Understanding the Field of View of Reflecting and Refracting Telescopes

[21joanna12]

I don't quite understand the concept of field of view of a telescope... Specifically, I am trying to understand the advantages of reflecting and refracting telescopes compared with the other, and my textbook says that "Refracting telescopes have a wider field of view than reflectors of the same length because their angular magnification is less. Astronomical objects are therefore easier to locate using a refractor instead of a reflector of the same length." Firstly, I don't understand why the angular magnification of reflecting telescopes is greater for a qiven length of telescope. Looking at diagrams, it seems like this would depend entirely on the power of you objective and eyepiece lenses. Secondly, I don't really understand what it means by field of view... the Wikipedia article on this doesn't seem to have a great explanation.

Thank you in advance! :)

 

[davenn]

Secondly, I don't really understand what it means by field of view... the Wikipedia article on this doesn't seem to have a great explanation.

the field of view is as it sounds ... its literally the amount of sky that is viewed
field of view through any given scope is also determined by the eyepiece and the resulting magnification

The are newer eyepieces are have been designed that will give a much larger FOV compared to a normal eyepiece of the same size

Teleview, Nagler eyepieces are well known for that feature

here a page with a bunch of formula for working out various telescope things ... FOV, magnification etc
http://www.chuckhawks.com/telescope_formulas.htmcheers
Dave

 

[21joanna12]

the field of view is as it sounds ... its literally the amount of sky that is viewed
field of view through any given scope is also determined by the eyepiece and the resulting magnification

The are newer eyepieces are have been designed that will give a much larger FOV compared to a normal eyepiece of the same size

Teleview, Nagler eyepieces are well known for that feature

here a page with a bunch of formula for working out various telescope things ... FOV, magnification etc
http://www.chuckhawks.com/telescope_formulas.htmcheers
Dave

Thank you for your reply! But I was just wondering why greater angular magnification means smaller field of view- wouldn't you be able to see a larger image with the same field of view if the telescope objectives are the same diameter?

 

[wabbit]

"Refracting telescopes have a wider field of view than reflectors of the same length because their angular magnification is less."

This is incorrect. The magnification does not depend on the telescope being a refractor or reflector. It is true that it is easier to design a refractor with a very wide TFOV, but this is a separate issue having to do with the more complex optical path in reflectors.

Firstly, I don't understand why the angular magnification of reflecting telescopes is greater for a qiven length of telescope. Looking at diagrams, it seems like this would depend entirely on the power of you objective and eyepiece lenses.

This is correct.

Secondly, I don't really understand what it means by field of view.

There are two different notions of (angular) field of view:
- the true field of view or TFOV, which is the angle in the sky your image covers. If the full moon just about completely fills your view, then the TFOV is ~1/2°.
- the apparent field of view or AFOV, which is the angle you see through the telescope. This does not depend on what you look at, nor on the objective lens, it is determined by the eyepiece, and depends on optical design.

Note that TFOV ~ AFOV/magnification. If your eyepieces all have a 50° AFOV, the apparent size of the image will be the same in all, but the full moon will have an apparent size of 10° (1/5th of the TFOV) at 20x and 50° at 100x (filling the TFOV).
The more you magnify, the more you are zooming in onto a smaller and smaller area of the sky (smaller TFOV).

 

[russ_watters]

Thank you for your reply! But I was just wondering why greater angular magnification means smaller field of view- wouldn't you be able to see a larger image with the same field of view if the telescope objectives are the same diameter?

Half serious but hopefully still instructive: because if the field of view were too large, you'd have to turn your head to see all of it. Think about it: a nice, comfortable forward field of view for your eye is about 90 degrees. So if you take a 1 degree field and magnify it 360 times, you'd have to spin around to see the whole thing!

 

[davenn]

Thank you for your reply! But I was just wondering why greater angular magnification means smaller field of view-

Do you own a camera with a zoom lens ?
if so have you noticed that when you zoom in on an object, you loose all the background view and only see the object ( depending on the power of the zoom -- less of the background) ... your FOV is now so much smaller

 

[Chronos]

It appears the source you were reading was discussing angular resolution, not FOV. FOV is based strictly on focal length of the primary objective, focal length of the eyepiece and AFOV of the eyepiece. The angular resolution of a refractor vs a same sized reflector is superior because nearly all reflector designs have a central obstruction [CO] that reduces the amount of light that reaches the eye [or camera sensor]. The amount of reduction depends on reflector design. It can vary from about 10% for a classical Newtonian to 30% or more for a catadioptric design. For some discussion see http://www.chuckhawks.com/telescope_formulas.htm and http://www.telescope-optics.net/obstruction.htm.

 

[wabbit]

The angular resolution of a refractor vs a same sized reflector is superior because nearly all reflector designs have a central obstruction [CO] that reduces the amount of light that reaches the eye [or camera sensor].

This obstruction doesn't however lower the angular resolution for point sources (or marginally, depending on how you define it), it does so only for extended features.

 

[Chronos]

Try splitting double stars with a refractor vs a reflector with the same size objective and see what you think. See here for discussion https://www.astronomics.com/why-buy-a-refractor-telescope_t.aspx.

 

[wabbit]

This is the reason why I qualified my statement above :) it gets hairy when you look at the two airy disks and the first ring, in some cases obstruction can make the separation stand out more (the central airy disks are smaller), in some cases less (the disk plus first ring may look like just a bigger disk), and it depends on brightness... I'll check your link.

Edit - not a lot of detail there, maybe a bit more at http://www.telescope-optics.net/obstruction.htm - I think some time ago I saw one site with direct comparison for splitting stars, with obsruction changing (or even appertizing masks) but I don't recall where.