Understanding Telescope Magnification – How to Choose + Calculations


Disclosure: This post contains affiliate links and I may earn a small commission if you click through and make a purchase.

All you know is that when you look through the telescope, you can see planets and stars in greater detail than what you can see with your own eyes.

What if you want to see more?

It’s a valid question as many astronomers look through their telescope and are disappointed with the views.

Is it the scope or user error?

Unfortunately, the advice of buying a higher power eyepiece is really not advice at all. They have their own set of drawbacks and it doesn’t address the issue of what magnification is appropriate for your needs.

If only it were that easy to understand how telescope magnification works. . .

Well, it’s not easy, per se, as there is more than you would think that contributes to magnification. But, it can be laid out in layman’s terms so you can understand it better.

Let’s calculate telescope magnification together and what it will mean for your next observation.

Telescope Magnification

Magnification is not a fixed telescope principle. It can be changed with use of eyepieces and additional accessories. You can have low, medium, and high magnification, and these terms are general guidelines depending on the quality of views with any given magnification ability of your telescope.

For example, a smaller 60 mm telescope may be able to take advantage of lower magnification of 20x, but it won’t be able to provide the same amount of resolution and clarity at 200x. Therefore, 20x may be low for that telescope and 100x might be high. Other scopes may only be able to effectively use low power at 60x but provide high power at 300x.

Magnification and power are used interchangeably to mean the same thing – a level or setting that determines the parameters for an enlarged image scale.

Telescope Magnification Formulas & Calculations

To help you understand magnification, here are some formulas for calculating telescope magnification and other related formulas. Below will be a brief explanation of the formula factors needed to understand the calculation.

Calculating Telescope Magnification

The first calculation is a universal telescope magnification formula that gives you a magnification with any given telescope and eyepiece.

Magnification = Telescope focal length / Eyepiece focal length

Example: 48x = 1200 mm / 25 mm

Magnification = Aperture in mm / Exit pupil

Formula Factors Required for the Calculations

Telescope Focal Length

The telescope focal length is a distance measured in millimeters and tells you how far light rays must travel from the center of the objective lens or the primary mirror before they meet or converge. Where this occurs is called the focal point. Parallel light rays will fall onto a focal plane. This is the point where an eyepiece is used to see a formed, enlarged image of the object.

Eyepiece Focal Length

The focal length of the eyepiece is expressed in millimeters, and it’s usually the primary factor you’re looking for when using or buying eyepieces. Longer focal lengths imply a lower magnification while a shorter focal length will imply a higher magnification. The magnification provided will vary between telescopes with different focal lengths.

Aperture

This is the diameter of the objective lens in a refractor usually expressed in millimeters. It is the diameter of the primary mirror in a reflector usually expressed in inches. There may be some conversion between inches and millimeters and vice versa necessary to complete the calculations.

Exit Pupil

Exit Pupil = Eyepiece focal length / Telescope focal ratio (f/number)

The exit pupil pertains to the eyepiece. It is the diameter of the circle or beam of light that you can see if you were to look through it pointed up towards the sky. However, to calculate the exit pupil also requires that you know your telescope specs. The exit pupil will change depending on what telescope you’re using the eyepiece with.

Ideally, the exit pupil will be larger or at least the same size as what the human pupil can dilate to. This is somewhere between 5 mm to 7 mm depending on various factors that includes your age, quality of vision, etc.

Focal Ratio

Focal Ratio (f/number) = Telescope focal length / Aperture in mm

This is a telescope feature that mostly applies to imagers as it provides an indication of optical speed. Slow f/numbers of f/11 and higher produce a larger image scale with narrow fields of view. Faster f/numbers of f/5 and lower produce a smaller image scale with wider fields of view.

Visually with apertures of the same size, this is of no consequence, however, it’s essential for imaging.

Dawe’s Limit

Dawe’s Limit = 4.56 arcseconds / Aperture in inches

This is a formula that was provided by William Rutter Dawes in 1867. The Dawe’s Limit is 4.56 arcseconds or seconds of arc. This means that a telescope can provide up to a maximum of 4.56 arcseconds of resolving power in order to resolve adjacent details in an image.

For example, how much resolution does a telescope require to see two stars closest together and still see two stars? 4.56 arcseconds. Using the formula above, you can calculate your theoretical resolving power.

It’s important to note that most amateur telescopes will not be able to achieve optimal resolving power as other factors like star magnitudes, telescope performance, your vision quality, atmospheric seeing, and more affects resolution. So, don’t get too obsessed with the math.

TFOV & AFOV

TFOV = AFOV / Magnification

True field of view (TFOV) and apparent field of view (AFOV). The AFOV is a specification provided by the manufacturer that tells us how much “space” we can see when looking through the eyepiece when it’s not attached to the telescope. It is reduced when we attach it to a telescope.

The TFOV is the actual field of view that you would acquire when using that eyepiece with the telescope. FOV is usually expressed in degrees. The higher the number, the wider the FOV which means you can fit in large and entire objects or a wider space to fit in many objects within the “circle” that you see through the eyepiece. The smaller the number, the narrower the field of view which means you will have less “space.”

The Atmosphere and Telescope Magnification

Between the types of light pollution, industrial pollution, inclement weather, and the atmosphere, there’s a lot that can quickly get in the way of making a productive observation. You know what they say, you want to avoid an astronomer when there are clouds about – they’re likely in a bad mood.

This is why many will pack up and move their telescopes to a dark location outside city limits where there are less contending factors to deal with. What you’re left to contend with is air turbulence and the Earth’s atmosphere.

Clear, dark skies are ideal, but if your aperture is rather small, turbulence can turn those bright point sources into twinkling blobs intermittently whereas a larger one might just present a blob the entire time. To get a night when turbulence is low and seeing conditions are ideal seems like a tall order to fill.

If you can’t see clearly with your low power eyepiece in these conditions, a higher power eyepiece will not improve things. In fact, it will make it worse. Increasing magnification when seeing quality is poor will not solve your problems.

There are tips you can utilize to help improve factors within your control like waiting for planets and objects to be at least 30 degrees above the horizon where the atmosphere starts to thin out. Dress warm to take advantage of those cold, Winter nights that may offer clear skies and longer observations. Be willing to travel away from the city to maximize performance and productivity.

Telescope Magnification and Targets

What are you trying to see? More power isn’t always necessarily better, however, with the quality of eyepieces available, especially with fast telescopes, many large Dobsonians are not limited to making the most out of high powers and optical quality as it can achieve.

This brings us to the common saying of, “the highest useful power of a telescope is 50x per inch in aperture.” Essentially, it’s like saying a telescope with a 4” aperture can make use of 200x magnification. Fact? No. True? Maybe sometimes.

Really though, what is “useful?” Some small telescopes may be able to push that theoretical limit higher, but is it showing you “more” than what you can see at 50x or even 30x per inch in aperture? Not necessarily.

Some larger telescopes will bottom out at around 30x per inch due to either seeing conditions or optical quality regardless of the fact that they “should” be seeing more at higher magnifications. Larger scopes are more affected by atmospheric seeing while smaller ones are not as affected.  

Just as important as magnification is contrast, resolution, context, and exit pupil. In its relation to targets, let’s explore a the two most desirable targets.

Planets

To view the planets with considerable detail would require about 20x to 30x per inch in aperture. This is what the experts hover around. Increasing magnification beyond the point of achieved optical sharpness, clarity, and resolution would only degrade quality. “More” power doesn’t mean improved clarity.

While planets are bright, their features are hard to see because they’re low contrast. Adding more power would decrease contrast on already low-contrast targets. It’s best to use medium to high powers as far as the seeing conditions allow. You may have to come down in power to get a sharper image at the cost of a smaller image scale.

Refractors may provide excellent seeing of planets with their smaller apertures because of the good contrast and can make use of the entire aperture. However, the chromatic aberration may detract from sharpness and visibility.

Reflectors suffer from secondary obstruction, so they don’t get as much light as their unobstructed refractor alternatives. But, they don’t suffer from chromatic aberration and can be made larger in size at cheaper price points.

Galaxies and Nebulae

There is no substitute for aperture here. These are challenging to see as they’re very faint and their structures are spread out regardless of the fact they have millions of bright objects like suns. This is a perfect example of why observing DSOs (deep-sky objects) can be both a frustrating and rewarding experience.

The telescope and eyepiece must be able to deliver exceptional quality in seeing details and structures, and a large aperture is able to collect more light. High powers will help to provide increased contrast against the context. That dark, black background will work in your favor to help make constellations and other features stand out. High-power DSOs include double stars, small galaxies, globular star clusters, and small open clusters.

While seeing conditions may not affect your ability to locate the galaxy, it will affect your ability to observe its features and details. So, while high power is desirable, it will be limited to seeing quality.

Another factor is the size of the object measured in degrees. If it’s wider than 1-degree, you’ll want to use low power to widen the field of view so that you can see the entirety of the DSO within the eyepiece. This includes open clusters, diffuse nebulae, and large galaxies.

Where 50x to 100x magnification may get you a closer look at a stars features, you may need to come down to 20x or 30x to see the larger picture. Of course, this will take some experimentation and tweaking for any given night as these are not hard and fast rules.

Accessories that Change Telescope Magnification

There are additional accessories other than eyepieces that can change the magnification. Let’s quickly review a few of these.

Barlow Lens

There’s a common myth that Barlow lenses degrade optical quality. This is true of cheap, poorly-made Barlows that are not well-suited to use with modern eyepieces. The good, high-index Barlows can also provide good correction of astigmatism at the edges of the field of view.

What is a Barlow lens?

A Barlow is a telescope accessory that goes into the focuser or diagonal in place of the eyepiece. The eyepiece is then inserted into the Barlow. Having been placed before the eyepiece, it’s within the optical path and essentially does two things:

  • Increases magnification of any given eyepiece
  • Lengthens the focal length of a telescope

For example, a 2x Barlow will turn a 400 mm focal length telescope into one that behaves with an 800 mm focal length while the eyepiece has been doubled in magnification.

Another benefit has to do with eye-relief. Generally, shorter, high-powered eyepieces are uncomfortable to use, even for those who don’t wear glasses, because you must press your eyeballs right up against the cups to see the entire FOV. Does not sound enticing, right? But, it’s what we do to get that higher power.

A Barlow helps you use a shorter eyepiece more comfortably or a longer eyepiece with a little more forgiving eye relief. Since it does increase the effective eye relief, this could be an issue if your eyepiece already had generous parameters as it could kick you out of the “sweet spot” needed to see the entire FOV.

Focal Extenders

Focal extenders are a type of Barlow lens usually with better quality due to the additional glass elements used to correct for optical aberrations. This can include astigmatism, light-loss, and chromatic aberration.

Like Barlows, they increase the magnification and focal length of any given eyepiece. One difference is the fact that focal extenders do not change the effective eye relief of an eyepiece.

Due to the additional glass elements, focal extenders can be slightly heavier than a Barlow. Take this into account since your total loads (focuser, diagonal, extender, and eyepiece) can affect your imaging or stability quality.

Telecompressor or Focal Reducer

This type of telescope accessory provides the opposite effects of a Barlow and focal extender. Instead of increasing magnification and focal length, it essentially reduces it. As a result, you also get a wider field of view.

For visual use, it’s really not an important accessory to have. However, for imaging, they prove to be useful to speed up exposures and allow cameras to come to focus.

How do You Choose Magnification?

After all of that, you can clearly see that telescope magnification calculators and charts are extremely helpful in determining the optical parameters and limitations of your eyepieces and telescope. However, it’s also clear that things are not black and white. There are a lot of specs that are intertwined with using magnification and that it can change depending on the eyepiece, telescope, and other accessories.

How do you choose what is the right magnification for you?

Experience. It can be said on paper that X magnification is great for X type of astronomy, but you may find that seeing conditions will be the judge of that. The telescope’s aperture will have a lot to do with how much power you can use, but most of the time, the maximum useful magnification will be much less than what is claimed that you can achieve.

On a rare night, you may be able to push things past what you thought you could ever do. Most nights, you’ll be switching between eyepieces or accessories to find the right balance. Beyond that, ensure you have set yourself up to make use of whatever powers you can achieve. Buy a telescope that can be collimated, buy quality accessories, let your scope acclimate for 30 mins to an hour before observing, and do your research on when it’s best to view specific targets.

It comes with practice, trial and error, and the willingness to try and borrow various eyepieces to see what works best for your setup and goals.

Getting the Bigger Picture

Delivering images to your eye in a magnified form of objects millions of lightyears away is a tall order to fill. Light passes through the atmosphere, then through the telescope, and then is delivered to your eyes. There’s a lot going on to make that happen.

Think about it. Light is traveling across space, penetrating Earth’s atmosphere, various forms of pollution, and then through a series of glass and mirrors to then pass through a human’s built-in lens, filter, intensifier, and even a windshield wiper (eyelids) to send these signals to the brain – it’s a marvelous process.

To appreciate the complexities involved with magnifying an image through a telescope requires you to get the bigger picture. It may be a lifelong journey of trying to understand telescope components and how they work and tie into each other, but you’ll come into your own with experience.

You’ll know the right magnification on any given night on any given target when you see it. Perhaps it’s a good idea to keep an observational log, yeah? Good luck!

Further Reading:

About Fern

І lоvе nаturе аnd thе іnfіnіtе bеаutу wе аrе ѕurrоundеd bу. Тhеrе іѕ nоthіng mоrе саlmіng аnd уеt ехhіlаrаtіng аѕ lооkіng dеер іntо thе unіvеrѕе аnd ѕееіng thе rаw bеаutу аnd соmрlехіtу оf оur gаlаху. Whеn І аm nоt dоіng thаt І lоvе tо rеаd, lеаrn аnd еmроwеr mуѕеlf. - F