I have a quesiton about reflecting telescopes/?

Question

How does a reflector telescope that has a 19 inch tube have a 1400mm focal power and other reflector telescopes that have longer tubes have only 700mm. The 19 inch telescope claims to have a 1400mm focal length that is 55 inches long. Does this produce a higher degree of magnification than the 700mm with a longer tube?

Answer 1

It's difficult to answer your question, in some respects, because it's actually quite a few questions rolled into one.

The 19" tube with a 1400 mm effective focal length is probably something called a "Schmidt Cassegrain" or "Maksutov" design in which the light comes in one end, bounces off a primary condensing mirror in the back, focuses in on a SECOND condensing mirror at the front and then passes through a hole in the center of the primary mirror before entering into a tube with an angled mirror and an eyepiece at the base.

In this type of system, the light has two chances to condense. The focal length is best looked at not as the actual distance it travels to a focal point, but as a measure of the geometry of the primary mirror. Look at it as if we were measuring where the focal point WOULD be, if there was only the primary mirror to contend with.

Focal length plays a role in magnification calculations, but don't be too hung up on mangification as a measure of telescope capability. You're better off with a high quality optical construction at any size than one that is assembled with poor precision or flawed components.

The other thing you probably want to know is that astronomical telescopes (at least the ones you and I might want to have in our back yards) aren't primarily about MAGNIFICATION. They're primarily about LIGHT GATHERING.

Deep sky objects aren't tiny. They're dim. Some of them are actually quite large, encompassing regions of sky fully visible to the naked eye. They're just so faint that the amount of light that hits your eye isn't enough to let you see them.

In some ways, it's like making a rain meter. If you put a coffee can out in the rain, and its dumping big drops all night long, you'll come out in the morning and see a couple of inches of water in the can. You can then say, "it rained 2 inches last night," and you'll be approximately correct. However, if it drizzles lightly and in the morning there is only some scant water in the bottom of the can, you won't be able to measure the result. The rain is too "faint" to detect.

One way to detect "faint rain" might be to use a funnel. If you take a nice big funnel and put it over a narrow cup, you'll collect a LOT more rain. You'd be "magnifying" the effect. If you measured the diameter of the funnel mouth and did the same for the diameter of the cup, you could calculate an area ratio. Let's say that the funnel's mouth has 4 times the area of the cup. In the morning, if you have an inch of water in the cup, that means it really only rained a quarter of an inch!

The same thing is true of light. When you look into the night sky, your eye only admits the light that actually falls on a portion of your cornea over your pupil. It's barely enough for you to see faint stars. It's not enough for you to see distant galaxies (ok, there are one or two that if you REALLY STARE on a DARK DARK NIGHT, you can see them with the naked eye, but you've got to know where to look and when). It's certainly not enough for you to see a nebula. What you need is a "light funnel". That's what a telescope is!

The advantage of a mirror telescope is that you can make a comparatively large "funnel" that catches a circle of light 8 or 10 or even 16 inches in diameter in a relatively managable (and affordable) package. This is a HUGE amount of light amplification relative to your naked eye.

At the other end, where the light focuses down, you can put interchangeable "occular lenses" or "eyepieces" (the one your eye actually goes up to) with different focal lengths. The ratio of the telescope's focal length to the eyepiece's focal length is the actual magnifcation ratio. Your 1400mm scope with a 15mm eyepiece would have a magification of about 93x (1400/15). If you had an eyepiece with a 45 degree apparent view (you look through it and your eye experiences a 45 degree circle "window", then you'd be looking at about half a degree of sky. That's about the size of the moon.

The same eyepiece in the 700mm focal length telescope you're looking at would cause a magnification of 47x and would show you a circle about 1 degree wide of the sky.

If you wanted the same amount of magnification out of the 700mm scope, you'd use a different eyepiece and be fine.

For backyard telescopes, you have to balance the desire to have the BIGGEST light gathering surface you can get with the practicality of how much you're willing to go through to use it.

If you want to have a 28" mirror, you're going to have to put up with a telescope that will probably be about 20 feet tall. If you wanted to make it "faster" (i.e., focus at a shorter distance for convenience) then you'd have to make a much deeper curve in the mirror which means that the construction would have to start with a THICK piece of glass, and it would have to be a very massive construction project indeed.

8, 10 or 16 inch mirrors are about as big as most people want to deal with in backyard mirror telescopes. In going to a Schmidt-Cassegrain design, there is increased cost due to the complexity of the construction, but improved handling characteristics because the tube is much shorter.

I hope that helps.

Telescopes are fascinating!

Answer 2

The focal length isn't actually the length of the tube. It's the distance the light travels from the objective (lens or mirror) to the focal point (where the light rays converge and the light focuses to a bright little point).

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In many telescopes, such as refractors and newtonian reflectors, this is roughly the same as the length of the tube. However some telescopes have a folded design, such as the schmidt cassegrain, maksutov cassegrain, and ritchey chretien.

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In these telescopes, the light path is folded back onto itself by the secondary mirror and actually extends through a hole in the mirror in the back of the telescope so the focal length can be slightly more than twice the length of the optical tube itself.

They may also be adding additional length if the telescope comes with a barlow lens because a barlow lens will essentially double or triple the focal length of a telescope.

The longer the focal length the more magnification per eyepiece the telescope will yield. All other variables related to focal length also depend on the aperture of the telescope. Because of this, it's better to talk about focal ratio. This is the aperture divided by the focal length.

If the focal length is short compared to the aperture, you will require less exposure time when doing astrophotography, and you will have a wider field of view, but you will also have a lot of edge distortion, the telescope may be more difficult to collimate, and if it's a non-apochromatic refractor, you will have a lot of chromatic aberration.

However if the focal length is long compared to the aperture, the opposite of the above will be true.

Answer 3

Most of the answers you've received are wrong...that's par for the course when you ask a technical question on Yahoo Answers!

The reflector with the 1400 mm focal length manages to fit this inside a 19 inch tube by using some optical trickery. The focal length of the primary mirror is actually about 500 mm focal length, but the manufacturer has added a Barlow lens into the focuser which has the effect of lengthening the focal length to 1400mm. Unfortunately, this design has many faults, and gives very poor images and is very hard to adjust. Avoid it at all costs! The other designs are standard Newtonians with only two mirrors, primary and secondary, and no built-in Barlow. This is a tried and true design, and will give you much better images and will be easier to keep in collimation.

The telescope with the 1400mm focal length will nominally produce twice the magnification with a given eyepiece, but because of the severe optical aberrations mentioned above, that higher magnification won't actually be usable. Anyway, most astronomical observations are made at lower magnifications, 30x to 150x.

Answer 4

A Newtonian reflector has a single primary mirror; it's focal length is basically the same as the tube length. A Cassegrain reflector has a set of two mirrors; the light crosses the length of the tube twice, essentially giving you twice the focal length in a smaller package (at twice the price.)

Usually the magnification (or power) is found by dividing focal length by the size of the eyepiece. 25mm is common, so 1400/25 = 56X. With a longer focal length, you get higher power using the same eyepiece. The 25mm eyepiece on a 700mm scope will yield 700/25 = 28X.

However, higher power is pointless if your scope doesn't have the aperture to support it. An over-magnified image will be blurry and faint if the telescope isn't gathering enough light. A rule of thumb is your aperture in inches * 50 (or mm * 2) is the maximum power you should try. For example, on an 8" Newtonian, 8 * 50 = 400X is the best magnification you can hope for.

Answer 5

To fit 1400mm of focal length into a tube less than 500mm long requires compound optics. The reflector in question may be using some variety of Cassegrain optics (Schmidt-Cassegrain, Maksutov Cassegrain, etc.) These reflectors have a curved secondary mirror that increases the focal length, enabling a long focal-length instrument to fit in a compact package. There are also Newtonian reflectors (usually cheap ones) that have an auxiliary lens in the focusing tube that functions like a built-in Barlow lens, increasing the effective focal length.

Magnification is equal to the telescope focal length divided by the eyepiece focal length, so for a given eyepiece, the 1400mm FL scope will give twice the magnification of the 700mm FL one.

Answer 6

A reflecting telescope (also called a reflector) is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a "catoptric" telescope. The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas. In a symmetrical Cassegrain both mirrors are aligned about the optical axis, and the primary mirror usually contains a hole in the centre thus permitting the light to reach an eyepiece, a camera, or a light detector. Alternatively, as in many radio telescopes, the final focus may be in front of the primary. In an asymmetrical Cassegrain, the mirror(s) may be tilted to avoid obscuration of the primary or the need for a hole in the primary mirror (or both). The classic Cassegrain configuration uses a parabolic reflector as the primary while the secondary mirror is hyperbolic. However, variations exist where the primary is hyperbolic for increased performance, and where the primary and/or secondary are spherical or elliptical for ease of manufacturing. The Cassegrain reflector is named after a published reflecting telescope design that appeared in the April 25, 1672 Journal des sçavans which has been attributed to Laurent Cassegrain. Similar designs using convex secondaries have been found in the Bonaventura Cavalieri's 1632 writings describing burning mirrors and Marin Mersenne's 1636 writings describing telescope designs. James Gregory's 1662 attempts to create a reflecting telescope included a Cassegrain configuration, judging by a convex secondary mirror found among his experiments.

Answer 7

Reflecting telescopes have, as I'm sure you know, mirrors. In some designs, only one mirror is curved, and the other is flat (as in Newtonians). In other designs, both mirrors are curved (as in Cassegrains, Schmidt-Cassegrains, Ritchey-Chretiens, and others). In that latter category, the focal length is an "effective" focal length because the secondary mirror has an "amplification" factor due to its curvature. This means that the light reflecting off that second mirror has the effect of increasing the focal length. RCs usually have an amplification factor of about 2 while that of many SCTs is about 5. In SCTs, you get a very long effective focal length in a short tube.

Answer 8

Are you sure it's a reflector? If it's a catadioptric, then there are two mirrors inside, and the actual light path is longer than the tube itself.

And if the focal length is longer, then it will give more power (given that the same eyepiece is used in both scopes, that is).