Stargazing What is the difference between linear and angular magnitude in telescopes?

[Sobhan]

what is the difference between linear magnitude and angular magnitude

 

[davenn]

never heard of either in reference to telescopes
and I have used scopes for best part of 50 years

please give more information

 

[Sobhan]

my question is this:

 

[russ_watters]

Wait, do you mean magnification, not magnitude?

 

[Sobhan]

Sorry i meant magnification.

 

[russ_watters]

Angular magnification is what our eyes or a lens see, whereas linear magnification is what is projected onto a surface, so it is measured linearly.

See (second section):
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lensdet.html

Since this is a telescope question, I'm moving the thread to that section.

 

[Sobhan]

the other question i have got is: where do these formulas like field of view and ... come from

 

[davenn]

the other question i have got is: where do these formulas like field of view and ... come from

I assume you mean, how is it calculated ?

Field of View in a Telescope

by Mike Swanson
After spending just a little time exchanging the eyepieces in a telescope, you will soon find that more than just the magnification varies with each eyepiece. One of the primary differences is the total amount of sky you can see with each eyepiece, also known as the field of view or FOV.

The field of view is the circle of sky visible through the eyepiece. Generally speaking, as you exchange eyepieces to get a higher magnification, the field of view is a smaller piece of the sky. We measure FOV in degrees or fractions of a degree. Usually astronomers refer to the actual field visible in the eyepiece as the true field of view or TFOV. Knowing the TFOV of each of our eyepieces is very useful since we can then compare what we see in the eyepiece to printed or computerized star charts to help us identify what we are seeing.

Also, some objects require a wide field of view to show the entire object, so we need to choose the eyepiece that will let us 'take it all in'. Herein lies the popularity of wide field eyepieces. At any given magnification, they allow us to see more of the sky. Extended objects like open star clusters, many nebula and some nearby galaxies are only visible in their entirety with a wide-field view.

Calculating the TFOV is not too hard, but there are a few things we need to proceed. First, we need to know the focal length of our telescope and the subject eyepiece. These two are easy as they are generally marked on the side of the scope and eyepiece. But we also need to know the apparent field of view (AFOV) of the eyepiece. This is generally obtainable from the manufacturer of the eyepiece, but it is useful to know that most Plossl eyepieces (the most common type on the market) have an AFOV of 50 degrees.

Armed with this information, the calculations are quite simple. First calculate the magnification of the eyepiece:

MAG = Scope Focal Length / Eyepiece Focal Length

Then you can directly calculate the true field of view:

TFOV = Eyepiece AFOV / MAG

Let's say the focal length of our scope is 1000mm and the focal length of our Plossl eyepiece (50 degrees AFOV) is 10mm:

MAG = 1000 / 10
MAG = 100x

TFOV = 50 / 100
TFOV = .5 degrees

cheers
Dave

 

[Sobhan]

What is the prove of that FOV?

 

[russ_watters]

It's derived from basic trigonometry. The angles and objects form triangles.