Minimum limit to telescope magnification

Is there a theoretical minimum limit to how much magnification you can have from a telescope? Could you set one up to not even magnify at all? I know magnification is equal to the focal length of the telescope divided by the focal length of the eyepiece, and it would certainly look odd having an eyepiece as long as the telescope. But would it work?

forget it.

Sure. Heck, you can have magnifications of less than unity. Ever look through a small telescope backwards? In that case, the equation is inverted, because the roles of the lenses are reversed and things look smaller than with the naked eye.

If you have a really big telescope with a magnification of one, you might as well have not have a telescope because its pretty much just like having your eye by itself.

Many people think telescopes are for magnifiying, but light gathering (making faint things brighter) is just as important, if not often more so.

Imagine you have a telescope with a lens or mirror 4 inches across. You eye pupil at night is roughly 1/4 inch across.

The minimum magnification you can have without wasting light is 4 inches divided by 1/4 inch or 16X.

If you lower your magnification to 8X, you’ll only be seeing the light that would be gathered by a 2 inch wide telescope and the rest of the light would be wasted.

First thing I thought of on seeing the OP was looking at something through a used paper towel roll tube.


And can you make a pinhole telescope with a pinhole eyepiece and what would the magnification be?

Oddly enough, this isn’t really true. It’s surprisingly hard to build an optical device that does nothing. Sinclair Optics (which makes an optical ray-tracing program) had a competition to design the lens that did the least. It was amazingly hard – optical systems invariably have aberrations, and the way to fix all your aberrations is to add more elements, so these “nothing” lenses could get quite complicated.
I’ve been studying telescopes with unit magnification recently – they’re surprising devices. If you build a Newtonian Unit Telescope it has extremely interesting properties. It has zero power (since rays parallel to the axis leave with the same slope) and rays that enter at one height exit at the same height. Rays that enter at one angle leave at minus that angle (so if you look at a distant object it will appear the same size, but inverted).

BUT, if you place an object a finite distance in front of the telescope there will be a real image a finite distance on the other side of the telescope. This image will have magnbification of -1, meaning it will be the same size as the object, but inverted. Furthermore, the distance between the object and the image will be fized – it won’t matter where the telescope sits between the object and the image. (So if you place a naked bulb or LED as the object, then place the telescope, then place a screen at the location of the image, you can then slide the telescope back and forth and the image will remain in focus at the same place, at the same magnification. This is essentially how they build periscopes – they use a chain of such 1:1 telescopes to relay the image through the periscope tube. (Or they did, at any rate, until recently. Periscopes nowadays use cameras instead of direct optics viewing).

Bah, that’s easy to build. In fact, I sent you such a device last week. Didn’t you get it?


All interesting stuff…

But why do you think I put the word “pretty” in there?

Can you explain why the light would be wasted? Why wouldn’t I be able to have night vision with a 1x eyepiece in my 8" reflector? And is this true for all kinds of telescopes?

My point, though , is that a unit magnification telescope wouldn’t be pretty much like a window. If you make it Newtonian, the image will be inverted. If you try to build a Galilean, you’ll find that you can’t – that would require one lens to be atop the other. As the OSLO contest points out, even if you cunningly design your lenses to allow it, it still doesn’t come out right. As someone who labored to make coupled Newtonian unit magnifiers work right, it’s not really equivalent to just having your eye there. Any “zero effect” telescope is far inferior to actually having nothing there. The point isn’t that building a unit telescope is pointless – it’s surprisingly difficult. It’s not the sort of thing one easily and nonchalantly does.

If you take an 8" telescope and set it to 1x magnification, the diameter of the exit beam (“exit pupil”) will be 8". It won’t all fit into your eye.

And it’s impossible to amplify light with a 1x magnification telescope of any design, because that would violate the laws of thermodynamics. If you point such a device at a large light source, the beam coming out of the telescope will be more intense than at the light source, which means the object there can get hotter than the source. Energy flowing from a cooler object to a hotter object - that’s a spontaneous decrease of entropy.

Why would it have to be 8"? Why can’t the image be compressed by the primary mirror or lens before going through the eyepiece? Does the eyepiece have to be closer to the mirror for lesser magnifications and right on it for 1X?

If the previous statement is true, I understand this. I don’t understand why the light beam can’t be compressed at 1X though. Why can’t the primary act like a magnifying glass and the eyepiece pull the narrower beam back into focus?

Oh quit being pedantic.

I bow to your superior optics knowledge, even though you’ve told me nothing I already didn’t know.

Two reasons for telescopes used with eyeballs. Magnfication and extra light gathering ability. If you have a magnification of one, you also have zero extra light gather ability. Therefore you it offers no real ADVANTAGE to have one, though I sure we could come up with circumstances that it would.

But, again, the two BIG advantages are GONE.

Nothing wrong with giving more info, ignorance fought and all that stuff, but no use being jerky because someone didn’t respond to an optics question with an essay on optics. The main point still holds.

Are you by any chance familiar with this forum at all? Also, nobody’s being “jerky” here, despite your accusations, least of all Cal Meacham.

[quote=“snailboy, post:13, topic:486035”]

Why would it have to be 8"? Why can’t the image be compressed by the primary mirror or lens before going through the eyepiece? Does the eyepiece have to be closer to the mirror for lesser magnifications and right on it for 1X?


Disclaimer, this explanation is approximate, but good enough to get the point across I hope.

In telescope optics, you have whats called an exit pupil. Its a little “disk of light” thats behind/above the eyepiece. Where you put your eye.

Lets say you have your eight inch aperture/diameter telescope (200mm).

You have your magnification set at 10X. The exit pupil will be 200mm/10x or 20mm in diameter.

You have your dark adapted eyeball. Your pupil has a rough diameter of say 6mm.

Thinking for a moment, it should be pretty obvious that that you arent going to wedge that 20mm disk through that 6mm hole. That 20mm disk represents ALL the light collected by the telescope. The light that CAN’T get through your pupil is wasted light.

As you asked, that 20mm disk is sorta “compressed light”. You’ve taken light falling on a 8 inch disk and output it as a 20mm (.8 inch) “disk”.

Looking at the math, you can see there is a minimum magnification for a particular sized telescope such that the exit pupil isnt bigger than your eye’s pupil. And the bigger the telescope, the higher that magnification number is.

If you set your magnification for 1X, the math works out such that the amount of light that isnt wasted is the equivalent of having a telescope with a 6mm aperture/diameter. So, therefore whats the point? Unless you want you image upside down, or mirror reversed, or with optical defects, or just want to spend money on fancy optics.

Trust me, if amateur astronomers could keep all the light gathering ability of a big telescope while dropping the magnification below whats allowed in the simple relationship described above, there are many circumstances in which they would do it.

Most non telescope savy people think its all about magnification. Amateur astronomers with large telescopes are often using their telescopes near or at the LOWEST magnification they can without wasting light. For them its often about GATHERING light. And some objects in the sky actually look better in smaller telescopes, or larger scopes that have their magnification set WAY low (which can cause other practical problems if carried too far).

As an aside, if you’ve never really got a chance to really play with a good telescope, search out a local astronomy club, go to a meeting or two, and find out when they are having a good star party. Most of the folks love having visitors and showing off their telescopes and stuff in the sky to beginners. Even if the bug doesnt bite you, you’ll most likely be very glad you did it. Just make sure to gasp just right when you see the moon or saturn through a telescope for the first time :slight_smile:

Okay, I didn’t know that. But is there a simple explanation for why that is so? I’ve been wanting to get a larger eyepiece so I can better view large celestial objects, like Andromeda galaxy and the Pleiades. My largest eyepiece currently is a 25 mm which, if I remember correctly, is 44X. I was hoping to get a 40 mm or so. Although they’re hard to find, some places do have them. (That’s basically what spawned this question, the fact that I’m having trouble finding eyepieces that large and larger ones just don’t seem to exist.) But now you’ve got me wondering if there’s some catch to them. I’m not so worried about not being able to get all of the light into my eye as these are bright objects anyway but I’m wondering if my telescope might not even support them.

There are pratical implications to your setup.

Do you have a 1.25 inch setup/star diagonal or a 2 inch ? I take it you have a Scmidt Cassegrain telescope? Or what do you have?

Be aware that you’ll never get as good a view of the Andromeda galaxy as the photos you’ve seen. A photograph (whether film or CCD) can brighten an image just by leaving the exposure open for longer, but there’s nothing equivalent you can do with your eye. So the fainter outer regions won’t show up to your eye, but might in a photo.

It’s an 8" Newtonian reflector as I mentioned earlier. It has a focal length of 1219 mm (f/6). I have 4 eyepieces: 25 mm, 15 mm, 10 mm, and 6.5 mm, along with filters. Like I said before, I’ve looked at getting a ~40 mm eyepiece for viewing larger objects and perhaps finding certain things like Uranus and Neptune. (I’ve found both of them before, but it’s difficult since they look like stars. It would be easier with a wider viewing range though.) I’d get more use out of one than the 6.5 mm. I’m just wondering what my practical lower limit for magnification would be.

And I know a few things about amateur astronomy. I’ve had this telescope for several years and before it, I had a small refractor. It’s not a new hobby to me. It’s just the details of the optics that I’m not so familiar with. Just so you know.