How do Telescope lens work and what are the different magnifications?

Question

I have just come into possession of a used Telescope and am having trouble figuring out how it works. I have it assembled but can't see anything through the eyepieces that were provided. there are 3 (25mm, 4mm, 2.5mm) and since it is winter the skies are cloudy and I can't tell which lens is stronger so any info would be appreciated.

Answer 1

The smaller the number on the eyepiece, the stronger the magnification. Therefore, begin with the biggest number 25 mm (it will be the easiest one to focus and the smaller magnification will make it easier to point the telescope to a known object).

Take the telescope outside during the day. (do NOT point it at or near the Sun -- the heat from the sun is enough to damage the scope, the eyepiece and, most importantly, your eyes).

Looking down the tube, point the telescope to so far away objects that are surrounded by other objects.

Not at the sun.

Let's say you found a row of houses three streets away. (far enough away that people will not phone the police to report a peeping tom).
With the higher numbered eyepiece (lowest magnification), try to focus until you see some detail (a wall, a door, a shrub). Remember that the image is normally upside down in a telescope.

Compare what you see (for example, the red house with the blue door) with what you get in the pointer (either a smaller scope, a tiny tube or just a notch outside the main telescope tube). This will tell you how accurate (or not) the pointer is.

Don't do this using the Sun as a target.

When you are doing astronomy at night, always begin with the lowest magnification in order to find the object in the filed of view (telescopes have very narrow fields of view and it is sometimes difficult to get the object in the field of view). Only after you are confortable with following the object at lowest magnification should you try a higher magnification.

Higher magnifications are OK for the Moon and for planets. They will not give you much joy on stars (except, perhaps, to help you separate bright double stars).

In your case, you'll find that going from 25 mm to 4 mm will be quite a jump (the field of view will be cut down to only 1/40 of what you started with. Make sure that the object is well centered before going from 25 to 4 mm. In time, you may want to buy yourself a 12 or 15 mm as an intermediate size.

Oh, just in case I forgot to tell you: don't point the telescope at the Sun.

Answer 2

Telescopes generally fall into three catagories. Refractors, reflectors, and catadioptrics.

Refractors use lenses. Lenses work by collecting "bending" light by a phenomena called refraction. Reflectors, on the other hand, have an open front with a small mirror mounted near the front in the middle of the tube, and a large mirror mounted in the back. The large mirror in the back collects the light and focuses it to the secondary mirror up front, which bounces it into the eyepiece.

Catadioptrics, light schmidt cassagrains, have generally short tubes, a large mirror in the back, and a glass plate up front with a round area in the middle, which is where the secondary mirror is mounted. They work in a similar fashion to reflectors but the specifics are a little different.

For any telescope, the operating principals are generally the same.

1. Take the cover off.
2. Put in the lowest powered eyepiece...this is your 25mm one.
3. Point the telescope at a ground based object in the distance.
4. Looking through the view finder, adjust it using the adjustment screws on it's bracket to get it to be pointing at the same object you see in the eyepiece.
5. Put in a higher powered eyepiece and repeat.
6.Put the lowest powered eyepiece back in.

Once you've aligned the viewfinder properly you can try pointing the telescope at something inthe sky. I'd start with the moon, and then try a bright planet like Mars.

How to tell eyepiece strengths. Eyepieces are listed by their focal lengths. The largest number is the lowest power, and should be the eyepiece which you will get more use out of. The higher powered eyepiece you use, the more difficult it will be to get a good image because you magnify the atmosphere more, you approach the limits of the physical capabilities of your telescope's optics, and the motion of the Earth becomes far more apparent, making it difficult to keep things in your field of view long enough. They are also generally not very comfortable to look through.

Answer 3

Your telescope has a focal length, or a distance from the front lens to the place where the image is focused. Find out what that length is -- it may be marked on the scope or it may appear in the owner's manual.

Divide the focal length by the eyepiece measurement. This will give you the power of the scope.

So, for instance, if you have a 1000mm focal length telescope, using the 25mm eyepiece, you'll be magnifying things 40 times (1000/25=40). Using the 4mm eyepiece, you'll be magnifying 250 times (100/4=250). Again, this is dependent upon your focal length.

More magnification is often not better. Usually it means less light, more distortion, and a telescope that's tougher to point accurately. A 25mm eyepiece is a good general eyepiece. I usually don't use more than a 9mm eyepiece with my 2000mm focal length telescope.

Answer 4

Hi Gary I used to own an Orion 4.5" Starblast. It is a nice little scope, called a rich-field. It is made for wide views at low magnification. Not for high magnification views of planets. I believe somewhere about 75x is the most I used it at.