The eyepiece is a magnifier. It enlarges the image formed by the objective or speculum. There are positive and negative eyepieces. The focus of the positive is in front of the field lens. As it is outside of the eyepiece, this type can be used as an ordinary magnifier, to enlarge objects placed in front of it in its focal plane. The focal plane of a negative ocular lies between the two lenses, so it cannot well be used to enlarge anything but optical images, which pass into it through the field lens.
[Note: Mr. Pierce considers the Huyghenian eyepiece to be a "negative" ocular which it really isn't as the image is still inverted unlike a true negative lens would be in such a situation. The Huyghenian eyepiece uses the fieldlens inside of the focus of the objective to focus the light to a shorter point and then looks at that virtual image with the eyelens. Thus, it is the only eyepiece design with the field stop inside the lenses. Bob May]
The power of an eyepiece, F, is shown by its EFL or equivalent focal length. This is the focal length of a simple lens giving equal power. The simplest eyepiece is a single lens. A plano-convex is the best, as its aberration is less than a double convex lens. The curved surface is placed next to the eye. A single lens gives a very brilliant image, but only the center of the field is clearly defined, the edges being somewhat hazy. Stars near the edge of the field also show considerable color, one side of the image being red and the other side blue. The eye must be held some distance away from the lens for clear sight.
The Ramsden positive ocular is recommended for use in telescopes having a focal ratio of F10 or less. (Focal ratio = Focal length of Objective divided by Diameter of Objective) Fig. 7 shows a Ramsden in a simple wooden mount with the lenses held in place by wads of chewing gum. If care is taken to "square on" the lenses when seating them in the mount, this eyepiece will give very good results. The Ramsden eyepiece consists of two plano convex lenses of the same focal length with their convex surfaces adjacent. Focal length of the lenses is 5/4 F, the separation of the lenses is 3/4 F, aperture of the field lens is 3/4 F, aperture of eye lens is 3/8 F, F being the desired final focal length of the eyepiece.
Fig. 8 shows a standard Ramsden ocular in section. The lenses are crimped int brass cells that screw into the tube. The cap is a protection to the eye lens and helps to center the eye. It can also be used to hold a piece of dark or colored glass when looking at bright objects When the cells are turned in the lathe, a thin edge of brass is left as shown in Fig. 8. After the brass pieces are blackened, this is crimped to hold the lens in place. Screw the cell into the tube, put into the lathe, insert the lens and hold it in place with the finger while the lathe slowly rotates, and turn down or "spin" the lip by pressing it with a smooth piece of steel while it turns. Be careful about scratches on the glass. Be sure that the lens is securely crimped into the cell. Brass may be blackened by heating it red hot and dipping into a saturated solution of copper nitrate, after which it is reheated. Repeat if necessary.
A mount which can be made without a lathe is shown in Fig. 9. It is made from brass tubing, which can be purchased, and washers, which can be drilled and cut from sheet brass or zinc with tin shears. A hack saw, file, soldering kit, and hand drill are needed. While this mount is shown holding the optical parts of a Hughenian ocular, it is just as suitable for the Ramsden type.
The Kellner eyepiece is essentially a Ramsden with an achromatic doublet replacing the plano-convex eye lens. It gives good color correction at the expense of good definition. The excellent magnifiers of Dr. Hastings and Steinheil make good eyepieces of limited range. [Note: The Kellner eyepiece also gives a wider field of view than the Ramsden as the color error is very low and thus the errors of field curvature and astigmatism come more into play at the edges of the image which are further from the center than the Ramsden type. I think that the complaint of less definition is mostly due to the additional surfaces causing less light to pass through. Bob May] The orthoscopic type has a double or triple cemented element with a simple lens close to it, next to the eye. It has a long eye distance [eye relief Bob May] making it suitable for high powers with the astronomical telescope or with low powers on a rifle telescopic sights.
All of these complicated eyepieces are expensive and require a refinement in design and construction far beyond the technical and manipulative skills of most of us amateurs. Conrady in his Applied Optics, Vol. 1, however, makes a suggestion that we may use: by making the focal length of the field lens 1 1/4 times that of the eye lens, and spacing them at .7 the focal length of the eyelens apart, he improves the flatness of the field considerably. Color correction is also excellent.
The Huygenian, a negative ocular, is best for long focus telescopes. The two lenses are of different focal lengths, and are place with their curved surfaces turned away from the eye. A diaphragm or stop is placed between them in the focal plane of the eye lens. This stop cuts off stray light and limits the field. In the eyepiece shown in Fig. 9, the lenses are held between washers which are separated by the lengths of the smaller tube. All are held together in the eyepiece by the retainer ring, which is pinned, soldered, or held in place with friction. The stop is pushed until it is in focus when viewed through the eye lens. For a Huygenian ocular, have the eye lens focal length = 3/4 F, with an aperture of 3/8 F. The field lens has a focal length of 1 1/2 F, with an aperture of F. Separation is 9/8 F.
Fig. 12 is a terrestrial eyepiece for a refractor. It shows erect images with such a telescope. It is of little use with a reflector where the image may be at almost any angle, depending upon the position of the plane of the diagonal prism or mirror and the horizon. Actually, with practice in observing, you soon become so used to seeing things upside down that the brain rights them unconsciously. The Newtonian reflecting system with an astronomical eyepiece shows objects erect if the observer stands with his back to the scenery with the telescope horizontal and the eyepiece on top, and so observes.
Lenses can be made as described in Hobbygraf #1 or may be purchased. If our cells are to be screwed into the tube, get 1 1/4" outside diameter brass tube with 1/16" wall; if as in Fig. 9 get both 1 1/4" and 1 3/16" tube with 1/32" wall. A 1 5/16" tube of this gauge is right for the is right for the eyepiece tube of the telescope as it takes the 1 1/4" eyepiece.
Many refinements in eyepieces have been worked out to suit special instruments and optical needs. The use of coated lenses has reduce losses due to internal reflections and has eliminated the "ghosts" cause by these reflections in certain types of ocular. Many of these eyepieces are available from firms dealing in war surplus materials. These are usually for special purposes and do not give the same excellence of performance when used with our astronomical telescopes.
More data on eyepieces may be found in the following references:
| Pierce Hobbygraf #1 | Lens Making |
| Hobbygraf #3 | Achromatism |
| Hobbygraf #13 | RFT and RFT eyepiece design |
| Hobbygraf #14 | The Barlow Lens |
| Ingalls Amateur Telescope Making, | pub. by Scientific American [now published by Willman-Bell in organized form. Bob May] |
| Amateur Telescope Making, | pub. by Scientific American [ibid.] |
| Scientific American Magazine | June 1938 Telescope page. Overcorrecting a speculum to compensate for eyepiece aberrations |
| Bell The Telescope, McGraw Hill | Chap. 6 - Eyepieces. |
| Baker Thick Lens Optics, Van Norstand | Simple treatment but good. |
| Southall Mirrors Prisms and Lenses, McMillan. | Very interesting development of fundamentals. Practical computations. |