How can I find out the maximum magnification of my telescope?

How can I find out the maximum magnification of my telescope?

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I have a Meade DS-2080AT-LNT. It's an inexpensive scope that I've had for several years. It has a couple of eyepieces with it, but nothing that will let you get a really good look at planets. I doubt I can see any Messier objects as more than a blip.

But it's more than good enough to take up and leave at my family's cabin in the far North and I think all of the kids that visit will enjoy it. But to get more out of the scope I was thinking about getting a Barlow lens. I see lenses of 2x and even 5x for sale. I don't know enough about telescopes to know what "x" of Barlow I should get for this scope.

The specs are as follows:

  • 80mm (3.1") Aperture: Plenty of brightness to reveal planets, clusters, nebulas and more.
  • Sturdy Fork Mount: Lightweight and solid aluminum mount and tripod for stable views.
  • 494 AutoStar® Controller: Automatically locates more than 1400 objects and points the telescope toward them for you at the push of a button.
  • Series 4000 Super Plössl 1.25" Eyepieces: (26mm and 9.7mm) Enjoy low and high power viewing with crisp, wide fields of view.
  • Altazimuth Set-up: Easy-to-use mount moves up/down, left/right.
  • SmartFinder™/Red Dot Viewfinder™: Makes stars and other objects easy to find. Electronic level sensor, north sensor, and precision internal clock help get your scope aligned with the heavens quickly.
  • AutoStar Suite™ DVD: Amazing planetarium software and instructional video will help you learn about the night sky and how to use your telescope. Print out star charts. Plan observing sessions. Displays over 10,000 night sky objects. Operates on any Windows®-based PC

I appreciate any help!

The magnification is given by (focal length of telescope)/(focal length of eyepiece). The spec I saw said the telescope focal length is 800mm, and the maximum magnification is 160x, so to get that would need a 5mm eyepiece.

Barlow lenses have the effect of increasing the telescope focal length: the 2x barlow would therefore give an effective focal length of 1600mm, and using the 9.7 mm eyepiece would give magnification of 160 (approx). However be aware that the occasions when 160 magnification gives really clear views will be quite rare, as it is at the limit of what an 80mm scope can achieve. The rule of thumb for maximum useful magnification for a good scope is 50x the diameter of the objective lens (in inches).


Maximum Useful Magnification:

The maximum useful magnification of a telescope is the highest magnification that a particular telescope can handle. This is a fairly theoretical limit, and in reality is usually much less. The highest useful magnification is more likely to be around 20-30 times the diameter, instead of 60. This limit of useful magnification is due to turbulence in the atmosphere, often referred to as “seeing“.

One thing to keep in mind is that the higher the magnification, the darker the image will be. Planets are most often viewed under high magnification, while DSO’s (deep space objects) are viewed using less magnification.

Max Useful Magnification = ( 60 ) × ( D_scope[inches] )

My telescope has a 130 mm diameter which is roughly 5 inches. Therefore…

Max Useful Magnification = ( 60 ) × ( 5 ) = 300x

Max Useful Magnification = ( 20 ) × ( 5 ) = 100x

Lowest Useful Magnification:

The lowest useful magnification is the lowest magnification a telescope can utilize while having an image that is still resolvable to the human eye. Low magnification is great for observing large areas of rich star fields and for pulling details out of DSO’s. The general rule is 3 – 4 times the diameter of the telescope. Any less than this will cause the exit pupil of the eye piece to be larger than the pupil of your eye. The result is an image that your eye can’t adapt to.

Using the same telescope specifications as above ( 130 mm diameter ≈ 5 in ).

Lowest Useful Magnification = ( 4 ) × ( 5 ) = 20x

What this means is that it would not be wise to use an eyepiece with this telescope that provides less than 20x magnification.

As a beginner, you&rsquore going to be keen to get a handle on everything out there in that night sky.

A bit of everything to start with would be the Moon, some planets, galaxies, nebulae, globulars, and open clusters.

The Moon View

The distance to the Moon is just over one light second and the Moon is second to the Sun as the brightest object in our sky. So, it&rsquos a good place to start and because it is so close, you should easily see its features.

Craters, the Maria, and rilles (long narrow depressions in the surface) are features you&rsquoll see. Your first impression of the Moon will be that craters characterize its surface. Then you&rsquoll see the many other features.

We always see the near side of the Moon. Because the Moon&rsquos rotation is in synchrony with the Earth that&rsquos the side we see. We never see the far side of the Moon.

Tip: In viewing the Moon, you&rsquoll get a brighter image and more detail with a higher aperture size.

The Planets of Our Solar System

There are seven planets in the solar system other than the Earth. Most can be easily spotted by the naked eye.

Mercury and Venus

These two planets are relatively close to Earth. Mercury is the smallest planet in the solar system.

Through binoculars or a travel scope, you&rsquoll likely see these planets as small disks and you&rsquoll be able to observe their phases, but, you won&rsquot necessarily see detail without using a telescope suited to viewing planets.

Mars is the second smallest planet in the system. On average, it is 12.5 light minutes from Earth.

Mars will appear as a small reddish disk. If you want to see more you&rsquoll need to invest in a good quality eyepiece and have decent magnification with clear atmospheric conditions. With this, you may see its tiny white-ish pole and you may see tiny dark markings.

A telescope to see Jupiter

A better view will be seen with an 8&Prime aperture and taking time to observe.

On first look, you&rsquoll see a whitish disk and possibly some brownish bands. Relaxing your eyes and allowing them to respond to the faint markings can present you with an observation of the different shades of the Jovian disk. Jupiter&rsquos four main moons will appear like very bright stars.

You will see more detail of this planet as you get better at visually observing it and the more you observe, the more you will get better at it. More detail on this in our article on seeing Jupiter through a telescope.

Using a telescope to see Saturn

Take your time and with a 3&ndash8&Prime telescope you will pick up the rings of Saturn and up to five of its moons. With the larger telescopes in this range, you may even see on a good night, Cassini&rsquos Division, which is a black circle appearing between Saturn&rsquos A and B rings.

Uranus and Neptune

Of our solar system planets, these are farthest from Earth.

They are not visible with the naked eye. They will appear as specks through the telescope.

With decent magnification, they may appear as disks and you may detect some color, such as a pale blue or greenish color.

The Sun

You can see sunspots and the internal structure with proper white light filters. Warning: Never look at the Sun without proper filtration.


Alpha Centauri is the next closest star system to Earth. It is some 4.4 light-years away.

Through a home telescope, Alpha Centauri A and Alpha Centauri B, the two brightest stars of the Alpha Centauri system, can be seen, but you wouldn&rsquot see Proxima Centauri, which is the third but faint object of this system.

Galaxies and nebulae

These will appear fuzzy. However, by training your eye with practice and with much patience, you may eventually see more.

Filters can help with viewing nebulae, especially narrowband ones like UHC and OIII filters.

These deep space objects can be classified in three ways:

For these guys, as a rule of thumb, the attributes most relevant are surface brightness and size.

How to: Finding Telescope Magnification

Finding your telescope’s magnification is an important part of a purchase process. However, working out the magnification of your telescope or working out the magnifications when choosing a telescope to buy can seem difficult and confusing. Most people generally look for higher magnifications when looking for a telescope but have no idea how to find the magnification. Don’t worry guys, you’re not the only one. In fact, working out the magnification isn’t that hard at all. Most of them are easy to obtain with the information given on the specifications list. Just add a little maths! This process also enables you to get to know your telescope more.

Magnification = Focal length (mm)/size of the eyepiece(mm)

If the Focal length was not provided and you only know the focal ratio

Focal length=F-ratio x aperture of your telescope

Example 1:

Saxon 1309EQ2 Reflector Telescope w/Steel Tripod

From the above specifications, we gather that the focal length the Saxon 1309EQ2 is 900mm.

Three eyepieces are supplied with this telescope, the 4mm, 10mm and 25mm.

When a 25mm eyepiece is used:

When a 10mm eyepiece is used.

When a 4mm eyepiece is used.

Example 2:

Celestron Nexstar 8 SE Computerised Cassegrain Telescope

We can see from the specifications list the focal length of this telescope is 2032mm

Magnification = Focal length(mm) / Eyepiece size (mm)

With a 25mm eyepiece used:

**Other eyepieces can be used to reach much higher magnification**

Example 3:

Saxon Hyperion 1021EQ3 Refractor Telescope

We can see from the specifications list the focal length of this telescope is 1000mm

Magnification = Focal length(mm) / Eyepiece size (mm)

Is higher the magnification the better?

**As shown in the above examples, finding telescope magnification is not difficult after all. However, please keep in mind that a telescope’s magnification is not everything. Is higher the magnification the better? The answer would be it depends, the ideal type of telescope for you depends on your needs and what you want to see.**

For lunar and planetary observation, the refractor would be the ideal choice as the refractor is made of lenses and is able to provide images with higher clarity when viewing to moon and the other planets.
For fainter stars and deep-sky objects like the nebula and galaxies, reflectors are the ideal choice since they’re relatively inexpensive to reach the aperture needed to view those objects.
For much fainter stars and deeper space objects the cassegrain telescopes are the ones to go for since they have more efficient light gathering power as oppose to the reflectors, the cassegrain telescopes are more compact, requires a smaller aperture for the same result a reflector would deliver.
Highest Practical Magnification

However, we also need to be mindful of the Highest Practical Magnification. Typically, on an average viewing night, the highest practical viewing power for each inch of aperture of your telescope is around 25x-30x magnifications. This means for a 3 inch (76mm) aperture telescope, the highest practical magnification on an average night is 90x magnification etc.

Maximum magnification

I have a choice between a 2x and a 3x barlow(with a 10mm eyepiece) and I will buy later on a zoom eyepiece. Also I should note that I will buy a sturdier telescope in a few months.

#2 DLuders

You may know that Magnification = (Focal Length of Telescope) / (Focal Length of Eyepiece), so:

With 10mm Eyepiece and no Barlow Lens, Magnification = (650mm) / (10mm) = 65x.

With 2x Barlow, Magnification = (650mm) x 2 / (10mm) = 130x.

With 3x Barlow, Magnification = (650mm) x 3 / (10mm) = 195x.

There is an astronomical "Rule of Thumb" whereby the Maximum Effective Magnification of a telescope is 30x-50x the diameter of the Primary Objective (measured in inches). 130mm Objective = 5.1 inches, so

For "average" Seeing Conditions, 30 x 5.1 = 153x (say, 150x). So, a 2x Barlow used with that 10mm Eyepiece would be suitable.

For "better-than-average" Seeing Conditions, 40 x 5.1 = 204x (say, 200x).

For rare, "perfect" Seeing Conditions, 50 x 5.1 = 255x (say, 250x).

If you over-magnify, the resulting image will be an overblown, blobby mess.

Edited by DLuders, 13 April 2021 - 09:58 AM.

#3 ShaulaB

If I read this correctly, you want to know if you should buy a 2x or 3x Barlow lens? I would only use a 2x, and not very often. Actually, you can see Saturn's rings, features on Jupiter, etc. using only your 10mm eyepiece, 65X,

Hopefully, you have a 20mm-ish eyepiece also. Get to know the deep sky objects available with lower powers.

As you said, with the shaky tripod, and I assume no motorized tracking with the mount, going to higher magnifications to view planets would not give you the best views.

BTW, usually the aperture goes first when describing a telescope, so 130/650 please.

#4 sg6

Expect every answer numerically possible, sorry.

I would say 130x, basically an eyepiece of focal length equal to the f number. The numbers work nice that way.

Will also say that to me at least the most you want is the see Saturn decently and my best Saturn was at 125x.

Most DSO's will be better at the 60x maybe 80x but using M42 it is one and a bit degrees, so in a 60 degree eyepiece that means 60x or less, say 40x. The double cluster is again 1 degree so again 40x to 60x.

You will find that the maximum it might be capable of is very rarely used. Also the"rules" given are not actually concerned with the capabilities of the scope but to operation of the eye.

Hate to say it but in some ways Astronomy is a form of Science Fiction. There is both some science in there and some fiction.

But the easy answer is a values around equal to the aperture in mm. It may deliver more, maybe not always, but that is a good guide. Personally I would put a 6mm in and hold off on a 5mm.

#5 Mr. Pepap

Theoretically, any telescope is capable of unlimited magnification. However, as magnification increases, the brightness goes down. There is a certain point where objects will become too dim to see at some magnification. This point is called the maximum useful magnification. Its formula is as follows:

. where X is the magnification and A is the aperture diameter in inches. The 50 comes from seeing conditions. Use 10 for bad seeing, 20 for poor seeing, 30 for average seeing, 40 for better than average, and 50 for perfect. For example, an 8-inch telescope would have a maximum useful magnification of about 400x under perfect seeing conditions, which turns out to be roughly 50x per inch of aperture. It would be 240x under average conditions, 320x under better than average conditions, etc.

You can then use these numbers to derive magnifications depending on the type of object you are observing.

25x-30x per inch is suitable for stars and planets, giving you roughly 200x max useful magnification (for an 8-inch scope, change up the numbers and plug them in to the formula above to find information on any scope!)

Clusters and nebulas are best observed with about 12x to 15x per inch.

Galaxies are best observed with about 8x per inch.

Edited by Mr. Pepap, 13 April 2021 - 03:19 PM.

#6 Stellar1

Technical aspects aside which have been covered above, you’ll find that whatever magnification you’re looking to use may work on one night and not the next. You’ll figure that out soon enough through practical use, I would set my eyepiece/ Barlow collection to stay within the 30-35x per inch of aperture and even then that’s under good seeing. Magnification and telescopes are a finicky thing, what works tonight won’t work tomorrow as atmospheric conditions change.

#7 SeattleScott

#8 RobertMaples

Hello I have a 650/130 reflector on a not very sturdy mount so I was wondering what is the maximum magnification you can use on planets and the moon on an average night? (ignore the aperature limit)

I have a choice between a 2x and a 3x barlow(with a 10mm eyepiece) and I will buy later on a zoom eyepiece. Also I should note that I will buy a sturdier telescope in a few months.

The problem with answering your question is an "average night" varies greatly from place to place. On most nights I can use 230x on the planets, but can't go much above that. Some people can rarely get above 150x and some people often go over 300x.

#9 NintendoGamer

Thank you for your responses.

I read that at 200x and up you can start to get a good look at the planets (Jupiter Saturn and Mars), so how often would I be able to hit 200x magnification(how often will weather allow it)? I really want to get the best views of planets that I can.

Also off topic question about collimination, since I got my telescope i was wondering how to collimate it since I couldn't find it in the manual nor the internet. The back side where the mirror is is black with no "collimination" "screws", only black plastic cover on which is written "Our telescopes are colliminated out of factory before shipping"

Sorry if I am being stubborn

#10 SeattleScott

Lol collimated at the factory, then a long bumpy ride across Pacific and getting tossed around by mailman. Just assume EVERY Chinese reflector will need to be collimated upon arrival.

You can probably remove a back cover to access collimation screws. Collimation is just routine maintenance so they have to provide access to the screws.

If you provide your location, someone might be able to give you an idea of useful maximum magnification for your area.

Edited by SeattleScott, 13 April 2021 - 11:54 AM.

#11 NintendoGamer

Thanks for the info. I left my telescope at an island but i remember there are 5 black crews around holding the black cover so i guess I'll remove it and try to colliminate. I don't have any laser equipment for it so can I do it without any special equpment for it ?

I guess it isn't a problem sharing my location, I live in Croatia,Zadar.

#12 dmgriff

If the primary mirror is "fixed" in position, hopefully, the secondary has three collimation screws.

#13 sevenofnine

Another way to look at it is the general rule that the highest magnification eyepiece equals the f/ratio of your scope. So if your scope is say f/8 then an 8mm eyepiece is going to be it on average. Like others have said all depends on the sky conditions in your area. In my Northern California area this general rule seems to apply. Good luck with your scope!

#14 Dave Mitsky

The following articles discuss how "seeing" affects astronomical observing:

#15 Barlowbill

If you want a Barlow, get a 2X. If you want a "good" Barlow, get one around 2X.

#16 JohnnyBGood

With my 130/650 scope I find that it's really hard to focus above 160x or so, so that was my limit for a long time. It also needs to have very accurate mirror collimation for sharp high power views.

To begin with I used a 10mm eyepiece with 2x Barlow to get 130x, which isn't too hard to focus. Later I got a 7-21 zoom and used it with the 2x Barlow to be able to adjust the max magnification to match the sky conditions. The sweet spot for focus at high powers is very, very small. Once I added a helical microfocuser I was able to get satisfactory focus up to about 200x (6.4mm eyepiece with 2x Barlow). More than 200x just isn't happening.

Honestly, I'm not sure it's worth spending the money on a modification like that. I only did so because I really love that particular scope. The scope is more built for low power, wide angle viewing and it really excels at that. For looking at the moon and planets I usually use a different scope with a longer focal length, like my ETX90 that has a much easier time focusing at high power.

#17 rhetfield

You may know that Magnification = (Focal Length of Telescope) / (Focal Length of Eyepiece), so:

With 10mm Eyepiece and no Barlow Lens, Magnification = (650mm) / (10mm) = 65x.

With 2x Barlow, Magnification = (650mm) x 2 / (10mm) = 130x.

With 3x Barlow, Magnification = (650mm) x 3 / (10mm) = 195x.

There is an astronomical "Rule of Thumb" whereby the Maximum Effective Magnification of a telescope is 30x-50x the diameter of the Primary Objective (measured in inches). 130mm Objective = 5.1 inches, so

For "average" Seeing Conditions, 30 x 5.1 = 153x (say, 150x). So, a 2x Barlow used with that 10mm Eyepiece would be suitable.

For "better-than-average" Seeing Conditions, 40 x 5.1 = 204x (say, 200x).

For rare, "perfect" Seeing Conditions, 50 x 5.1 = 255x (say, 250x).

If you over-magnify, the resulting image will be an overblown, blobby mess.

I have that same scope. In my midwestern haze, Jupiter and Saturn look good at 130. On a good night, Mars and Saturn are good at 195x, but Jupiter is not.

I bought a 4.5mm eyepiece and a barlow that can do both 2x and 1.5x. This gives me more options. In the case of an OP, a 3x that the lens assembly can come off and screw into the end of an eyepiece would work well. The 3x with the 25mm eyepiece that most 130/650's come with would give an 8.3mm equivalent (about 83x).

#18 gnowellsct

As you get to 1x magnification per mm of aperture (130x in our case) you begin to "stress" performance in a variety of ways.

1. The mount may not be stable enough.

2. Your hand or the mount may not be steady enough to hand track (if you don't have a drive)

3. You reach the limit not just of the sky but of the optic. When you get to 2x magnification per mm and above, as a general rule, only your most precise optics (expensive, as a rule) deliver a usable image. But weaknesses in your less expensive optics begin to emerge.

4. You reach the limit of the focuser. Yes, once you get to > 2x per mm of aperture you're going to find it more difficult to focus. This is why primo refractors have primo focusers with a separate fine focus knob.

5. The good news is that since we have an atmosphere you'll hardly ever be using that 8" (200 mm) at 400x or that 10 inch at 500x.

Keeping expectations to 1.0 to 1.5x per mm of aperture is a more reasonable course. I would say 1x per mm (as someone recommends above) is very conservative, but perhaps realistic, given constraints of mass production. I think the standard advice of planning on 1x to 2x per mm of aperture is relatively good, with the understanding that the upper limit will only be reached very rarely.

Generally speaking aged eyeballs provide another limit, floaters and other gunk become very irritating at 1.5x per mm and higher. It doesn't do you any good to have Jupiter at 2x per mm if there is a soap bubble floating in front of it.

You can do whatever you want with the moon. It is very tolerant of even bad optics. I have had it at 2000x and seen features. Not well, but there are features. But if you're using more than 2x per mm, even on the moon, you're not improving the view.

#19 Tony Flanders

I find 130X useful very often with my 130-mm f/5 Newt, but there are also plenty of times when I get better results using a 4-mm eyepiece to obtain 162.5X. Trying to go any higher than that is generally a recipe for frustration neither the optics nor the focuser can handle much more than that.

So I would say with a 10-mm eyepiece you want either a 2X or 2.5X Barlow.

#20 NintendoGamer

Thank you very much for the responses. In that case I will be getting a 2x barlow, and possibly later a zoom eyepiece which will extened the magnification limit a bit.

#21 Joe in Gatineau

Forget about maximum magnification for now - until you can get a sturdier mount, it is mostly irrelevant. There are many ways do steady a mount, ranging from hanging a weight from the centre column, to fabricating beefier legs. I don't know what you have, but you can find many examples and resources on this site.

Good luck & keep looking up!

#22 gnowellsct

Thank you very much for the responses. In that case I will be getting a 2x barlow, and possibly later a zoom eyepiece which will extened the magnification limit a bit.

Yes good choice, but these comments indicate a fascination with magnification. Which is very common in the beginners' forum, and to be expected. Cheap telescopes come in boxes which stress the magnification. Cut outs from Hubble space photographs bedeck the box. Expensive telescopes come in brown boxes, maybe with a logo, that don't say anything at all.

Magnification fixation quickly wanes as you discover the limits of optics and the limits of the sky and the limits of your eye. It's a lesson we all have to learn for ourselves.

I confess that in my first five or six years I had a strong preference for looking at the Ring Nebula at 300x or higher. But in the past five or six years my preference is to view it at 16x. You see it as a tiny pearl adrift in the sea of the lower half of Lyra. You have to know where to find it at that magnification.

If you want to see more, study the view. That does more than magnification. Especially on planets, once you are up in the 150x to 300x range the gain is in sticking with the viewing over time, catching the infrequent moments of steady sky, and learning to see details.

You could probably save yourself some money by getting a 2.5mm eyepiece and setting your scope up outside and see what you think of the view compared to say a 5 or 10 mm.

All that said, I do occasionally "stress test" an optic with high magnification. I took my 92mm out the day I got it and pushed it up to something ridiculous, like 500x. The views were pretty good. It wasn't any ordinary 92mm. But looking at a distant tree branch is not the same as trying to get a good view of Jupiter. I was looking for false color in the image--chromatic aberration. High magnification on a branch against a blue sky brings that out (in a refractor). But this scope didn't have any.

Eye relief

Eye relief is the distance that you have to hold the lens in your eyeball from the top lens in the eyepiece so that you can see the entire field of view that the eyepiece offers you. The rule of thumb is that the eye relief of a Plossl is generally 2/3 of the focal length. So, a 10mm Plossl will have about 7mm of eye relief, which is okay. However, note that a 6mm Plossl will have about 4mm of eye relief, while a 4mm Plossl will have less than 3mm.

Eye relief that’s that short is pretty tight. And very uncomfortable. Because of seeing conditions, which I described above, you have to hold your eye and head steadily and very still, right up close to the eyepiece, for 5-10 minutes or more as you observe until the atmosphere quiets down and gets real still. Holding your head that still for that long can lead to neck and back strain in those that are prone to it.

Eye relief is particularly important for folks who need to or prefer to wear glasses while observing. Although normally, you can take your glasses off while observing and let the telescope become your glasses, that isn’t true for people suffering from astigmatism. Obviously, if you’re wearing glasses, you can’t get your eyeballs as close to the eyepiece because your glasses are in the way. People who wear glasses while observing are typically looking for about 16mm of eye relief at a minimum 20mm is better.

For eyeglass wearers, as well as the rest of us who just prefer to be more comfortable at the eyepiece, most eyepieces beyond the introductory Plossls have eye relief that is significantly greater than the tighter relief offered by high magnification Plossls: 13mm, 16mm, or more. Again, the eye relief for any particular eyepiece is noted under the Specifications tab for that eyepiece.

5. Try To Use Orthoscopic Eyepiece For Planets

Orthoscopic eyepieces are specifically designed to give you the best possible view of planets. Their focus is very high but only in the center of the eyepiece. I don’t recommend to use them when you want to see a larger object where your eyepiece is full of details like the moon, but they are great for viewing planets. More from the center the viewing object is the more blurry image you see. However, the center is very bright with high focus, so small planets are with a lot of details in high definition.

This is just a suggestion, and it is not mandatory equipment to view planets. You can do that with a regular eyepiece, but if you want to squeeze the maximum from your telescope and you focus only on observing planets, it is a good idea to try them. Keep in mind that orthoscopic eyepiece is not very comfortable to use especially with the glasses and you need to train your eyes to get used to them.

There are many brands you can choose from, and I went for Baader Planetarium Classic Ortho 10mm Eyepiece. I’m very happy with it. It is a decent orthoscopic eyepiece on the budget.

How powerful of a telescope do you need to see Mars?

Looking for a telescope to view Mars with enough power to see its surface features? Telescopes of apertures of 5 inch upwards to 8 inch in a reflector telescope are ideal to see surface color, polar caps, and noticeable dark features of Mars (as well as the moons and bands of Jupiter, and rings of Saturn).

Mars through 8 inch telescope will give you great views but don&rsquot be too disappointed if you have something smaller. You should still see major dark surface areas and polar caps of Mars with a 3 &ndash 5&Prime aperture.

If you are wanting to see clouds you are going to a need an 8 inch reflector and upwards to 14 inch, which isn&rsquot the most practical.

The above is a guide that includes what you are likely to see of Mars through telescope observations based on aperture size. What you actually see, however, will depend on a few variables, including light pollution, quality of optics, and atmospheric conditions.

You may be wondering about power (magnification).

Magnification of telescope to see mars&rsquo features

Magnification is how much a telescope enlarges its subject. You get this based on the focal length of the telescope and that of the eyepiece you&rsquore using.

Magnification = telescope&rsquos focal length ÷ the eyepiece&rsquos focal length.

Mars is a small object and contrast is not an issue so you can go full throttle with the magnification.

This means use the highest useful magnification of your telescope.

As a guide, you can easily work out this maximum useful magnification from the aperture size&hellip

It is expected somewhere within 50× the aperture in inches (or 2× the aperture in mm) of the aperture. Be aware that this is the optimal amount and the amount decreases with declining atmospherice conditions. So include a bit of leeway in this case and try for 25x to 30x the aperture size.

So the power of a telescope to see Mars&rsquo surface depends on the aperture size of your telescope as well as the eyepieces and focal length of your scope. I hope this helps.

Can mars be seen at night without a telescope?

Yes, you can see Mars at night without a telescope. But you&rsquoll need a telescope to see Mars surface (or at least a suitable set of binoculars).

Mars through binoculars

Mars can be seen through binoculars for astronomy. To find out more see my article about selecting the best astronomy binoculars. I also cover tips about using binoculars for astronomy.

Just how big is Mars?

The visual included above shows the size of Mars with respect to Earth. It is about half the size of Earth.

Does Mars have rings?

No, unlike Saturn, Mars is ringless.

What is Magnification

Magnification is measured by how many more times larger an image appears than the naked eye. For example, a magnification of 100x will make the object 100 times larger than what our eyes see.

Figuring out what magnification you need to see planets depends on both your telescope and the choices of eyepieces .

How to Calculate Magnification

Telescope Focal Length ÷ Eyepiece Focal Length = Magnification

Apart from magnification, you'll also want to know your telescope's maximum useful magnification, so you don't use too high magnification. Please contact us if you would like help figuring this out!

How to Calculate Maximum Useful Magnification

Take your telescope's aperture in millimeters and multiply it by two to get your maximum useful magnification:

Telescope Aperture (in mm) x 2 = Maximum Useful Magnification

Even after determining the best magnification, it can vary by clear night, dark skies, and the planet’s best observable nights. Give different levels a try and find the right one for you.

Our astronomy events calendar of 2021 includes the best nights for the planets this year!

Astronomy Formulas Explained with Sample Equations

Math and science both use numbers in their respective field, but how do you think they differ? Here is a quick reference chart at what is covered more in-depth further along.

In Math, numbers are mere figures manipulated and computed to produce another number. In contrast, Science associate numbers with a reference which result in what we call “units of measurement”.

If we recall, Astronomy is the science that studies stars, planets, and other celestial bodies that makes up the universe. This field is not just about staring into the dark skies but rather a scientific method of observing those heavenly bodies.

In studying Astronomy, we have to educate ourselves with the common formulas used in this field. To know these formulas, we have gone over the main formulas needed in a simple discussion with examples for more natural understanding. So, people like me can get it.

Download our FREE equation calculator sheet. It shows some common popular brands and you can put your telescope in to find out all of the equations talked about here. Click here to get to the download.

Focal Ratio

The focal ratio or f/stop is considered as the “speed” of the optics in any telescope or device which uses a lens mechanism. This measurement is computed by dividing the focal length by the lens’ aperture.

Sample Computation:

You have a telescope whose lens aperture is 102 millimeters and focal length of 1000 millimeters.

When solving for the focal ratio, make sure the units of the focal length and aperture are the same so that they will cancel out.

After that, perhaps, your next question would be, which should I prioritize when choosing a telescope? Is it the aperture or the focal length?

If you plan to go for deep space observation, then aperture must be your priority of choosing. However, if it is just space objects in the solar system, the aperture will not be that important.

The next thing you need to consider is the size of the celestial body you aim to observe. For smaller bodies, you need to have a longer focal length and high magnification telescope. Whereas, for larger bodies, you need to have a telescope with shorter aperture and low magnification rating.

A telescope with a focal length of 400 mm will enable you to observe objects such as large clusters of stars, the core of Andromeda galaxy, lunar phenomena (low magnifications) and the Orion nebula.

Meanwhile, a 900 mm telescope will enable you to observe planets in the solar system (Jupiter, Mars, Saturn, and Venus) lunar phenomena (high magnifications) and M57 the Ring-Shaped Nebula.

If you were looking at the aperture specs of a telescope, a 60 mm and 70 mm aperture would be too small for it. The telescope’s aperture affects two factors:

  • Amount of Light: A telescope with a larger aperture is more able to capture huge amounts of light from the object.
  • High Resolution: To have a feasible computation for the maximum resolution of a telescope, you multiply the lens aperture in millimeters by two. Example: 60 mm times two is equal to 120x magnifying power.

A telescope with focal length 400 mm and aperture 10 mm has a magnification of:

Field of View

Field of View (FOV) refers to the amount of space in the sky which you can see in the telescope’s eyepiece. This measurement is either expressed in degrees or a fraction thereof.

When observing through the eyepiece, the amount of space in the sky you see is the True Field of View (TFOV). Understanding this is essential so that you can compare the things you see in the eyepiece with the printed star charts produced by computers.

For instance, two stars separated by one-degree can be viewed at the same time when using a combination of telescope and eyepiece which provides a TFOV of one-degree or higher.

In calculating for the TFOV, you need to grab the measurement of the telescope’s focal length, eyepiece, and the apparent field of view (AFOV). No worries, as these three variables are often specified in the telescope already. Nevertheless, the general AFOV for the majority of Plossl eyepieces is 50 degrees.

Below are the formulas used to compute for the TFOV:

Sample Computation:

For instance, you have a telescope with a focal length of 1200 mm and an eyepiece with a focal length 25 mm. The AFOV specified in the Plossl eyepiece of the scope is 50 degrees. Compute for the TFOV of the scope.

Based on the computations, the true field of view for that specified scope is 1.0417 degrees. You can also express this answer in arcminutes by multiplying 1.0417 by 60 equals 62.50 arcminutes or 62.50’.

What Are Some Setbacks From This Method?

  • The eyepiece is a spherical surface, and calculating the TFOV assumes it is a plane surface. Consequently, the translation of AFOV to TFOV is not that accurate and expected to have some distortions even if the AFOV is accurately given.
  • It is a typical case where the specified AFOV in scope would differ around +, or – 5 degrees.
  • The specified and actual focal lengths of the telescope and the aperture differ by around 5 percent. This slight variation can affect the real true field of view perceived by the observer.

Despite these setbacks, still, this computation provides a good estimation for the actual field of view for any scope. It is a practical and straightforward method.


The magnification or power of any telescope is its capacity to enlarge the size of objects. Comparing, magnification, and field of view, the latter is most important when choosing a scope.

Computing for the magnification of a telescope is direct and straightforward. Below is its formula.

Sample Computation:

For instance, you have a telescope with a focal length of 1000 mm and an eyepiece with a focal length 20 mm. Compute for its magnification.

Exit Pupil Diameter

Exit pupil diameter is frequently confused with magnification, but it shouldn’t be because of the difference in their concept. This refers to the size of the light cone which exits from the eyepiece and enters the observer’s eye.

The formula for Exit Pupil Diameter:

Sample Computation:

For instance, you have a telescope with an aperture focal length of 100 mm, eyepiece focal length 10 mm, scope focal length 500 mm, and magnification power of 50x. Compute for the exit pupil diameter of the scope.

Theoretical Maximum Magnification

The theoretical maximum magnification of a telescope refers to the ceiling limit of its power to enlarge objects. This can be computed by multiplying the scope’s aperture in inches by 50 or aperture focal length in millimeters times two.

Consequently, a 3-inch scope has a theoretical maximum magnification of 150x, whereas a 5-inch scope has 250x theoretical maximum magnification power.

Sample Computation:

For instance, you have a telescope with an aperture diameter of 100 mm. Compute for its theoretical maximum magnification.

Why Can’t We Magnify Objects As Large As It Needs To Be?

There are two main reasons why this is not possible.

  • The telescope itself limits the amount of light coming into the eyepiece and its magnification.
  • The telescope itself limits the accurate resolution of the object’s details as it is being enlarged tremendously.
  • The Earth’s atmosphere is one major factor affecting the accurateness of your viewing. If the atmosphere is turbulent, then you would experience a hard time seeing a clear image of the object.

Minimum Magnification

If there is a theoretical maximum magnification, its counterpart is the minimum magnification. This feature is based on the exit pupil diameter of the telescope.

Exit pupil diameter is the size of light which exits the eyepiece and enters the observer’s eye. To have a brighter more vivid image, the exit pupil diameter should be larger. Anything over 7mm is wasted due to the human eye can not accept more light than the diameter of a dilated pupil.

If there is a rule of thumb for theoretical maximum magnification, there is as well for minimum magnification. The rule of thumb for minimum magnification, a focal length of aperture in inches multiplied by 3.6 or aperture in millimeters divided by 7.

Sample Computation:

You have a telescope with an aperture focal length of 4 inches or 101.6 mm. Compute for its minimum magnification.

Resolving Power

Resolving power of a telescope or any optical instrument is its ability to measure the slightest distance between two objects which can be observed as two separate things.

A telescope with high resolving power can allow you to clearly see the smallest minimum distance between two objects that are close to each other.

Below is the formula for calculating the resolving power of a telescope:

Sample Computation:

For instance, the aperture width of your telescope is 300 mm, and you are observing a yellow light having a wavelength of 590 nm or 0.00059 mm. Compute for the resolving power of the scope.

Stellar Magnitude Limit

The Stellar Magnitude Limit of the scope refers to its ability to allow the observer to see the faintest star in the sky.

Hundreds of years ago, the brightness of a star is expressed in “magnitudes” by Astronomers. This system was invented by the Greeks, where the brightest star is the first magnitude while the star with the weakest light is the sixth magnitude.

This method of measurement was very vague and broad, but when telescopes and other scientific inventions came, the accurate measures were slowly established.

Light grasp (GL) is the term for the amount of light a telescope can collect in its optical system.

The formula for the GL is:

GL= light grasp
DO= diameter of the objective
Deye= diameter of the eye pupil

Meanwhile, the formula for the Stellar Magnitude Limit is:

Sample Computation:

You want to observe a particular star with a magnitude of 8.8 using your current telescope with an objective diameter of 100 mm. Do you think your telescope is capable of viewing this star?

Based on the computation, the magnitude of the faintest star your scope is capable of viewing is 12. Consequently, the star with magnitude 8.8 can be viewed using your current telescope.

Need More?

Stargazing or observing the heavenly bodies can be an enjoyable and fun learning experience. To fully understand and appreciate this field though, you do need to have certain math skills (sorry), which are relevant to properly selecting the telescope which will better suit your needs.

Likewise, this field definitely sharpens your mind and imagination as you explore things out there. If you liked this article, you might like these two articles, “What is a Refractor, or What is a Reflector Telescope.” Astronomy and telescopes are scientific “things,” and you just can’t do science without math.