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Refracting vs Reflecting Telescopes

hello friends if you go telescope

shopping you'll notice that there are

two main types on the market refracting

telescopes and reflecting telescopes the

first telescopes popped up in Europe in

the early 1600s these designs used

lenses to enlarge distant objects this

type of telescope relying on lenses

alone is called a refracting telescope

the first refracting telescopes use a

design that is now known as a Galilean

refractor a convex lens objective and a

concave lens eyepiece are used to

magnify distant objects apparent

diameters these telescopes are quite

easy to use as they produce an image

that is built the right side up and true

in the left or right sense today

Galilean refractors are mostly sold as

novelty items and are sometimes called

spy glasses if you were to purchase a

modern refracting telescope it would

almost certainly be constructed using a

slightly modified design a capillary and

refractor capillary and refractors still

use a convex lens objective but have a

longer focal length with a convex lens

eyepiece set behind the focal plane this

creates an image that is inverted both

vertically and horizontally but the

capillary InDesign is still preferred

over the Galilean because it allows for

a larger apparent field of view

Keplerian refractors are great for the

amateur astronomer and are super common

but refracting telescopes do have their

limitations the main three of which are

spherical aberration chromatic

aberration and limb sag spherical

aberration occurs because spherical

lenses do not bring parallel light rays

to a perfect focus each part of the lens

refract light somewhat differently this

causes a slight blurring of images but

can be corrected by using an objective

lens that is parabolic rather than

spherical chromatic aberration is caused

but the simple glass lenses refract

shorter wavelengths of light more than

longer wavelengths this problem can be

mostly corrected by employing compound

lenses these are lenses made with two

different types of glass each with

different refractive properties

Lim's AG occurs only in the largest of

refracting telescopes but it's the main

limiting factor of refracting telescopes

as they get larger and larger glass is

not perfectly rigid and as lenses are

made larger and larger they become so

heavy that the lens shape becomes

distorted by its own weight and there's

really no way to fix lens back except to

not use a lens to capture focus a light

enter the reflecting telescope

Isaac Newton discovered that if you use

a concave mirror as your objective your

image is free from chromatic aberration

and better yet because mirrors can be

supported from behind they can be made

much larger than lenses without the sag

problem and remember the larger the

objective the more light that can be

gathered the newest design is still used

in many reflecting telescopes today a

concave mirror is placed at the end of a

large tubular housing the other end of

the tube is open to let in the light

within the tube a small flat mirror is

placed at a 45 degree angle this small

mirror reflects light out through an

opening in the side of the telescopes

housing containing a convex eyepiece

there's just a few limitations to this

design this little mirror does block

some of the incoming light slightly

reducing the effective surface area of

the objective but usually only by a few

percent at most also some people find it

awkward to look sideways at objects one

way to fix this latter problem is by

using a different method to get light to

the eyepiece the Cassegrain reflector

uses a small mirror that is not angled

but is instead convex reflecting light

back towards an opening in the center of

the objective mirror that contains the

convex lenss eyepiece as you can imagine

both the small secondary mirror and the

opening in the objective reduce the

effective surface area of the objective

mirror but just like the newtonian

design this reduction is rather small in

general reflective telescopes are

preferred by professional astronomers

because the larger objective allows for

more light to be captured this way

dimmer objects can be seen with more

detail and resolution however there's

absolutely nothing wrong with refracting

telescopes there Graper hobbyist as they

do a pretty good job and don't require

much maintenance at all anyhow I hope

you've enjoyed learning about the

differences between refracting and

reflecting telescopes there are many

different specific designs out there

including many variations on the four

designs I showed you in this video but

hopefully this has given you enough to

feel informed about the subject you know

what to do if you feel this videos

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thank you for making all the way in this

video I sincerely hope you enjoyed my

name is Randy Dobson and I'll see you

next time