INSTRUMENTS

ASTRAL TELESCOPE

BASIC NOTES

page 11

OCULARS

OCULARS
Oculars or eyepieces as they are sometimes called, are relatively sophisticated magnification lenses. The objective of a telescope forms an image sufficiently to permit examination of detail.
The ocular consist of a series of lenses (2 elements in a simple ocular, up to 4 elements in the more sophisticated oculars), mounted in a metal casing suitable for insertion in the ocular  holder of a telescope. While it is the telescope that determines the resolution and the quality of the detail in the image, the ocular remains a key element in the optic system of the telescope.
Use of the correct ocular allows the observer to note a level of detail that would otherwise have seemed impossible to him. Oculars are available in varying diameters and according to different optic schemes, according to the interest or means of the observer.

DIAMETERS
Standard diameters are 24.5 mm, 31.8 mm and 50.8 mm. The first two are used mainly in amateur instruments, while use of the largest diameter makes sense only in larger instruments and therefore may be considered professional.
The rule that the greater the diameter of the ocular, the greater will be the apparent field hold true. It is worth nothing that in accordance with the focal , a noticeable increase in quality corresponds to the increase in size.

FOCAL LENGTH
Oculars are available in focal lengths ranging from 4 mm to 50 mm for highest to lowest levels of magnification.

OCULARS SCHEMES
A wide range of oculars are available, to suit all types of observation and all price-ranges:

Kellner (K) A scheme with three lenses, quite suitable for telescopes with a mid to high focal ratio. The Kellner ocular gives good quality images and low to mid-level magnification..

Orthoscopic (OR) The orthoscopic scheme with four elements gives high quality images in a range of focal that goes from 4 mm to 18 mm and with some increase in the field of vision compared with Kellner or MA types. Orthoscopic oculars are advised for general use with all types of telescopes

Huygens (H)
This has two lenses with reduced aberration and the chromatic correction is good. I is suitable for sun observation. The simplest and most economic types give best results with a focal ratio of F10 and above during sun observation.
EYE RELIEF
The eye relief is defined as the distance between the surface of the lens of an ocular and an observer's eye when the observer is positioned in order to encompass the entire field of view. A high level of eye relief is particularly recommended for those who wear glasses. Oculars with a short focal generally have a lower level of eye relief than those with a short focal.
For those who wear glasses to correct presbyopia (longsightedness) or myopia (shortsightedness) can remove the glasses when using the telescope, without losing any of the image's resolution. It will only be necessary to get the image into focus. Those suffering from astigmatism, a defect in eye symmetry, particularly those in whom the condition is pronounced, will need to wear their glasses, when using the telescope. Otherwise the image will appear deformed.
A reasonably high level of eye relief is recommended, as otherwise the eye will be placed too close to the lens, thus running the risk of eye fluid staining the lens.
APPARENT FIELD
The Apparent Field of View of an ocular is the angle (in degrees) threat the diaphragm of the field of the ocular holds against the eye, when the ocular is fitted to the eye. The apparent field of an ocular therefore becomes wide in reality when the ocular is inserted into the telescope.