choosing a telescope

Very often, beginner amateur astronomers underestimate the importance of the eyepiece, considering that the main thing in the telescope is the diameter of the objective. Nevertheless, the correct choosing of the eyepiece will allow you to use the maximum capabilities of the telescope with a variety of types of observations. In this article I will talk about what eyepieces are, how they differ and which one is better to buy an eyepiece.

 The basic parameters of the eyepieces:
1. focal length
2. barrel diameter (1.25 … 3 inches)
3. field of view (from 38 to 120 degrees)
4. removal of pupil
5. eye cup type
6. optical design
7. coating
8. brand (manufacturer)

Through the eyepiece we consider the image that forms the lens of the telescope in the focal plane. To make it easier to understand, imagine two magnifying glasses, a large one as a lens, and a small one as an eyepiece.

Many newbies who have just bought a telescope immediately put the maximum magnification on it and then are surprised that nothing is visible except darkness. The fact is that some celestial objects must be observed with a large increase (planets, moon, double stars), and others with a minimum or medium (galaxies, nebulae, clusters). Remember – the higher the magnification of the telescope, the lower the brightness of the image and the worse the contrast. Putting an excess increase in the observation of the planets, you will not see anything but a blurry dull spot.

 

1. Focal length

One of the most important parameters of the eyepiece is its focal length. Usually it is indicated in the name and marking of the eyepiece (for example, Explore Scientific 11 mm 82 degrees). Here the logic is simple: less focal length of the eyepiece – more magnification of the telescope. The magnification of the telescope can be calculated by dividing the focal length of the telescope by the focal length of the eyepiece. So, if the focus of the telescope is 1000mm, and the eyepiece is 10mm, then the magnification is 100x. The focal length of the eyepieces can vary from 56 to 2 mm.

The maximum magnification of a telescope depends on its diameter of its lens and is approximately equal to 1.5 * D … 2 * D., where D is the objective diameter in mm. So, for a 150mm telescope with high-quality optics, the limiting magnification is about 300x.

There is also a minimal telescope magnification, which can be calculated by the formula D\7, where D is the objective diameter in mm. For example, a 150mm telescope has a minimum magnification of 21x. Using a smaller magnification (for example, 20x) is impractical, since the light beam from the eyepiece will have a larger diameter than the pupil of the observer, and the light will pass by the eye. However, the use of exit pupils of more than 7 mm is allowed if it is necessary to obtain a larger visible field of view. The brightness of the image will be the same as with lowest magnification, but the central part of the lens will actually work.
The size of this output beam (the so-called exit pupil) can be calculated by dividing the diameter of the telescope by magnification. For example, the exit pupil of a 300mm telescope with an increase of 100x is 3 millimeters.

Different magnifications are used to observe various celestial objects:

Magnification	Name	Exit pupil (mm)	Objects
D/5…D/7	lowest	5-7	search eyepiece, large nebulae
D\3	умеренное	3	Messier catalog objects
D\2	average	2	bright galaxies, nebulae
0.7*D	permeable	1.4	small galaxies, planetary nebulae, clusters
1*D	big	1	Moon, Sun, planetary satellites
1.4*D	resolution	0.7	details on the surface of the moon, planets, the sun
2*D	extreme	0.5	double stars, moon

As a rule, for observing practically all types of space objects, two or three eyepieces with different focal lengths and a good Barlow lens are sufficient.

In turn, eyepieces are both with a fixed focal length (the so-called “fixes”), and with variable (the so-called zoom-eyepieces). In Zoom eyepieces, the range of change of the focal length usually does not exceed three times (8-24 mm, 7-21 mm, 3-6 mm, 2-4 mm.)

2. Barrel diameter

On the telescope itself there is a special device where the eyepiece is inserted. This device is called a focuser. The barrel diameter of the eyepiece is usually indicated in inches (″). Among amateur telescopes eyepieces with diameters of 1.25″, 2″, less often 0.965″, and even less often – 3 are the most common. Accordingly, with 1.25″ focuser, you can use 1.25″ eyepieces, with 2″ focuser – both 2″ eyepieces and 1.25″ (through a special adapter). Under the 3 ″ eyepiece focusers are not so much – except that the famous Explore Scientific 30mm 100 degrees. The focus of 0.965″ is usually equipped with simple telescopes with an aperture of up to 50mm.

2″-eyepieces allow you to see a larger field of view at the same focal length. For example, a 30 mm 1.25″ eyepiece has a maximum field of view 1.6 times smaller than the 30 mm eyepiece with a 2″ size.

 

3. Field of view.

In addition, the field of view of the telescope also depends on the eyepiece. There are several types of field of view.

1) the field of view of the eyepiece is the angular size of the image visible through the eyepiece (angular size of the diaphragm).
The field of view is usually declared by the manufacturer, but in some cases the numbers may differ from the real value. The field of view of different eyepieces can vary from 38 to 120 degrees. The most common eyepieces, the Pössl system eyepieces, have a field of view of about 50 degrees. However, not always the stated field of view can correspond to the real one.

Eyepieces with a field of view from 66 to 82 degrees are also called wide-angle, from 82 to 120 degrees – super-wide-angle. The field of view is indicated either in the specifications or directly in the name of the eyepiece (for example, Explore Scientififc 24 mm 68 degrees).

 

2) the true field of view is the angular size of the portion of the sky visible through the eyepiece used with any telescope and with a corresponding magnification.
To calculate the true field of view of the telescope, it is necessary to divide the field of view of the eyepiece by magnification.
For example, the field of view of the eyepiece is 40 degrees, the magnification of the telescope with this eyepiece is 40 times. We get the true field of view 40 \ 40 = 1 degree (2 angular diameter of the moon).

 

With variable focal length eyepieces (zoom eyepieces), the field of view also changes. As a rule, at the maximum focal distance the field of view is minimal (about 40 degrees), and at the minimum focal length it is maximum (50-66 degrees). When changing the focal length of the eyepiece, you can see how the field of view of the eyepiece itself changes.

4. Eye relief

Another important parameter that newbies rarely pay attention to. The removal of the pupil is the distance from the last lens to the eye, at which the entire field of view of the eyepiece is visible. If the removal of the pupil is small (less than 10 mm) – the observations become uncomfortable, it is necessary to press the eye too tightly to the eyepiece, the eyelashes stain the lens, the eye lens sweats up, and in the cold you can also overcool the eye. The most comfortable removal of the pupil – 15-18 mm. Especially important is the large removal of the pupil for people who make observations with glasses (for example, for the correction of astigmatism).

A typical mistake for beginners is to cuddle too close to the eye lens, even if the pupil is more than 15 mm away. In this part of the field of view “falls.” Try to find a comfortable eye position and do not move it when observing from the axis.

As a rule, in the plössl \ kelner \ erfle system eyepieces, the pupil can be calculated using the forum 0.7*F, where F is the focal length of the eyepiece. We get that in the eyepiece 20 mm of one of these schemes, the pupil’s takeaway is about 14 mm, and in the eyepiece 4 mm – only 2.8 mm.

There are short-focus eyepieces with increased pupil relief (so-called Long Eye Relief). In essence, they are a combination of a long-focus eyepiece and a negative front component (something like a Barlow lens). By the way, the simble non telecentric Barlow lens also slightly increases the removal of the pupil.

 

5. The type of the eyecup.

Most of eyepieces are equipped with a special light-shielding device – the eyecup. The eyecup can be either soft (made of rubber or rubber) or hard rubber / plastic. In addition to the light shielding function, the eyecup also centers the eye so that it does not have to catch the exit pupil. Some eyepieces are not equipped with an eyecup – if you wish, you can make it yourself (for example, from soft thermal insulation for plumbing pipes).

6. Optical scheme.

For 400 years since the invention of the telescope, eyepieces have undergone significant changes. In the twentieth century with the advent of electronic computers, new methods for calculating eyepieces appeared. In addition, the glass melting technology also did not stand still. Currently, more than a few dozen different schemes are known.

Initially, a single collecting lens (Kepler eyepiece) or a single scattering lens (Galilean eyepiece) was used as the eyepiece. Now these eyepiece circuits are practically not used, except in toy telescopes and binoculars. The two-lens eyepieces of the Huygens and Ramdssen systems proved to be more perfect. They are still used in low-cost binoculars and microscopes. The marking usually indicates the letter “H” or “R”, respectively (H20, R10).

Almost every budget telescope is equipped with a three-lens eyepiece of the Kellner system. The eyepiece consists of a single lens and achromatic glue. The main advantage of this eyepiece is the low price. The oculars of the Kellner system work well with non-powerful telescopes. Kelners are marked with the letter “K” (for example, K20).

The next step is the Plossl eyepiece. The optical scheme of the eyepiece consists of 4 lenses – two glues facing each other with positive lenses. Therefore, it is also called symmetric. Marking – “PL” (PL 12.5).

Cheap wide-angle eyepieces are mainly represented by Erfle scheme. This is a five-lens eyepiece with a field of view from 60 to 90 degrees. The advantages include low cost and low F\D ratio. By cons – poor image quality across the field when using fast telescopes (f\5). Eyepiece system Erfle better to use on telescopes with low aperture. A good option to “try” wide-angle eyepieces at a low price. Personally, I myself started with such eyepieces, then switched to higher quality wide-angles. Marking – SWA, SWAN, UW, sometimes UWA.

 

7. Coating

To reduce lens flare, increase light transmission and improve image contrast, eyepiece lenses are covered with a thin film (“coating”). The simplest and cheapest eyepieces can be without any coating at all, which is not good. As a rule, the darker the glare from the eyepiece, the better the coating. The color of coating can be very different – blue, purple, green, orange, red (“ruby”). In good eyepieces glare from the lenses calm green or lilac color.

8. Brand (manufacturer).

The main brands of eyepieces:
Sky-watcher
Celestron
Meade
Baader Planetium
Long perng
Orion
Levenhuk
William Optics
Explore Scientific
Nagler

Levenhuk, Orion, William Optics, Meade are not manufacturers. They only buy a batch of eyepieces from other manufacturers (Synta \ Sky-Watcher, Long Perng, UO) and sell under their label. Often the same eyepiece can be in different packages under different brands – for example, William Optics UWAN 28 mm 82 degrees and Levenhuk Ra UWA 28 mm 82 degrees, or Celestron X-Cel LX \ Meade HD. So be careful!

The range of prices for eyepieces can be quite large – from 3-4 to 1200 dollars. It all depends on the characteristics and brand.

General tips and advice on choosing an eyepiece.

First, you should not immediately after the purchase of the telescope throw out the telescope’s complect eyepieces and run after the new expensive ones. Complete Kelners / plossl with focal lengths of 25, 10, 6.3 mm are quite good. Observe first with complete eyepieces – they are more than enough to familiarize yourself with the sky. Barlow complete lenses useless – they only degrade image quality. Good Barlow’s lens I advise you to purchase separately.
If you even decided to take a separate eyepiece, determine the focal length, field of view and price. If things are bad with the budget – take the usual plossl, but with a focal length of 10-7.5 mm. To observe the planets, use them in conjunction with a good Barlow lens.

There is a general rule: the higher the focal ratio of the telescope, the higher the requirements for the quality of the eyepiece, namely, the degree of correction of the own aberrations (distortions) of the eyepiece. For example, on the Newton’s high-aperture telescope (f \ 5), eyepieces with the same focal length, but with different optical schemes, will be shown with the same magnification, but different image quality. At the same time, on an slow telescope (for example, Maksutov-Cassegrain) and a simple, better eyepiece will show approximately the same.

If you want a wide-angle eyepiece, then the rule is this – for incompatible (f \ 7 … f \ 15) telescopes you can take inexpensive wide-angle eyepieces (such as Deepsky WA, SWA or UW – field of view 60-80 degrees). If a high-aperture telescope (f \ 4-f \ 5) – better-quality wide-angles are desirable (Explore Scientific 68-82 degrees, Televue Nagler, Televue Panoptic), and a telescope of the Newton system is also a coma corrector (GSO, Televue).