Telescope aperture comparison :: Aperture vs Focal Length

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Every telescope can be chosen on the basis of two criteria because the two main characteristics that will be taken into account are either aperture or focal length. Aperture in a telescope is the objective length diameter for a refracting telescope or the primary mirror when it is in reference to a reflecting telescope. So, if you switch to greater focal length, it is clearly better to observe remote objects in space such as the moon, planets, etc. or if you require better observation of DSO (deep space objects) such as galaxies getting a device with larger aperture figures is better.

Telescope Aperture

The diameter of the area which collects the light is defined as aperture. There are two general areas. First is the objective length diameter that can be found in refracting telescopes and the second is the prime mirror that can be found in reflecting telescopes. Aperture is responsible for determining the maximum usable magnification power or MUM factor. This means that when a telescope is pushed to its maximum 50x then it loses the object’s clarity. In order to get better clarity of remote objects, the aperture has to be higher so that the MUM power is not affected and you can clearly see the objects. For instance, to view Saturn properly, the aperture diameter of 280x will be fine.

Aperture is responsible for determining the maximum usable magnification power or MUM factor

Telescope aperture explained

The viewing conditions always have to be taken into account. So, in perfect conditions, 1 inch of aperture will get you 20x to 50x of magnification quite easily. Then, a 4-inch scope will usually provide an astronomer with 200x magnification, whereas 10 inches will offer you the chance to gain 500x. Afterward, atmospheric scintillate will degrade an observed image and clarity will be much lower. Consequentially, it is understood that observation of close planets will be fine with anything that varies from 50x to 200x. Observation of remote planets will require 300x and above. Observation of the universe will always require the maximum aperture possible.

Telescope Aperture comparison

It is important to mention that in most cases you will see the desired objects in the solar system or in the universe if the aperture figure is low. In simple words, an aperture is responsible for clarity.

20x to 50x magnification will make sure that you can perfectly observe the details of the Moon but although the planets will still be visible, its clarity will be bad.
100x magnification will ensure that details are visible of planets like Mars, Venus, and Mercury. Jupiter may require higher apertures.
280 magnification will ensure decent observation of Saturn. Planets located further away will require more MUM power.

Is bigger aperture better?

It has been proven that the bigger the aperture, the better it is for an observer to see objects clearly and in more detail. It can be explained by the fact that when more light enters a telescope the faster its focus will be. A much smaller depth of field will be provided, which is also a great advantage. Higher quality resolution will be ensured too. Wide aperture allows more light to fall on the sensor, so there is greater versatility. However, aperture is not the most significant factor for beginners to consider.

Focal length of a telescope

When the light hits the mirror or shines through the length of a telescope, it is then directed by the device’s optics to be focused at a certain point. This is called the focal length of a certain object. The focusing length of a telescope is expressed by numbers that indicate the maximum focusing range of a lens when it is focused at infinity. For instance, if a certain telescope has 1200 mm of focal length as well as 20 mm eyepiece, the overall figure of magnification will be 60x.

What does the Focal Length of a telescope?

The focal length plays a crucial part in the whole observation process. It indicates an actual distance from the objective to a place where an observed image is formed. The higher the focal length, the greater the magnification will be. The focal length is important for the quality of an observed image because the further away it is located the greater focusing will be required.

Is a longer Focal Length better for a telescope?

It always depends on the objectives of each astronomer and their level of experience. The long focal length provides a narrow view of a certain point of the sky but the actual viewed object will be larger and seen better. The short focal length is most suited for looking at large objects as well as star fields. The long focal length will also be ideal for studying small objects and seeing them in more detail. It includes the Moon and planets.

What is a good focal ratio for a telescope?

The long focal length of a telescope will be ideal to watch the stars, it usually should be 1000 mm to 1200 mm in diameter. The short focal length is ideal to view the Moon and planets, the recommended range is from 80 mm to 150 mm. Therefore, if you are a beginner to astronomical observation of the sky, it is probably a good idea to start with 80-150 mm telescopes. When choosing a telescope, remember to choose the focal length with a combination of the diameter and the length that is expressed in a zoom ratio, which is stated as D. It is proven that the best D for the majority of telescopes is 1.6-2D. The chance that you will not see anything with a high D ratio when buying short-focus eyepieces is very high, so please choose wisely.

The long focal length of a telescope will be ideal to watch the stars, the short is better to view the Moon and planets

Formula of telescope Focal Length

Using the focal length formula is important to know a telescope’s power and capability. For accurately calculating the power of a telescope you will have to divide the telescope focal length by the eyepiece focal length. The result of the calculation will give you the X power. So if a telescope has 600 mm of the focal length and 20mm eyepiece, then 600/20=30. It means that you get 30x of power.