Nothing works visually without eyepieces. But which eyepieces fit to my telescope – and how many do I need actually?

Did you know that the French astronomer Adrien Auzout is said to have said that he wanted to build a telescope with a length of 300 meters because he hoped that the magnification would then be so high that he could see animals on the moon with it?? In 17. In the nineteenth century it was common to worry about telescopes with high magnification.

From this you can see that there has always been a desire to experience the objects in space in a larger size.

Every day we meet them, hear about them and pity them. People who have chosen the wrong eyepieces. But with this strategy, you’ll make the right decision right away.

This article is for you, if you have already asked yourself how to find the right eyepieces as easily and immediately as possible, without reading masses of books or getting lost in a jumble of theory and formulas.

Because once you have remembered these things, you can find the right eyepieces for any telescope.

#### This is how you calculate magnification:

Focal length of the telescope/focal length of the eyepiece

Eyepieces are like lenses, they magnify the image produced by the telescope and provide us with a viewing experience.

#### Basis 1: The big ones, the small ones and what to use them for

With the eyepiece sizes you must pay attention only to little. Eyepieces are only available in two sizes, which are standardized for astronomical telescopes. This means: You only have to put the eyepieces into your focuser and you are done!

But is it really that simple?

**The thick ones with the wow effect**

Eyepiece diameters are given in inches, not usually millimeters. The big 2" Eyepieces are 50.8mm in diameter and provide a wonderful view at low magnifications. We choose them when we want to seek out objects, observe extended objects, or enjoy a huge overview. When you look through such an eyepiece for the first time Attention: Normally only telescopes from 150-200mm objective diameter have an eyepiece focuser for 2" Eyepieces.

**The slim ones**

The smaller 1.25" Eyepieces with 31.7mm diameter are the standard and simple designs are usually included in the accessories of the telescope. 1,25" eyepieces are used for medium and high magnifications and are useful when observing lunar craters, planets or globular clusters.

Each object needs different magnifications, but which one makes sense for which object?

#### Basis 2: Three magnifications to see everything

Pay attention to few good eyepieces in the beginning, with about 3 to 4 eyepieces you can get a good start, as long as you choose a small, a medium and a slightly higher magnification. As a rule, you can use them to cover the entire spectrum of astronomical objects. Better take three very good eyepieces with which you get sharp images and a good contrast, instead of seven mediocre bad eyepieces.

#### Basis 3: Why the exit pupil is so important

The exit pupil is the bundle of rays coming out of the eyepiece into the eye. You will see it mostly as a small bright circle in the eyepiece, if you look at it from thirty centimeters distance. The exit pupil, simply called AP for short, becomes an important parameter for us when we want to calculate which eyepieces we need for which object.

And this is how you do it: **AP =opening of the telescope/magnification**

The larger the AP, the smaller the magnification.

And vice versa: the smaller the AP, the higher the magnification.

We will still need the AP, keep it in mind.

#### Basis 4: minimum, optimum and maximum magnification

You don’t need more than these three sizes. Why? Because it allows you to cover a wide range of magnifications and see everything. With more eyepieces you only refine these ranges.

**The minimum**

A small magnification is more important than a high one. But there is a lower limit below which observation is not useful. It is the minimum magnification. For them, choose an eyepiece with the longest possible focal length. If you want a 2" If you have an eyepiece focuser, use a 2" telescope Eyepiece one, which usually provides a large field of view. But they have a smaller focuser? Then take a 1.25" eyepiece.

To calculate the min. magnification: **Öff aperture of the telescope in mm/7**

Let us take a look at an example. If you have a telescope with 200mm aperture, then the minimum magnification is 28x. The combination with the eyepiece produces an exit pupil (AP) of 7mm.

This is the diameter of the light that enters our eye from the eyepiece. Important: Seven millimeters is exactly the maximum aperture of the pupil of the human eye. With a larger AP, our pupil would be an aperture and other light would be lost.

##### Tip 1:

You do not have to choose an eyepiece for your telescope that gives exactly the minimum magnification. It is sufficient if you take this as a guide and choose an eyepiece with low magnification.

**Tip 2:**Always search for an object with this eyepiece, because a small magnification offers you a large field. With a wide angle eyepiece you can extend your field of view even more. Small magnifications are also suitable for galaxies, open star clusters and hydrogen nebulae.

**The optimal magnification**

At a medium to higher magnification, at which the theoretical resolving power of the telescope is reached and utilized, we speak of the optimal or conducive magnification. We reach it when a light bundle of 0.7 to 0.8mm diameter passes through the eyepiece. By definition the star is then a minimal small slice.

If we magnify higher, we do not gain any more details, but only object size.

**Optimal magnification:aperture/0.8**

A telescope with a diameter of 200mm has a useful magnification of 285x. You can use this as a guide. It is well suited for planets or planetary nebulae.

**The maximum magnification**

This is where opinions often differ. How high may or should we magnify? Let’s say it again about the exit pupil: if it is 0.5mm, we reach the magnification limit of a telescope.

##### The rule of thumb is: objective aperture x 2

A 200mm telescope would reach maximum magnification at 400x. But in practice this makes only very seldom sense. The image is darker than at low magnification, suitable only for bright objects. And the air rest would have to be perfect. Until these perfect conditions the eyepiece usually stays in the case.

Now we are ready to find the right eyepieces.

#### Why focal length and aperture are important

To choose the right eyepieces we need one more thing: the aperture and the focal length of the telescope. From these two values the aperture ratio is calculated. Example: A 200mm telescope with 1.000mm focal length has an aperture ratio of f/5.

The calculation of the different magnifications – and in this case also the focal lengths – works ingeniously simple:

For the minimum focal length calculate 7 x f/5. In the 200mm f/5 telescope this would be an eyepiece with 35mm focal length. In practice, almost no one can reach a pupil opening of 7mm, so it is better to subtract 1-2mm for your focal length. Then we end up with 33mm and an AP of 6.6mm.

Calculate the optimum focal length… Wait, you don’t have to calculate it at all, because it corresponds exactly to the aperture ratio. So at f/5 equals 5mm.

The maximum magnification can be found with the simple formula:focal ratio/2. Here we are at 200/1.000mm telescope at 2.5mm eyepiece focal length.

#### Which eyepieces for which object?

It is important to know what you can observe with a certain focal length. For large nebulae small magnifications with 7 – 6mm are suitable, if the nebula is very bright also 4 – 3,5mm are possible. Open star clusters and galaxies you like to observe between 3.5mm and 1.5mm. For globular clusters it may be a higher magnification with an AP between 1.5 and 1mm. Double stars can be magnified really high, between 0.7 and 0.5mm. On the clear table you can see the simple formulas for magnification and object.

#### Which eyepieces for my telescope?

Do not buy seven okualare of inferior quality. Better buy three to four good eyepieces. Because unfortunately it is like this: Only then your telescope shows its full performance.

Good eyepieces are a win-win situation for you and your telescope.

Make sure you have enough eye relief, good edge-to-edge sharpness, a rather large field of view and high light transmission.

At the beginning, take an eyepiece for a low magnification around the minimum range, a medium magnification with about 1.5mm AP and a higher magnification at about 0.8mm.

In the 200mm telescope this gives 28x, 133x and 250x. You can’t see animals on the moon with this, but if you don’t want to be animalistic, choose good to excellent eyepieces and do without the enclosed set in the telescope.

For this, you need to invest at least fifty euros and more per eyepiece. But believe me: Every night observation will be a beautiful experience, which you will still rave about during the day.