens.

pg. 1 This eyepiece is effective in reducing chromatic aberration , doing a better job with longitudional achromatism than with lateral. It is quite effective in reducing spherical aberration and distortion, and removes the problem of coma. In standard use, the objective lens will form an image at the first surface of the field lens.

Orthoscopic Eyepiece

This eyepiece is characterized by its wide field of view and high magnification. It finds application in high-power telescopes and range finders. It corrects well for distortion. The triplet corrects well for chromatic aberration. ECEN 4616/5616 4/5/2013

Erfle Eyepiece

This eyepiece has two achromatic doublets, even three in some variations. Its wide field of view (up to 70¡) enhances its use in high quality astronomical telescopes.

Some raytraced designs from Wikipedia.com/eyepieces:

pg. 3 Galillean: Exit pupil is virtual

Huygens: Lens ahead first image compacts the rays and allows smaller lenses, at the expense of shorter eye relief. ECEN 4616/5616 4/5/2013

Simple Eyepiece:

Layout

4/5/2013 Total Axial Length: 48.06215 mm LENS.ZMX C o n f i g u r a t i o n 1 o f 1

Any simple lens can be an eyepiece – it is just a magnifier where the eye is placed a distance behind the lens and defines the allowable rays. It is usual to design eyepieces backwards, as above, with the “exit” pupil as the entrance pupil, and the rays converging to the image. This makes it much easier to see the flaws in the design.

The above “design” (just a stock achromat) has an obviously curved image plane. This is not a fatal flaw for an instrument designed for looking through, as the eye will accommodate changes in focus as you look around the field of view.

pg. 5 More complex eyepieces:

Layout 10X EYEPIECE 4/5/2013 Total Axial Length: 59.96940 mm C_001.ZMX C o n f i g u r a t i o n 1 o f 1

Layout modified Wild eyepiece 4/5/2013 Total Axial Length: 86.73723 mm C_007.ZMX C o n f i g u r a t i o n 1 o f 1

Don’t design eyepieces like this yourself (unless you have to) – look for designs on the web, or in databases. If you need to use a complex design in your model, many companies (Edmund Optics, for one) will send you the prescription for their products, after you sign an NDA (Non-Disclosure Agreement). ECEN 4616/5616 4/5/2013

How an eyepiece is actually used:

An objective lens added to create a telescope.

The objective creates an image of the distant object, and the eyepiece acts as a magnifier, allowing the eye to get closer to the “prime focus” than you could ordinarily achieve. Here the size of the eye’s pupil has been made the stop, and limits the amount of light hitting the objective that can make it into the eye.

pg. 7 The same design as above, but with the eye’s pupil expanded to its maximum diameter of ~7mm. The telescope’s magnification is ~10 , so the 7mm pupil can accept light from ~ 70 mm diameter of the objective. If the objective was only 40 mm diameter, the amount of light would stop increasing when the pupil’s dilation reached 4mm, and beyond that no extra light would be received. Instruments designed for use in dim light need to take the eye pupil range into account.

The previous telescopes would have an inverted image. This can be solved by using an image relay, which rights the image, but adds length to the design.

Microscope: ECEN 4616/5616 4/5/2013

The eyepiece in a microscope works exactly like the one in a telescope (they can often be interchanged). The intermediate image is now created by a short focal length lens placed close to the object. The magnification is the magnification of the real image produced by the microscope objective times the magnifier magnification of the eyepiece .

Bad design from the Internet:

The “artist” has a ray magically changing direction in mid-air at the focal plane of the objective lens. (For some reason, people tend to draw rays where they would like them to go, not where the physics says they must go.)

The simple microscope:

pg. 9 The eyepiece and objective are at opposite ends of the optics tube (usually 160 mm long). Other objectives are mounted on a turret for easy changing of the magnification. The magnification can also be changed by exchanging eyepieces of different focal lengths.