You must understand the limitations of any telescope. It is possible to calculate many of the parameters. Fortunately, only a few variables are required to make these calculations. However, finding an online calculator to do the calculations quickly is easier. On this page, you will find an easy-to-use telescope calculator.
Before using the calculator you must find a few numbers that are unique to the telescope and other equipment that you use. The most important telescope specifications are the aperture and focal length. You should find these two numbers on the optical tube of your telescope. If you can not find it perform a search using your telescope make and model. Enter the two numbers in the calculator below.
The calculator will update as soon as any number is changed.
With those two numbers entered the first three calculated fields are valid for your telescope. These values are focal ratio, maximum theoretical magnification and resolving power of the telescope. The relevance of these will be discussed below the calculator if you are scratching your head.
If you wish to know the remaining three parameters you need to enter the focal length and field of view of the eyepiece you are using and the strength of any Barlow lens that you may be using. The focal length of any eyepiece is written on the side of it. The field of view may also be written on the eyepiece but it may not be. If it is not written on the eyepiece, try searching for its specifications online. If you can’t find it leave the value as 60 degrees as that is a good option.
The power of any Barlow lens will be written on it. It will be something like 2x or 3x. If you are not using a Barlow lens leave the value at 1.
With those numbers entered the form is complete, and all values are calculated
Telescope Calculator
For a description of the numbers required, see the previous section.
What Are the Numbers in Telescope Calculator
We stated that it is important to know these numbers but not why they are important. In this section, we describe what the numbers represent and why they are important.
We will describe each of the calculator’s numbers as they appear in the telescope calculator.
Aperture
The aperture of a telescope is the diameter of the primary lens or mirror. You will find the primary lens of a refractor at the front of the optical tube. In a reflector, the primary mirror is located at the bottom of the optical tube.
A telescope’s aperture is the most important feature of any telescope. The wider the aperture, the more light that is allowed to enter. More light means that the image will be brighter allowing it to be magnified to larger amounts. Also more light will allow you to observe more distant objects.
A doubling in aperture will result in the telescope collecting four times that amount of light.
Be aware that the effective aperture of reflector telescopes is slightly reduced due to the presence of spider veins at the front. This is not accounted for in our calculator but is in the order of a few percent.
Focal Length
The focal length of a telescope is measured from the primary lens or mirror to the point at which the light is focused.
There is an important relationship between a telescope’s aperture and its focal length. This will be discussed in the section about the telescope’s focal ratio.
Eyepiece Focal Length
Like telescopes, eyepieces have a focal length, which is the distance at which they focus the light that enters them.
Eyepieces come in a range of focal lengths from 3 mm to about 50 mm.
Barlow Power
A Barlow lens is an optical device that diverges light. This is different from the other optical devices in a telescope that brings light to a focus point by converging light.
The effect of a Barlow lens is that it increases the telescope’s focal length by a set amount. For example, a 2x Barlow lens will double the telescope’s focal length.
It increases the magnification of a given telescope and eyepiece combination and the telescope’s focal ratio.
Native Focal Ratio
As indicated in previous section the focal ratio of a telescope is an important specification. It is a measure of the amount of light that will be available for magnification by an eyepiece.
The focal ratio is the telescope’s focal length divided by its aperture. As an example, a telescope with a 200mm aperture and a focal length of 1000mm has a focal ratio of 5. It is represented as f/5.
The focal ratio of telescopes is measured in the same way as lenses used in photography.
As stated focal ratio is an indication of how much light the telescope delivers to the eyepiece. The lower the ratio the more light there will be. A telescope with a low number (up to f/6) is said to be fast, while one with a large number (above f/9) is said to be slow. This comes from photography and is in relation to the shutter speed required.
Because there is more light available in a fast telescope it will allow for higher magnification.
You may ask why not all telescopes are made to be fast. The answer is two-fold. Because they have short focal lengths the magnification provided by a given eyepiece will be lower. This is not optimal for brighter objects like the Moon and planets. Also, with a shorter focal length, the lens or mirror needs to converge the light to a greater degree. This introduces aberrations, which are expensive to mitigate.
Fast telescopes are great for observing dim objects that occupy a reasonable amount of the sky. Slow telescopes are great for observing planets and the Moon as they are bright.
Maximum Magnification
An important parameter in the telescope calculator is the maximum magnification. In practice, it is the theoretical maximum magnification, as other factors can reduce the amount of magnification.
As discussed in the previous section, a telescope can only deliver a certain amount of light to an eyepiece. That amount is determined by the focal ratio. The more light that is delivered the more magnification can be applied and still produce a satisfying image. If the image is magnified too much it will be dim and blurry.
Because magnification is calculated using the focal length we only use the aperture to determine the maximum magnification. To find the maximum magnification, take the aperture in mm and times it by two. If you are more comfortable with inches multiply the aperture in inches by 50.
Telescope Resolving Power
Like a digital camera, telescopes have a minimum size they can see. If two objects are closer than the minimum separation, it will not be possible to see them as two distinct objects. This is what resolving power is.
A telescope’s resolving power depends on the wavelength of the light and the aperture of the telescope. The formula is: resolving power = 1.22 x wavelength of light divided by aperture (Light gathering and resolution). As the light we see through a telescope is a range we are best to use an average of the light. With this average, we can reduce the formula to resolving power = 116 derived by the aperture in mm.
If two objects are farther apart than this angle it is theoretically possible to see them as distinct objects. This is evident more when observing binary stars. Obviously, objects closer together will appear as a single object.
Magnification with Selected Eyepiece
Magnification is a function of the combination of the telescope, Barlow (if fitted) and the eyepiece.
To determine the magnification multiply the telescope focal length by the multiplication factor of any fitted Barlow lens. Then divide the result by the focal length of the eyepiece. If no Barlow is used just divide the telescope focal length by the eyepiece focal length.
Once you have the result compare it to the maximum magnification for your telescope. If it is larger, or getting close to, that number it is probably not a good combination to use. This is because the image formed will not be pleasing.
Field of View
The field of view is the amount of sky you can see at any time. It is measured as an angle.
To determine the field of view, you must find the eyepiece field of view. This may be stamped on the eyepiece, or you may need to search for the specifications of your eyepiece. Divide this number by the magnification of the telescope and eyepiece combination to get the effective field of view.
The field of view will determine the maximum angular size of an object that you can view at once.
Exit Pupil
The final calculated value in the telescope calculator is the exit pupil. The exit pupil is measured in mm and is the size of the light cone emitted from the eyepiece. If this number is larger than your pupil you will not be able to see the whole scene. If the exit pupil is too small observing may be difficult.
The size of the exit pupil is determined by dividing the eyepiece’s focal length by the telescope focal ratio. If using a Barlow lens multiply the focal ratio by the Barlow lens power.
Final Thoughts
Hopefully, you have derived some benefit from our telescope calculator and have a good understanding of how the specifications are calculated.
With this knowledge, you can use your telescope to a higher level and improve your observational skills.
If you are interested in learning more about the mathematics involved with telescope specifications see Telescope Math.
I found astronomy while working in dark rural locations. Initially, I explored the night sky and learnt the constellations before purchasing a pair of binoculars to further my knowledge of the sky.
My first telescope was a 200 mm Newtonian reflector on an equatorial mount. I found that this telescope had a steep learning curve but was a rewarding experience.
As time progressed, I became interested in astrophotography. This resulted in purchasing a 110 mm refracting telescope and a dedicated monochrome-cooled astronomical camera. This resulted in another very rewarding steep learning curve that far surpassed the experience with my first telescope.
I have joined Telescope Guru to share my knowledge of telescopes and astronomy.
