telescope explanations? focal length etc...?

Question

can somebody give me a basic explanation of a reflecting telescope and a refracting telescope please?

i think im right in saying a wider aperture gives a better telescope because it catches more light, is this correct?

however, does the length of a telescope make a difference?
and by extension of that question, will a longer focal length make a better telescope?

when i talk about better im talking about resolution and magnification. i understand what these words mean. so, what effect would it have on these 2 things if i was to increase the focal length of my telescope and why?

and does it work one way for a refractor and one way for a reflector? if so, then why?

Answer 1

Reflecting - cheaper for a given aperture

Refractor - more sturdy and portable

Long focal length - capable of more magnification

Short focal length - brighter image, wider field of view

Resolution - depends on aperture

Magnification = focal length of primary divided by focal length of eyepiece

For a first telescope I like a short focal length refractor. Sturdy, portable easy to use. This would be my first choice (even if it wasn't on sale):

http://www.telescope.com/control/product/~category_id=refractors/~pcategory=telescopes/~product_id=24688

More experienced amateurs often like larger aperture reflectors to see dimmer objects.

I have a small short focal length refractor for portability, and a larger reflector (actually a catadioptric) for home use.

Answer 2

Okay...here goes...

A Reflecting telescope accepts light down a tube which then strikes a concave mirror. The concave mirror focuses the
captured light into a beam which is then reflected by a smaller mirror about 90 degrees over to an eyepiece and focuser arrangement on the side of the tube. The focal length is the total length of the distance from the concave mirror to the smaller mirror and the eyepiece. A long focal length means that the various elements have to be farther apart. A long focal length also means that the mirror will be closer to "flat" than a similar sized mirror with a short focal length.
Flatter mirrors are easier to grind and "sometimes" have fewer distortions in the view that is produced. Reflector telescopes depend upon a very fine silvery coating on the surface of the mirror glass for their performance. Some mirrors even come with an ultra high gloss and protection coating on the mirrors themselves for optimum performance.
Reflector telescope mirrors become rather heavy and unweildy at diameters of somewhere around 12 to 14 inches.
Their cost also goes up dramaticaly above 11 inches. A ten inch reflector telescope might cost $ 2,000 and a 14 Inch reflector telescope might be around $6,000. The difference is only 4 inches, but you pay for every inch rather dearly.


Refractor telescopes use a glass lense to capture the light and bend it toward a focus point and on to an eyepiece mounted on an adjustable focuser rack. The glass lense has two curved surfaces and thus represents twice the grinding effort. Also, since the light rays must pass through the glass lense, the quality and purity of the glass is most important for good or "the best" optical transmission. Precision grinding and polishing is also needed for best performance, along with special coatings to protect and enhance the views. Refractor telescopes get very pricey at lense diameters of 6 inches and above. 8 and 10 inch Refractors cost over $10,000 and are quite large and heavy.

Built into all of these discussions is the weight of the telescope which must be carried by some kind of suitable, adjustable mount. The mount must be heavy and solid enough to hold the telescope firmly and not allow wiggles which would make the views blurred and unuseable. At some point the mirror plus tube weight or the lense plus tube weight make the telescope quite unsuitable for portable work and the telescope is only viable for a fixed installation such as inside of a personal observatory or a large university observatory installation perhaps on a mountain top.

Given that you may now own a certain telescope, it is not possible to change its focal length normally.

Given that you might wish to get better pictures than you presently are receiving from your present telescope, you would need to sell the current one, collect the money, add more money and take the total cash to pay for a good used model which is larger than your present one.

In General, and those words are very important, in general, you wish to purchase the largest mirror or lense telescope you can afford within the constraints of needed portability.
The "in general" qualification comes in when we discuss really fine optical quality. Some 6 inch refractors are two or three times the cost of others because they have more precision optics and mechanical features like really smooth focusers which greatly enhance the performance of a given scope.

Hope that this helps some.
Zah

Answer 3

Im not very confident with telling you about refractors, but I do know a little about reflectors.

Basically, when you buy a reflecting telescope, you'll gasp because some are huge. They come in different lens sizes that are measured in inches. You have as small as 4.5 inches to maybe 16 inches. Basically, the bigger the better. This is because the bigger the lens is the more light you collect to make whatever celestial object look brighter. Focal ratio is based on how fast the telescope will observe the image. For example, if you have a telescope that specify a focal ratio of f/4.7 then you're telescope would be better at veiwing deep space objects. IF you were to have a telescope that would specify a focal ration of ...lets say.. f/9... then you're telescope views objects at a slower rate.. this would be good for planets and the lunar surface. A good way to better your views at a planet or the lunar surface with a fast telescope (f/4.7) would be to get an instrument that fits in your eyepiece called a barlow. What they do is help slow down the focal ration to get the result of a slower focal ratio.

Aperture is key. The more you have the better.
Magification is based upon the eye pieces you purchase. Some come with 10mm and 25mm. Basically the smaller the mm the better for planetary views and lunar views... the bigger the mm the better for deep space objects, such as the 35 or 42 mm.

I hope I could help.
Jansen

Answer 4

A telescope is a light bucket. The larger the bucket (aperature) there more light you can gather, and thus the more detail you can see.

The focal length of a telescope is important because it is a factor in how powerful it is, or how much magnification you're getting. To find the telescope's power, divide the focal length by the eyepiece's measurement.

For instance, in my 2000mm focal length telescope, I might use a 20mm eyepiece. Thus, 2000/20=100X magnification power. In the same telescope, a 40mm eyepiece would get 50X magnification (2000/40=50).

Refractors bend light. They're usually better image quality, but it comes at a higher price. Each side of the telescope lenses must be shaped properly, which makes them more expensive.

Reflectors (as the name implies) reflect light using a mirror. While image quality is not as fine, only one side of the surface must be shaped properly, reducing the cost of producing these types of scopes.

While there are quality differences, today they are such minimal difference so as to be indistinguishable to the typical user.

There are other types of telescopes, as well. Many of the blue Meade telescopes that have become popular recently are Schmidt-Cassegrain scopes; these utilitze both mirrors and lenses. There are other types as well, but these are generally for more advanced astronomy applications.

Answer 5

(1.) A refracting telescope uses transparent curved lenses to bend (refract) the light. A reflecting telescope uses an elliptical mirrored lens to reflect the light. For a given size, the reflector is cheaper, since it requires shaping one side of one piece of glass versus shaping both sides of two pieces of glass. It is also more compact.
(2.) You are correct, the larger the aperture, the higher usable magnification available (assuming equal optics). It goes up as the square. A 12" diameter lens has four times the light gathering power of a 6" diameter lens.
(3.) Focal length in theory has no effect on image quality, but since the magnification is the ratio between the focal length of the telescope and the focal length of the eyepiece, a shorter focal length telescope means you have to use an eyepiece with a shorter focal length, and very small eyepieces with focal lengths such as 4mm and 6mm are much harder to see through.

Answer 6

As you realize, aperture is the most important factor in determining what a telescope will show. Equally important is the quality of the optics: are they made by a reputable manufacturer?

Focal length is relatively unimportant, unless you are using the scope for imaging, where a shorter focal length will mean shorter exposures. For visual observation, in theory you can get any magnification with any focal length telescope, by choosing the right eyepiece. In practice, you're limited by the laws of physics.

By and large it is easier to make long focal length scopes than short focal length scopes, so longer ones tend to be better. However a short focal length scope (such as my 11" f/4.3 Newtonian) made by a superb optician (Carl Zambuto) can equal just about any length of scope, except maybe a long focal length scope also made by Carl Zambuto.

Short focal lengths cause optical problems: chromatic aberration in refractors and coma in Newtonians. They also require extremely high quality eyepieces to perform at their best. Long focal length scopes of both types are free of serious aberrations (if well made) but have relatively narrow fields of view; they work well even with relatively inexpensive eyepieces.

The best advice is to stick with something in the middle: f/6 to f/8 for a Newtonian; f/10 or longer for an achromatic refractor. This works OK for smaller apertures, but portability and mounting becomes a problem in larger apertures. So some compromise is needed, which is why there are so many scopes available on the market.

Answer 7

I agree with Nyx that you should invest in the book Nightwatch. This is one of those scopes that is bridging the gap between the toy scopes that are completely useless and real instruments that can be used to actually see things. I suspect that this scope does not fit well into either category. You mentioned aperture - that is very important, and it is one area where this scope is lacking. The way to understand aperture is like this: Most of the deep sky objects that are enjoyable to see are not too small to see - they are instead too dim to see. The Andromeda Galaxy is 4 times the size of the full moon as it appears in the night sky. You could easily see it with the naked eye if your eyes were big enough to collect enough photons to create a coherent image. If your eyes were 10 inches in diameter, it would appear very big and bright, because your eyes would be big enough to collect all those photons at once. That is what a telescope does. It gathers a lot of photons and funnels them into your eye to make those dim objects appear brighter. You can use different eyepieces to also magnify the image, but that is not as important as you might think. Magnification over 100x is increasingly difficult to use and also less appealing visually. You are spending most of your money here on a computer GoTo system. That means you are sacrificing decent aperture and optics and a good mount for a computer. I am not sure why you would want to do that. Half the fun of amateur astronomy is in knowing where and how to find things in the night sky. If you are in a light polluted environment, you may have more need for the computer, but if that's the case, you won't be able to see much with this small (4 in.) aperture anyway; you will have to take it to a dark location where the computer is less essential. All things considered, what you are looking at here is a whiz bang computer driving a too-small aperture scope with questionable optics on a shaky mount. It absolutely cannot be used for astrophotography that is worthy of the name (ADDED: I suspect you are not ready for astrophotography yet). That would not be my choice. My recommendation would be an 8 inch Dobsonian (10 inch if you can afford it) from a reliable source (e.g. Orion). You will have to learn the night sky and figure out how to aim it, but at least you will be able to see something once it is aimed properly. If you still have questions, I would recommend that you look up your local astronomy club on line and attend a few star parties before buying a scope. I personally think you could do a lot better.

Answer 8

Hi. Increasing the focal length has the effect of improving the image quality due to reduced coma at the expense of brightness of the image. Just as in a camera, f/1 is faster optically than f/10. This is true for retractors and reflectors. Any help?