A short focal length gives a wide angle of vision, but the images will appear smaller. Long focal lengths offer narrower fields of view, but the images appear larger. Shorter focal lengths are ideal for viewing starfields and larger objects. Small objects and studying fine detail, for example, on the planets, are best captured with long focal lengths.
The focal length of a telescope is an important thing to consider when choosing what telescope will be best for you. In this post, we will be discussing which focal length is better when comparing long and short focal lengths.
We will also go over other frequently asked questions on telescope focal lengths to make an informed decision on which focal length you prefer.
Focal Length Definition
A telescope works this way: light is gathered by the telescope’s aperture and transmitted to the focal point (focuser), where it is focused for the viewer to see objects through the eyepiece.
The focal length of a telescope is the distance (in millimeters) through which light travels from its entry point, the aperture, to the exit point (the focuser).
The focal length of a refractor or reflecting telescope begins at the aperture (objective or primary mirror lens) and finishes at the focuser, where light rays converge. When looking at a telescope, you can generally assume that a long tube equals a long focal length and a short tube equals a shorter focal length.
However, some telescope models, like the Schmidt-Cassegrain, have a distinct optics setting and secondary mirrors that reflect the light before sending it to the focuser. This implies that they have way longer focal lengths than Newtonian telescopes, despite the fact that their tube appears to be smaller.
Conventional refractors and Newtonian reflectors must be as long as their focal length since the focal length is a linear measurement of the scope’s light path. The light path of a catadioptric (CAT) telescope is reversed and folded inside, resulting in a shorter structure physically than the optical focal length.
This enables CATs to be more portable than their competitors.
How Does Focal Length Influence Images?
Any telescope’s angular field of view is determined by the focal length of the lens system. Longer focal length scopes have a smaller field of view than shorter focal length scopes.
This is demonstrated by shooting photos through two scopes with different focal lengths from the same location with cameras affixed directly to the back of their optical tubes at focus. This is known as prime focal length photography.
The image from the scope with a shorter focal length will have a larger field of view no matter what aperture or telescope design you use.
In simpler terms, regardless of the objective lens or primary mirror’s diameter, the focal length determines a telescope’s field of view.
How Does Focal Length Affect Magnification?
If you know the focal length of your scope and the focal length of the eyepiece attached to it, you can easily determine the magnification. For example, if you have a scope with a focal length of 2000mm and the eyepiece has a focal length of 10mm, then its magnification is 2000mm divided by 10mm, which comes to 200x.
So the longer the focal length of scope, the greater its magnification can be, and vice versa. But it’s important to remember that scope with an extreme focal length (long or short) will not always produce views as sharp as those seen through a medium-length telescope.
This is because such “extreme” telescopes rarely deliver images over their entire field of view at maximum power due to image degradation caused by imperfections in the optical path like specks of dust and atmospheric turbulence affecting image clarity.
Is a Short Focal Length Telescope Better?
A shorter focal length implies a larger photographic f/stop when comparing equal clear apertures; this implies shorter exposure times. For example, an f/6 optical system has an 80mm aperture and a focal length of 480mm. An f/10 optical system has an 80mm aperture and a focal length of 800mm.
Newtonian reflectors and refractors with short focal lengths are often simpler to store, move, and set up than longer focal length scopes. The field of view for shorter focal length scopes is larger.
One of the benefits of long focal length, reducing chromatic aberration, is removed with the great color correction of an APO refractor.
In other words, with technology and high-quality equipment, certain disadvantages of short focal length may be overcome.
Is a Long Focal Length Telescope Better?
In reflectors and catadioptric scopes, shorter focal lengths for a given clear aperture give proportionally larger central obstructions. This leads to degraded contrast and optical quality.
Long focal lengths, on the other hand, allows you to magnify objects more using any given eyepiece. As a result, your shortest focal length eyepiece will provide greater magnification.
Benefits of Shorter Focal Length Telescopes
From the above discussion, we can highlight a few benefits of using shorter focal length scopes. They include:
A larger photographic f/stop
A shorter focal length means a larger photographic f/stop and a shorter exposure time. For example, an f/10 optical system has a 10mm aperture and a focal length of 100mm. An f/16 optical system has a 20mm aperture and a 320mm focal length.
Short focal lengths allow for shorter scopes.
A smaller telescope is easier to store, move, and set up than longer focal length scopes. Also, they are easier to set up.
Large field of view for shorter focal length scopes
A smaller telescope also has a larger field of view. This is because the narrower the beam, the more it will spread out as it travels through space.
A wider beam means stars stay in their position over time, and you can find objects like comets easier with this advantage.
Drawbacks of Shorter Focal Length Telescopes
As discussed earlier, shorter focal length telescopes have a wider field of view. However, this is not always an advantage, as objects tend to look small in such scopes because they are closer to the edge than at the center of the field of view.
The shorter the focal length, the less sharp and detailed objects appear at maximum magnification.
Benefits of Longer Focal Length Telescopes
Easier to manufacture
Long focal length scopes ease problems with optical designing during manufacture. So manufacturers are able to make superior power scopes at low costs, and the consumer doesn’t have to pay a lot for such scopes.
Better magnification
Sharp, detailed maximum power views are easier to achieve in long focal length telescopes because they can be designed with fewer lenses or mirrors that must be precisely positioned.
Drawbacks of Longer Focal Length Telescopes
Unfortunately, long focal length scopes are limited by their size and weight because larger mirrors or lenses must be used to achieve better magnification without image degradation.
The narrower field of view also means that you can only focus on a small area at a time.
Long vs Short Focal Length Telescope FAQs
What is the best scope for viewing planets?
Since you will be focusing on a specific planet at a time, you are better off with a refractor or a short focal length telescope.
Longer focal length scopes are better for deep sky objects because it requires less magnification to view them clearly, and thus more valuable light can be collected from each object in the night sky.
All of these factors suggest that long focal length telescopes will give you high-quality views of planets, while shorter focal lengths may provide insufficient detail in such situations.
What is the best scope for viewing galaxies?
A refractor provides better resolution than any other type of telescope at higher magnifications giving sharp details across large areas on the main lens or mirror – making them excellent choices when looking into deep space where faint fuzzies abound!
With this quality, performance comes some additional bulk and weight as well, but we think the results make it worthwhile.
Long focal length scopes also provide a field of view that is much wider than shorter scopes, making them great for deep sky objects in general. The high magnification levels mean you have to spend more time looking through the eyepiece, but generally, this means less observing time per object.
What size telescope do I need to see the rings of Saturn?
You do not need a very powerful scope to view the rings of Saturn. A scope with magnification as small as 25x.
However, considering that Saturn is pretty far away from Earth at all times, you will need a long focal length in order to get the best view possible in the shortest amount of time.
Final Thoughts
So which one is better? A short or long focal length telescope?
As we have seen, either scope comes with its pros and cons. It boils down to personal preference and the situation you are in, and the objects you want to view.
Ever wonder if you can use a spotting scope for Astronomy?
Please be careful and use at your own risk
None of the authors, contributors, administrators, or anyone else connected with StarryNova, in any way whatsoever, can be responsible for your use of the information contained in or linked from these web pages.
