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Help on Terms When Buying?

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4 replies to this topic

#1
BeeeKaaay

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Listed below are some things I'd like to know about telescope descriptions:

•What is "40mm", "80mm", etc.?
•What is the difference between a refractor and a reflector telescope, and what are their uses?
-Also, what is Achromatic Refractor?
•What is focal length? (Such as 400mm)
•What is Aperture? (Such as 80mm)
•What is Focal Ratio? (Such as f/5)

Please answer in fairly simple terms as I am still a beginner.
Thanks!(:

#2
Islander

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40mm 80mm etc refers to the diameter of the object lens (the lens at the other end of the telescope to the eyepiece) or primary mirror (the one at the bottom of the tube)

A reflector uses mirrors to bring the light to focus at the eyepiece, a refractor uses lenses to do the same. An achromatic refractor brings two colours of light to a common focus but not the third - this produces the effect known as chromatic abberation which manifests as colour fringing on bright objects. An apochromatic refractor brings all colours to the same focus and does not suffer from chromatic abberation. Reflectors don't suffer from chromatic abberation either.

Focal length is the distance that the object lens in a refractor and the primary mirror in a reflector bring light to focus in. It's used to calculate magnification by dividing the focal length of the scope by the focal length of the eyepiece (e.g. 1000mm FL scope with a 10mm eyepiece gives 100x magnification).

Aperture is the diameter of the object lens in a refractor or primary mirror in a reflector. The larger the aperture, the more light the scope can gather.

Focal ratio is the focal length of the scope divided by its aperture (e.g. a 1000mm FL scope with a 100mm object lens has a focal ratio of F10). The smaller the F number, the 'faster' the scope.

Hope this helps :)
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#3
umadog

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•What is "40mm", "80mm", etc.?
Depends on what it applies to. Probably objective diameter.

•What is the difference between a refractor and a reflector telescope, and what are their uses?
http://www.astronomy... telescope.aspx


-Also, what is Achromatic Refractor?
http://en.wikipedia....matic_telescope
http://www.gaherty.c.../achrefract.htm

•What is focal length? (Such as 400mm)
http://en.wikipedia....ki/Focal_length

•What is Aperture? (Such as 80mm)
Diameter of objective (lens or mirror)

•What is Focal Ratio? (Such as f/5)
focal length divided by objective diameter (in the same units).
http://www.astronomi...qa8l1jqu/Page/1


Perhaps you should check out the excellent book "Nightwatch"? which describes things in detail from the ground up. When you're getting into a learning-based hobby like this, there is nothing to rival a good book.
9" DIY job, Sumerian 10", Webster 18", Lunt LS60/PT
M:109/109, C:72/109, H400:154/400, Mercury->Pluto

#4
faulksy

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i take it the 40 - 80mm you are on about will be the diamater of the end of the telescope the bigger the better as it will gather more light for dim objects.ie dso,s not planets due to them being bright enough.a refractor has a tube withe lenses in it and no mirrors, just a straight through telescope, a reflector gathers light(object) at the bottom of the tube and the light bounces back up the tume to a 45 degree mirror which then hits you in the eye.i will let some one else have a go now

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#5
BeeeKaaay

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40mm 80mm etc refers to the diameter of the object lens (the lens at the other end of the telescope to the eyepiece) or primary mirror (the one at the bottom of the tube)

A reflector uses mirrors to bring the light to focus at the eyepiece, a refractor uses lenses to do the same. An achromatic refractor brings two colours of light to a common focus but not the third - this produces the effect known as chromatic abberation which manifests as colour fringing on bright objects. An apochromatic refractor brings all colours to the same focus and does not suffer from chromatic abberation. Reflectors don't suffer from chromatic abberation either.

Focal length is the distance that the object lens in a refractor and the primary mirror in a reflector bring light to focus in. It's used to calculate magnification by dividing the focal length of the scope by the focal length of the eyepiece (e.g. 1000mm FL scope with a 10mm eyepiece gives 100x magnification).

Aperture is the diameter of the object lens in a refractor or primary mirror in a reflector. The larger the aperture, the more light the scope can gather.

Focal ratio is the focal length of the scope divided by its aperture (e.g. a 1000mm FL scope with a 100mm object lens has a focal ratio of F10). The smaller the F number, the 'faster' the scope.

Hope this helps :)


Thank you, that helped alot! :)




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