What’s the Difference Between A Fast and Slow Telescope?

Introduction

It’s common to see a telescope referred to as either fast or slow in the description on a retailer’s website.

In our own telescope reviews, we often talk about a telescope being either fast or slow (or somewhere in the middle, a ‘jack of all trades’) with a brief explanation of what that means.

However, when you look to astronomy forums to clarify what fast or slow means, and what impact that will have on your backyard astronomy, it’s likely you’ve come away confused with all the competing answers.

So, in this article, we’re cutting through all the ‘thoughts’ on the subject and rely instead on plain old scientific facts.

By the time you get to the bottom of the page, you’ll be super clear on:

  • What the terms fast and slow mean when applied to a telescope
  • How the physics that goes into telescope design cause them to be fast or slow, and
  • The impact choosing one over the other has on your backyard astronomy

If you just want the Cliff notes version, use the clickable table of contents below to get straight to the information you need. Otherwise, grab a coffee, and let’s dive in.

A (Very) Brief Guide to the Physics of Telescope

It doesn’t make sense to dive straight into the telescope speed debate without first having some clarity on the facts and figures of telescope design.

We’re going to tackle this quickly and simply, with the minimum of math, so stick with us because it will make the rest of the article a lot easier to follow.

Important Telescope Measurements

First of all, let’s define the key measurements and what they mean.

Aperture – This is the large opening at the end of your telescope which points at the sky and lets light into the telescope tube. Its size is the biggest single factor determining how much detail your telescope can see. A bigger aperture means more details become visible.

Focal Length – This is the distance light has to travel from the first lens or mirror it hits in your telescope to the point you look at it in your eyepiece. Unless you have a compound telescope, longer focal lengths mean longer telescope tubes. A compound scope, however, uses a combination of mirrors and lenses to fold the focal length into a smaller tube.

Focal Ratio – For our article, this is the most important measure because it’s the focal ratio that determines the speed of your telescope. The focal ratio is the ratio between your aperture and focal length. It is simple to work out, just divide the focal length by the aperture. Focal ratio is written as f/x.y (where x and y are numbers).

For example, if you have a telescope with a 75mm aperture (3″) and a focal length of 525mm, then we know its focal ratio is:

525mm/75mm = f/7.0

Some more examples are given in the table below.

Aperture (mm)Focal Length (mm)Focal Ratio
(F.L /aperture)
75525f/7.0
100600f/6.0
150600f/4.0
2001800f/9.0
2503000f/11.0

Magnification – this is perhaps the most common measurement we want to know when we’re stargazing. It’s your eyepiece in combination with your telescope’s focal length that sets magnification. To work it out, simply divide the focal length of your telescope by that of your eyepiece.

For example, if you have a telescope with a focal length of 525mm and an eyepiece with a focal length of 25mm, then the magnification is:

525mm/25mm = 21x

To better understand the impact of your telescope’s focal length on magnification, all of the examples in the table below use the same 25mm eyepiece in a variety of different telescopes.

Eyepiece (mm)Focal Length (mm)Magnification
2552521x
2560024x
25100040x
25180072x
253000120x

As you can see, a longer focal length provides more magnification for the same eyepiece than a shorter focal length.

Use the following link to read more details about telescope magnification, which opens in a new tab.

Field of View – when we look through a telescope, this measure tells us how much sky we can see. Just like magnification, this measure is decided by your eyepiece and the focal length of your telescope.

Each eyepiece has an ‘apparent field of view’ (AFOV) measure attached to it. An average Plösll, for example, might have a TFOV of 52°. We work out the actual or true field of view (TFOV) by dividing that 52° by the magnification.

For example, let’s stick with our telescope with a focal length of 525mm and an eyepiece with a focal length of 25mm. We know from above that we end up with a magnification of 21x. If we now assume that this same eyepiece has an AFOV of 52°, then we work out the true field of view as follows:

52°/21 = 2.48°

This means the patch of sky we’re looking at with this telescope/eyepiece combination is a circle with a diameter of 2.48° and everything inside that circle is magnified 21x.

Since focal length affects magnification, it indirectly affects the field of view. To show how this happens, we’ve copied the table from above and added an extra column to show how the TFOV changes using the same eyepiece in different focal length telescopes.

Eyepiece (mm)/AFOVFocal Length (mm)MagnificationTFOV
25 / 52°52521x2.48°
25 / 52°60024x2.17°
25 / 52°100040x1.30°
25 / 52°180072x0.72°
25 / 52°3000120x0.43°

You might not be surprised to see that as magnification increases, the true field of view decreases. For our discussion of fast and slow telescopes, keep in mind that magnification is a function of the focal length, i.e., longer focal length telescopes produce smaller fields of view for the same eyepiece.

Before we leave field of view (which you can learn more about here), make sure to take away that aperture size has no impact on a telescope’s field of view.

Resolving Power – tells us how much detail we can see through our telescope. More specifically, it defines the smallest gap between two objects that we can see, a double star is the classic example.

This is simple to calculate, just divide 120 by the diameter of your telescope’s aperture in millimeters.

For example, let’s assume we have an average 6-inch aperture telescope, which is 300mm, then its resolving power is given by:

120/300 = 0.40 arcseconds

The actual formula for resolving power is quite a bit more complex but this works for our discussion of slow telescopes vs fast. In reality, Earth’s atmosphere rarely allows us to split stars that are less than 1 arcsecond apart.

The key takeaway here is that a larger aperture will have greater resolving power than a smaller one and that resolving power is entirely independent of focal length.

Light Gathering Ratio – this final measure tells us that a telescope with an aperture twice the area of another telescope collects four times the light.

This is based on the inverse square law and is the reason why, for example, a seemingly small jump from an 8″ telescope to a 10″ telescope results in your light grasp increasing by 56%.

The details are not as important as understanding that larger apertures collect lots more light than smaller ones.

Irradiance (Brightness) – is our last measure. We all understand what we mean by an image being brighter or dimmer but the proper term for brightness is irradiance.

Stars are classed as a point source of light so the telescope’s aperture is the only measure that determines their brightness at the eyepiece. The larger the aperture, the brighter stars will appear.

For everything else which is not a point source, e.g., planets and nebulae, different rules on iridescence come into play.

In these cases, lower focal ratios create high iridescence (brighter) images on the focal plane independently of aperture size. A squaring of the focal ratio halves the iridescence of an image in the eyepiece and the size of your telescope’s aperture has no bearing on this.

We measure iridescence using watts per meter squared (W/㎡).

Simply put, smaller focal ratios don’t spread out light as much as larger focal ratios, so telescopes with smaller focal ratios deliver brighter images. The video below does a good job of explaining this simply.

Say That Again… in English Please

Okay, let’s take a breath because even though that was a brief tour of telescope measurements, there was still a lot to take in.

Now, we’re going to take a step back and convert that all to an analogy that’s easier to understand.

For a short while, we’re going to replace our telescope with a long room. You are standing at one end of the room and, at the other end, is a large, circular window. Just like in a telescope though, all of the light from the window will be focussed on your eye.

In this analogy, the window is the aperture of our telescope and you are the eyepiece. The length of the room from the window to you is our focal length.

Imagine this situation:

The window has a diameter of 1 meter and you are standing 10m away from it. The focal length of the room is 10÷1 = f/10.0

Let’s make some changes and see what happens…

1) You move a further five meters away from the window and the window stays the same size. The focal length has increased to 15m, making the focal ratio f/15.0.

The same amount of light is coming into the room but less of it reaches your eye, so the image is dimmer. Also, the window appears smaller now because it’s further away from us.

In the language of telescopes, our field of view has got smaller and the image has got dimmer.

2) Next, we stay 15m away, but we increase the size of the window’s diameter to 1.5m. We’re now back to a focal length of f/10 (15÷1.5) and the window appears the same size as it did when it was 1m wide and we were 10m away from it.

The amount of light reaching our eyes is also the same, so the view through the window is just as bright as it was when it was 1m wide and we were 10m away from it.

However, there is now 2.25x more light is coming into the room because the aperture has grown. This means we can see finer details through the window because resolution increases as the window gets larger.

3) Finally, we move very close to the window. It’s still 1.5m across, but we’re now just 6m away from it, giving us a focal ratio of f4/0 (i.e. 6÷1.5).

We’ve got exactly the same light-gathering power as we had before, and the same resolution, because the window’s size hasn’t changed. But more light from the window hits our eyes because we’re much closer to it, so the image we see through it is brighter.

We notice two other things now. Firstly, the window offers a much bigger field of view because we’re closer to it.

Secondly, we’ve begun to notice image quality at the edge of the window has deteriorated compared to the center. This is because the light rays coming from the edge of the window have to be severely bent to hit our pupils.

Let’s summarise this so we’re finally in a position to move on and talk about what makes a telescope fast or slow and the differences between them.

  1. Image magnification and field of view is controlled by a combination of your eyepiece and the focal length of your telescope
  2. Image brightness is determined by your focal ratio (except for stars)
  3. Light gathering power, the brightness of stars, and image resolution are all defined by the aperture of your telescope

What is Meant by a Fast or Slow Telescope?

The terms fast and slow in reference to focal length come from photography and make more intuitive sense when applied to telescopes used for astrophotography.

To take an image of the night sky, your DSLR or CCD collects photons of light. The trick to good astrophotography is to catch as many photons as you can as quickly as possible. Speed is important because longer exposures increase the risk of noise in the image sensor and your telescope drifting off target.

Another way of saying we want more photons in a shorter amount of time is that we want a brighter image. After all, the brightness of an image is measured by the number of photons per unit area.

As we saw in the section above, brightness is determined by the focal ratio of your telescope. The smaller the focal ratio, the brighter the image on the focal plane. For example, an f/2 telescope will produce a brighter image than an f/6 telescope and brighter images build up the pixels in the sensor more quickly so the exposure time needed to take an image is shorter.

What is a Fast Telescope?

A fast telescope is one with a smaller focal ratio. It is called fast because it produces a brighter image with more photons per pixel which results in shorter (or faster) exposures being needed to capture an image.

Traditionally, telescopes with focal ratios of f/5.0 and below are considered fast.

What is a Slow Telescope?

A slow telescope is one with a larger focal ratio. It is called slow because it produces a dimmer image with fewer photons per pixel which results in longer (or slower) exposures being needed to capture an image.

Traditionally, telescopes with a focal ratio above f/8.0 are considered slow.

Telescopes between f/5 and f/8 don’t fall into slow or fast camps and are seen as middle ground, ‘Jack of all trades’ optical tubes.

What are the Differences Between a Fast and Slow Telescope?

Now you’ve taken the time to come through all the introductory parts of this article, the difference between fast and slow telescopes might already feel intuitive.

If not, think of it by working through a side-by-side example.

Remember that with the same eyepiece (or camera) the field of view is the same in telescopes with the same focal length, so let’s compare two telescopes trying to image the same part of the sky.

They both use the same CCD camera and each of them has a 600mm focal length. From everything we’ve learned so far, we know that they will both image exactly the same area of sky (field of view).

Now, assume one of these telescopes has a focal ratio of f/5 and the other is f/8.

Working backward, we can calculate that the f/5 telescope with a 600mm focal length must have an aperture of 120mm (600÷120=5) and the f/8 model has an aperture of 75mm (600÷75=8).

We can now see that the f/5 model is faster because it is collecting more photons (bigger aperture) and squeezing them into the same area (field of view). This makes for a brighter image and a faster (shorter) exposure time.

Okay, that hopefully all makes sense but it all relates to photography and astrophotography. Our eyes don’t work like a camera, they don’t add layer upon layer of photons, so is it important to think about fast and slow telescopes for visual astronomy?

We’ll look at that in more detail next.

Are Fast or Slow Telescopes Best for Visual Astronomy?

We’re not taking astro images with our eyes, so fast and slow telescopes don’t directly affect us that way, but there are other elements baked into either a fast or slow telescope that we do want to pay attention to.

Fast Telescopes for Visual Astronomy

As we’ve now discovered, a fast telescope is one with a lower focal ratio. Lower focal ratios occur when the telescope’s focal length is relatively small compared to its aperture.

Another way of saying this is that for any given aperture, the focal length of a fast telescope is shorter than the focal length of a slow telescope. Some examples which demonstrate this are given in the table below.

Aperture (mm)Focal Length at f/3Focal Length at f/7Focal Length at f/11
75 (3″)125mm525mm825mm
100 (4″)300mm700mm1100mm
150 (6″)450mm1050mm1650mm
200 (8″)600mm1400mm2200mm
250 (10″)750mm1750mm2750mm

Earlier in this article, we saw that shorter focal lengths mean lower magnifications and larger fields of view. That’s why we say that faster telescopes are better for viewing larger deep sky objects (DSOs) like galaxies and nebulae.

Any given eyepiece will have a lower magnification in a faster telescope which means you could find yourself using very short focal length eyepieces to achieve planetary levels of magnification. Such eyepieces tend to have short eye relief (you’ll need to have your eye practically touching the eyepiece to see in it) which makes observing hard work.

On a separate note, image quality can also suffer in fast telescopes.

When you think back to our analogy of standing in a long room with a window at one end, we discussed the impact of sharply bending light rays. White light is made of the different colors of the spectrum and these bend (refract) at different rates.

In a short focal length (fast) telescope, the mirror or lens focussing the light to your eyepiece has to bend the rays more sharply than in a longer focal length (slower) telescope. This results in coma and chromatic aberrations at the edges of the field of view, which is another reason why fast telescopes aren’t recommended for planet watching.

4979783-Chromatic Chart.jpg
Chromatic aberration chart (source)

It is also the case that mass-market eyepieces aren’t designed to cope with fast telescopes. If you opt to use a fast telescope, you’ll get the best mileage from it using higher-end eyepieces which are better corrected for it.

On the bright side, the small tube of a fast telescope makes them very easy to transport!

Slow Telescopes for Visual Astronomy

Unsurprisingly, slow telescopes offer the opposite benefits to fast ones for the visual astronomer.

Their longer focal lengths mean that mirrors and lenses don’t need to refract the light so much to bring it to a focus at the eyepiece, so we end up with crisper images and fewer aberrations.

Those longer tubes also deliver higher levels of magnification for any given eyepiece and a smaller field of view. All of which makes slow telescopes ideal for planetary and lunar observation.

(Remember though, more magnification is only useful if it reveals more detail and more detail comes from a larger aperture. High magnification in a small aperture scope just makes the image dimmer and more blurry.)

Since these are slower scopes, the payoff is brightness. Yes, images appear larger in a slow telescope than in a fast, but they are also dimmer. That’s not a problem for planets and the moon but does count against them when hunting large DSOs with low surface brightness.

One final thing to keep in mind is that slow telescopes are big. They have longer tubes, which makes them heavier, and they need better, more sturdy mounts to cope with their increased mass.

All Telescopes, Regardless of Speed

Aperture size, as we’ve already said, is usually the main consideration of astronomers when buying a telescope.

Regardless of whether it’s fast or slow, a larger aperture lets your telescope reveal more detail to you. If you love to look at planets and the moon, choose a large-apertured slower telescope for teasing out surface details under high magnification.

If your love is DSOs, then get a larger aperture on a fast telescope to achieve brighter images and larger fields of view with more detail in them.

However, if all of what the night sky has on offer appeals to you, stay in the middle zone of speed, say f/5 – f/8. These models can still have big apertures to reveal details but they are equally as comfortable with planets and galaxies.

Summary

Telescope selection is always a matter of compromise.

Whether a fast or slow telescope is more important to you depends on what you want out of your astronomy. The more specialized you become, the more important this choice will be.

If you’re a beginner and want to enjoy the whole sky, then something in the mid-ranges of focal ratios will be great.

Planet-watching enthusiasts or lunar hunters benefit from higher magnification and better image quality, so should be looking at a slower telescope.

Faster telescopes have brighter, smaller images but can’t handle magnification so well and are prone to color distortion, all of which make them great for pointing at galaxies and nebulae.

Finally, if you feel like venturing into the world of astrophotography, a fast telescope with apochromatic lenses to handle color distortions is the way to go.


Written by Adam Kirk

2 thoughts on “What’s the Difference Between A Fast and Slow Telescope?”

  1. Adam.
    Thank you for your articles. They are amazing in their explanations. Detailed and written so a newbie can still understand it. I look forward so much to your emails.
    I read this latest one about fast vs slow telescopes three times, and still get something more out of it!
    John Abbott

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