Not everyone knows what an eyepiece is. A few decades ago, ordinary fans of astronomical research did not even think about which option to enlarge the image to choose, they dreamed of any telescope. The real chic at that time was to get the factory lens and eyepiece.
Today, dramatic changes have taken place in the current situation, and now astronomy lovers pay attention not only to the telescope, but also to the optics installed in it.
Landing diameter
It is necessary to consider the main characteristics of the instruments used for astronomical research.
In order to understand what an eyepiece is, it will be necessary to isolate such a parameter as the landing diameter. It has standard indicators corresponding to the inner diameter of the telescope focuser. This characteristic is indicated in inches. Currently, a tube with two-inch eyepieces gives an insignificant increase, but differs in a good field of view, which can be considered its main advantage. The manufacture of such eyepieces is an expensive pleasure, so only wealthy people can afford to purchase such optics.
Features of selection
Depending on the inner diameter of the focuser, the magnifying eyepiece is used differently. For example, with an inner diameter of 1.25 inches, it can be used with 1.25 "eyepieces. With a two-inch focuser diameter, it is suitable for use as two-inch eyepieces. A two-inch telescope focusing device gives its owner the freedom to choose eyepieces.
A rather rare occurrence is the use by astronomy enthusiasts of eyepieces with a landing diameter of 0.965 ". This standard was in demand in the mid-twentieth century, but is not used in the manufacture of modern telescopes. When discussing what an eyepiece is, it should be noted that a device with such a landing diameter is the minimum field of view, so in our time it is not used even by amateurs.
Device increase
The focal length of the eyepiece is one of its most important parameters. Indicate this value in millimeters. With its help, you can determine the increase in the eyepiece of the microscope. For example, if the telescope has a focal length of 1200 mm and an eyepiece size of 10 mm. To determine the magnification, 1200 mm is divided by 10 mm, 120 is obtained. Currently, telescopes are offered to buyers, in which the magnification of the eyepiece is an impressive magnitude, and the focal length is in the range from 56 mm to 2.5 mm.
line of sight
Having found out what an eyepiece is, we can continue the analysis of the main characteristics. The field of view is the angular distance between the visible parameters. Depending on the optics scheme, the field of view of the eyepieces differs significantly.
Currently, manufacturers offer a lens and an eyepiece having a field of view in the range of 35 ° -100 °.
Modern tendencies
In the last decade, astronomy fans have been using eyepieces with a large rhenium field (from 68 °).
Considering that the main purpose of the eyepiece is to observe stellar fields, nebulae, clusters of stars, with a large field of view the observer has the effect of personal presence. The astronomer is convinced that, holding out his hand, he will be able to put the whole universe in his palm.
In addition, among the advantages of eyepieces with a wide viewing angle, professionals note the convenience of observing. Such telescopes are not equipped with an additional clock drive and mount. The observed object will be further in the field of view of the eyepiece, so there will be no need to constantly move the tube in the field of view of the optical device.
How many times the eyepiece increases directly determines the possibilities of astronomical calculations.
Allocate a real (true) field of view, which implies an overview of the entire telescope system, including the eyepiece.
Exit pupil features
By it is meant the distance from the outer lens of the eyepiece (ophthalmic) to a point located on the main optical axis. The eye is located in it so that you can see the entire field of view. Convenience of observation will directly depend on the removal of the pupil. For example, in the case of using the eyepiece at a very small distance from the lens, discomfort appears. Observations in the winter season can lead to significant damage to the ocular cornea, and even to frostbite. If eyelashes press against the lens of the eyepiece, traces may remain on the antireflective coating.
There is a direct relationship between pupil extension and focal length. Given that there is a similar problem, engineers create such schemes in which the exit pupil is located at a completely comfortable distance.
Such models have a fixed pupil extension, which is independent of the focal length parameters.
Excessive removal is also characterized by certain disadvantages. Its magnitude for a long-focus eyepiece is 30-40 mm, so the observer will have to “catch” the image with an eye. Currently, the most convenient to use are called telescopes in which the magnitude of the upper limit of the exit pupil is 25 mm.
For amateur astronomers with visual impairments, the developers of this technique recommend choosing telescopes with eyepieces in which the pupil spacing is 20 mm.
Eyepiece Characteristics
Analyze the main quantitative (connecting) characteristics of the eyepiece:
- focal length;
- removal of the exit pupil;
- fit size.
For example, to determine the focal length, a mathematical formula is used:
G = f'ob / f'ok,
here f'ob is the focal length of the taken lens.
The unit of measurement for the exit pupil is mm. The fit size is determined at 1.25 "or 2".
Quality characteristics
Among the important factors that affect the quality of the resulting image, it should be noted:
- Light scattering level. With its high rate, wide halos appear around the stars in question, the background brightness increases, and the image contrast decreases. The degree of enlightenment of the coating directly depends on the mechanical details and the quality of the polishing of the coating.
- Blocking occurs with incorrect calculations, when using low-quality antireflection coatings. Concentrated glare and streams of light are located around bright objects that do not fall into the field of view.
- The design of the eyecup should completely cut off extraneous illumination, fix the position of the eye in a certain position relative to the ocular lens of the eyepiece.
- Reliable mechanics, including smooth switching of the zoom ring, the installation of a safety recess for fixing the eyepiece in the focuser.
- Weight and dimensions.
Residual aberration of the modern eyepiece
The clarity of the image and the ability to observe small details in the starry sky depend on the quality of the aberration correction. Professionals consider the curvature of the visual field, chromatism, and also astigmatism to be the reasons for lowering the quality of the picture.
All eyepieces are characterized by field astigmatism, therefore, to the edge of the field of observation, the stars become dashes or crosses.
When drawing images of stars in a line that is directed from the center of observation, there is meridional astigmatism. With a perpendicular arrangement - the sagittal version.
For eyepieces, field aberration is also characteristic, which leads to the focusing of the object in the center of the field of view, and at the edges it looks defocused. Attempting to change focus instantly results in a change in focus of the center of the field of view.
It grows slightly to the edge of the picture, and in combination with astigmatism leads to the appearance of oval aberration spots. The result of chromaticity of magnification is the stretching of the star as it moves away from the center of the field of view into the chromatic spectrum, the center of which has a blue color.
Distortion to some extent causes a distortion in the appearance of the object, without making changes in the contrast and detail of the picture. For example, the planet’s disk is compressed or extended closer to the edges of vision. Perhaps this kind can be considered one of the most harmless aberrations of the eyepieces. But when conducting mathematical calculations of the real and theoretical field of angular vision, significant differences will appear.
Eyepieces that are serviceable with respect to distortion are called orthoscopic. Spherical aberration causes the appearance of symmetrical radially halos around one star. Many modern telescopes combine astigmatism and coma in the field of view. Between the center and the edge there is a dependence of the sign and size of the coma on the wavelength. The most unpleasant thing is that two aberrations can function simultaneously, and together with lens defects.
Conclusion
At present, there is growing interest among lovers of atronomical observations in telescopes. If in the twentieth century the main task of fans of the starry sky was to acquire a telescope of any size and type, then in the twenty-first century the situation changed radically. Now astronomy lovers can choose the highest quality optics, which is a guarantee of quality research.