Every part You Want To Know

One of many key technical specs you must at all times contemplate when choosing a brand new telescope is its focal size. It’s the second most necessary quantity for a telescope solely behind the aperture. However what does the focal size actually imply in a telescope and what’s it? Let’s reply all these questions simply and shortly.

The focal size of a telescope is the gap the sunshine travels contained in the telescope’s tube to get from the entry level to the exit level. The entry level is the entrance lens, and the exit level could be the focuser, which is the half the place the eyepiece goes so you may take a look at the picture being shaped.

So, if it’s the gap from entry to exit, does that imply the focal size is simply the size of the telescope’s tube?

Effectively, that’s true for some varieties of telescopes like refractors, nevertheless, there are extra fashionable and complex designs like Cassegrain telescopes the place the sunshine travels a number of instances backwards and forwards throughout the tube, extending their focal size on goal whereas retaining a compact design.

The focal size is often given in millimeters. The most typical sizes for house telescopes vary from 600mm for entry-level fashions to 2,000mm on the high-end ones.

What does focal size do in a telescope?

Focal length in a refractor telescope
Focal size in a refractor telescope

Okay, now we all know what focal size is. However what does it do and why does it matter?

The mechanics of how gentle transmits relying on the focal size of the gadget are a bit complicated and require a sure grade of physics data so we’re going to skip the small print, however right here’s what you have to know:

The focal size in a telescope is necessary as a result of it’ll play a giant half in figuring out two issues: the magnification of the picture, and the sphere of view.

The magnification is how a lot you might be “zooming” into the objects you need to see via a telescope. It really works similar to in your telephone’s digicam. Extra magnification means extra element within the picture you see, nevertheless, there’s a restrict at which you’ll not get any profit from including any extra magnification. That restrict is known as most helpful magnification and is decided by the aperture. You may get extra particulars about that in our article explaining the aperture.

However how do you attain that magnification?

Effectively, the magnification of the picture at any given time is decided by the next easy formulation:

magnification = focal size of the telescope / focal size of the eyepiece

Which means that a bigger telescope focal size, will increase the magnification. However the focal size of the telescope is static so the one factor you may change is the focal size of the eyepiece you might be utilizing. Due to this, it’s necessary to have a focal size that may suit your want from the beginning.

As for the sphere of view, it merely means how huge is the observable space of the sky you may see with a telescope in levels.

A big focal size usually means a smaller area of view. That is fascinating while you need to take a look at the Moon, or planets. Targets the place you actually need to amplify as a lot as you need to see the small print. Then again, a brief focal size leads to a bigger area of view. That is superb while you need to take a look at deep-sky objects like galaxies the place you need to seize a bigger space of the sky.

What’s the most effective focal size for a telescope?

This small tabletop telescope has a bigger focal size than many lengthy tube scopes

For newbie’s telescopes search for a focal size of at the least 600mm. Something lower than that may restrict your viewing expertise an excessive amount of and would solely be really useful for youths’ telescopes.

Past that, the reply goes to depend upon what sort of celestial objects will likely be your most important goal.

If you wish to observe planets or the Moon intimately, go for an extended focal size.

If you wish to use it for observing stars, galaxies, and nebulas, then a shorter focal size will likely be higher.

In fact, it is possible for you to to watch all objects indirectly with all focal lengths, so don’t fear an excessive amount of about it if it’s your first telescope, however do know that some objects are going to look higher than others relying in your decisions.

The focal size doesn’t at all times improve because the aperture of some telescope fashions will increase. There are telescopes with giant apertures and smaller focal lengths and vice-versa.

Some varieties of telescopes will naturally have bigger or shorter focal lengths, for instance, refractor telescopes will usually have a decrease focal size as they’re restricted by the size of the tube, whereas Maksutov-Cassegrain telescopes often have a big focal size regardless of their compact design.

For astrophotography, the foundations above additionally apply. Planets look nice on longer focal lengths (1000mm+), whereas shorter focal lengths are higher for deep-sky objects (600mm to 1,000mm)

The way to improve or lower the focal size of a telescope

Despite the fact that the focal size of a telescope is unchangeable, there are methods to artificially prolong it by utilizing sure equipment like a Barlow lens or a focal extender.

Each of those equipment are positioned within the focuser and work very equally, they improve the sunshine transmission distance between the entrance lens and the attention of the person, which successfully works as if the focal size was being elevated.

Barlows and extenders come in several sizes, named after the quantity of elevated magnification they provide, for instance, 1.5x, 2x, 3x, and so on.

We’ve a full article on Barlows and focal extenders if you’d like further particulars.

On the opposite finish, it is usually potential to lower the focal size of a telescope utilizing a focal reducer. Focal reducers are merely small mirrors or lenses with constructive energy that lower the focal size. The ensuing focal size will be calculated by multiplying the telescope’s focal size by the focal reducer’s multiplier. For instance, a telescope with a focal size of 800mm utilizing a 0.5x focal reducer will appear like a 400mm telescope.

Focal reducers can join on to an eyepiece or digicam and are usually extraordinarily low-cost as a result of their design is easy.

Whereas these equipment are wonderful instruments that offer you entry to a better vary of focal lengths, you must observe that each time you add another piece to the optical chain of your telescope, some gentle will likely be misplaced on the best way, leading to a dimmer picture, or including small optical errors. How huge that impact is will depend upon the standard of the accent.

Focal size vs aperture

There may be some debate on which quantity is a very powerful specification for a telescope. Aperture or focal size?

Most astronomers agree that aperture is certainly probably the most related specification in a scope. The explanation why may be very easy, the aperture goes to find out the quantity of sunshine the gadget can seize. The extra gentle you may seize, the higher your ensuing picture goes to be, it is so simple as that.

On high of that, there isn’t a technique to change or play with the aperture of a telescope. What you get is unmodifiable, whereas alternatively, as we talked about above, the focal size will be artificially modified, at the least as much as a sure level.

Aperture is certainly the specification the place you must make investments most of your cash on the subject of choosing a brand new telescope.


  • Focal size is the gap the sunshine travels to go from the entry to the exit level in a telescope.
  • The focal size is necessary to find out the magnification and area of view of the ensuing picture.
  • An extended focal size is sweet for observing planets, a brief focal size is sweet for observing deep house objects.
  • Focal size will be artifically modified utilizing accesories.

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