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Applicability of magnifications in an astronomical telescope

Magnification is the most misunderstood parameter of telescopes, and not only by beginners. New telescope users often assume that a larger magnification gives a better result. But they quickly find out that this is rarely the case, and even vice versa, a lower magnification almost always gives a better image.

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Planet observations, Sochi, 600 meters above sea level. (Photo: K. Radchenko)

Why is a large increase not always good?


There are several reasons why a large increase may not be preferable. The usual assumption of new amateur astronomers is that since we are trying to observe objects that are very far away, we want to enlarge them a bit to bring them closer. But most of the objects in the night sky, despite the fact that they are very far away, seem very large. For example, the Orion Nebula looks more than twice the size of the full moon, and the Andromeda galaxy looks six times larger. Although Andromeda is 70 trillion times farther from the moon, it is also 420 trillion times larger than our companion! A large increase gives a small field of view, which means that a large object may not fit into the field of view of the telescope.

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View of the Andromeda galaxy: on the right, at a larger magnification, but the entire Andromeda galaxy can only be seen in low multiplicity mode - on the left.

Another reason why the magnification is not to be greatly increased is due to the brightness of the image. The unsuccessful law of physics says that when the magnification doubles, the image becomes four times less bright. Most celestial objects are very weak, so making them dimmer than necessary is not recommended. This is why the most important thing in a telescope is the aperture (lens diameter), not the magnification. Brightness is the key to astronomical observations.

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Image of the Orion nebula: enlarged on the right, but also dimmer than at low magnification - on the left

Some objects, however, are small and bright and therefore can withstand large magnifications. Planets just fall into this category. Jupiter, despite being the largest planet in our solar system, is far enough away (644 million km.), And is visible as 1/36 of the size of the full moon. However, Jupiter is brighter than any star in the sky. Such large increases work well on Jupiter, Saturn, Mars and other bright objects such as the Moon.

How much is too much?


So why not just increase Jupiter as much as we want? If he looks better in the 200s than in the 50s, shouldn't he look better in the 600s or 1000s? No, and there are two reasons why.

The first is connected with the telescope itself. The brightness of an object depends on the size of the telescope and magnification. The more light you can collect (the larger the lens area, which depends on its diameter), the more you can increase the magnification of the instrument before the image becomes too dim. In addition, the resolution, or the smallest details that can be seen, also depends on the size of the diameter of the lens. This means that there is a theoretical upper limit on how much the telescope can increase before the image becomes faded and too blurry. This is determined by a very simple equation:

Maximum telescope magnification = D x 2
D - lens diameter in mm

For example, a 75mm telescope has a maximum theoretical magnification of 150x. A 150mm telescope can magnify 300 times and a 200mm telescope 400 times. However, this is strictly a theoretical maximum, because the telescope itself is not the main limiting factor.

The usual limiting factor at maximum magnification is the Earth’s atmosphere. Since we must look through the atmosphere to see anything in space, the more we increase the celestial objects that we are looking at, the more we increase the negative impact of the atmosphere. And if the atmosphere is turbulent, this turbulence will tend to blur the image. The stability of the atmosphere is called observation conditions. When visibility is good, the atmosphere is stable and the image looks very clear. When visibility is poor, the atmosphere is very turbulent and the image looks blurry. In a night of poor visibility, even a good telescope cannot provide more detail in the image.

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Jupiter in excellent visibility conditions

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Jupiter in poor visibility conditions

The real upper limit of magnification, no matter how large the telescope, is, on average, about 250x - 300x per night. On a bad night, you cannot exceed 100-150x. Please note that the observation conditions and transparency (purity of the atmosphere) are not the same. Often very dark, clear nights will have poor visibility conditions, while foggy nights with low transparency often give excellent visibility. This is caused by the fact that in the upper layers of the atmosphere the vortex flows, spoiling the picture, subside.

Well, if too much is bad, what about a low magnification?

A lower magnification gives a wider field of view and a brighter image. However, just as there is such a thing as a too large increase, there is such a thing as a minimal increase. The minimum magnification is determined by the exit pupil of the telescope system. The exit pupil is the diameter of a ray of light emerging from the eyepiece. The larger the beam, the brighter the image. At least until the diameter of the exit pupil of the telescope does not exceed the diameter of the pupil of the eye of the observer.

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Different sizes of exit pupils. The large exit pupil on the right is wider than the pupil of the observer's eye.

If the exit pupil is wider than the pupil of the observer’s eye, the brightness of the picture disappears. The effect is exactly the same as limiting the telescope aperture (aperture). The observer’s pupil size depends on whether the observer is adapted to the dark and how old he is (the maximum pupil size decreases with age). A typical dark-adapted pupil is 7 mm in diameter. The eyes of older observers can only open 5 or 6 mm. Assuming the standard size of the human pupil in the dark is 7 mm, there is a simple equation for minimum magnification:

Minimum usable magnification = D / 7
D - lens diameter in mm

Optimal magnification


The second problem is that decreasing the magnification reduces the image scale and detail. The best resolution of the human eye is achieved by using the smaller diameter of the exit pupil of the instrument. Observational experiments usually find that for observing deep space objects the best picture can be seen with an exit pupil from 2 mm to 3 mm. This will be an increase of 35-50 times on a 100mm telescope, 70-100x at 200mm and 120-175x at 350mm. A lower magnification may be necessary to cover the entire large object in one field of view. But when you try to observe small details in a galaxy, or nebula, or in a globular cluster of stars, average magnifications may be ideal.

To view the planets, you can use a higher multiplicity. Of course, every object, telescope and observer is unique, so certain magnifications may be better for certain combinations. Most astronomers have three eyepieces - one large, one medium and one low - to cover various observation conditions. Usually they range from 50x to 250x, as it covers everything from a wide field to high multiplicity. A large increase may be useful for great nights, but most likely it will be an eyepiece that is rarely used. Less power may be useful for wider fields of view.

Look at the magnification calculator to determine the magnification of any combination of eyepiece and telescope.

I hope this article will be useful to someone!
All clear skies and successful observations!

Konstantin Radchenko, editor-in-chief of the Open Astronomy group.

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