What are the differeent sizes of telascope eyepieces mean i.e. 2,5 mm 35 mm ect?

Question

What are the differeent sizes of telascope eyepieces mean i.e. 2,5 mm 35 mm ect?

Answer 1

They put different numbers on them to confuse people and create an excuse to raise prices.

Something to do with focal length, last time I checked.

Answer 2

The magnification power of a telescope is determined by taking the focal length of the objective (big) lens or mirror, and dividing it by the focal length of the eyepiece.

A typical 8" telescope will have a focal length of 2000 millimeters, or 80 inches.

With a 40mm eyepiece the magnification would be: 2000/40=50

At this power, you may be able to see the entire full Moon. The rings of Saturn will be visible, and you'll see the four larger moons of Jupiter, and some detail on Jupiter.

With a 25mm eyepiece you would have 80 power. At this magnification, you could expect to see slightly more than half of the full Moon, and you'd start seeing some detail in the rings of Saturn and the bands of Jupiter. The four larger moons of Jupiter star looking more like disks instead of points of light.

With a 12 mm eyepiece you would have 166 power. At this point, if the atmosphere is still, you will see Cassini's division in the rings of Saturn. If the atmosphere isn't still, you'll start feeling like you're looking through an aquarium.

With an 8mm eyepiece you can expect see more detail on the planets, but bear in mind, as power goes up, your need for a still atmosphere increases.

A 6mm eyepiece is usually the highest power you can expect to get a usable image with on a good (but not great) night. This is 333 power in our hypothetical telescope. If the air is

really clear, the things you saw at lower power will be bigger.

Naturally, if your telescope has a shorter or longer focal length, the math will be different.

Most experienced observers use the lowest power that will let them see the detail they're looking for. It makes finding objects easier, the telescope vibrations are less of a problem, and it's easier for your eyes to compensate for a smaller object than for a blurry one.

On the other hand, higher power will tend to darken up a light polluted sky to give you more contrast, so if you're in a very light polluted area, you might prefer wider field eyepieces at somewhat higher power.

As eyepiece focal lengths get shorter, the eye relief (distance between the eye lens and your eye to see the entire field of view) tends to get shorter too.  Designers can compensate for this, but it adds extra elements to the design, costs tend to go up, and more air to glass surfaces from extra elements tends to give more "light scatter" (halos around bright objects)

If you’re a beginner, you probably want to start out with two or three eyepieces. If you shop around, you’ll be able to find some reasonably good Plossl eyepieces for about $40 each. Space your eyepieces so you can step up and down in power by 50 to 100% at a time. If you can afford to buy more than four eyepieces, consider buying better quality ones

instead.

Answer 3

Those are probably the focal lengths.  The focal length of an eyepiece is the distance over which the eyepiece converges a parallel beam of light.  If you're holding an eyepiece upside-down over an ant, it's the distance that must lie between the eyepiece and that ant, in order to fry the ant.  (Actually, it's not frying, unless you've doused the ant in cooking oil.  Maybe we should say, "in order to roast the ant"?)

More seriously, do not point your telescope at the Sun (without a proper filter, placed in front of the telescope--the end pointed skyward).

Incidentally, an eyepiece is not a single piece of glass the way an ordinary magnifying glass is; it's multiple pieces of glass arranged just so.  Some eyepieces may cause light to converge *within* the eyepiece before it converges a second time outside the eyepiece.  Focusing the sun's light with such an eyepiece can cause heat to build up inside it, possibly ruining it, so please don't try this experiment unless you know what you're doing.

DO NOT POINT YOUR TELESCOPE AT THE SUN.  I can't emphasize this enough.

Anyway, the eyepiece goes into the focuser tube of the telescope, and the magnification yielded by this combination is the focal length of the telescope objective, divided by the focal length of the eyepiece.  So, if the focal length of the objective is 900 mm, and that of the eyepiece is 25 mm, the magnification is 900 ÷ 25 = 36x.

There is another property of eyepieces, called eye relief, which is the distance that you must hold your eye from the eyepiece when you are NOT POINTING THE TELESCOPE AT THE SUN, in order to see the entire field of view.  This varies from eyepiece to eyepiece, but for most introductory eyepieces, it's about three-fourths of the focal length, give or take.  This means that if you're using a 4 mm eyepiece, you better take off your eyeglasses, if you wear them.  There are various types of eyepieces where the eye relief is a comfortable 20 mm or so, no matter the focal length; these are useful if you like to observe with your glasses on.

Answer 4

The focal length of the lens in the eyepiece.

Magnification is measured by dividing the focal length of the main lens or mirror, by the focal length of the eyepiece.

Therefore, for a given telescope, the highest magnification will be given by the eyepiece with the lower focal length.