understanding eyepiece FOV

[rider]

Greetings all

OK, I’m relatively new at this and I am trying to get an understanding of the factors which influence eye-piece selection. (other than quality)

These are my assumptions. Please free to correct my misunderstandings.

Assuming the same scope is used, I think that the real view from a particular lens is a function of
-Magnification (as defined by the MM of the lens X the focal length of the scope)
- Apparent Field Of View (as defined by eye piece manufacturer)

If this is so,


Am I right in assuming that for a 2500 F.L. telescope, a 16mm lens with a AFOV of 82 degrees will provide much the same view as a 30 mm lens with a AFOV of 43 degrees

Both end up with a 0.5 -ish view

I think they will both have the same area of stars in frame, so would duplicate each other.

Am I missing something.


Rider

[programmer]

According to this calculator you're correct (.52 in the first case, .5 in the second)

[Dennis]

Hi Rider

The magnification and limiting magnitude would be different, shown in the example from SkyTools below:

16mm 82 degrees.
FOV 31.5 min
Magnification 156.2
Dawes limit 0.5”
Magnitude 15.6

30mm 43 degrees.
FOV 31 min
Magnification 83.3
Dawes limit 0.5”
Magnitude 15.3

Cheers

Dennis

[ngcles]

Hi Rider,

Rider wrote:

"I think they will both have the same area of stars in frame, so would duplicate each other." (emphasis added)

The calculations by Dennis are spot on -- but only tell half the story. To answer the second part of your question as reproduced.

The view through the 43 deg AFOV eyepiece while about the same size will be much less aesthetically pleasing, will have a slightly brighter background sky, the view will seem much more "compressed" and because of the narrowness of the AFOV, will seem a bit like looking down a sewer pipe.

The 82 deg AFOV ep while showing essentially the same slice of sky will be much more like looking out of a porthole, show a more contrasty view and more detail will be more easily discerned due to the higher magnification -- assuming it is a high quality occular -- which an 82 deg AFOV usually implies! All round much nicer!

Best,

Les D
Contributing Editor AS&T

[rider]

hi again all.

thanks for the succinct and enlightening explanations, I now understand enough to get me in REAL trouble.

my summary of the below answers.

option 1. I can spend merely copious quantities of money getting a particular FOV.
option 2. I can spend ENORMOUS amounts of money for a particular FOV.

if I want the "wide screen hi definition" view of a particular amount of sky, I must pick option 2.

goodbye bank balance...


Rider

Originally Posted by ngcles

Hi Rider,

Rider wrote:

"I think they will both have the same area of stars in frame, so would duplicate each other." (emphasis added)

The calculations by Dennis are spot on -- but only tell half the story. To answer the second part of your question as reproduced.

The view through the 43 deg AFOV eyepiece while about the same size will be much less aesthetically pleasing, will have a slightly brighter background sky, the view will seem much more "compressed" and because of the narrowness of the AFOV, will seem a bit like looking down a sewer pipe.

The 82 deg AFOV ep while showing essentially the same slice of sky will be much more like looking out of a porthole, show a more contrasty view and more detail will be more easily discerned due to the higher magnification -- assuming it is a high quality occular -- which an 82 deg AFOV usually implies! All round much nicer!

Best,

Les D
Contributing Editor AS&T

[ngcles]

Hi Darryl, Rider & All,

Thank you, but I have to decline any appellation of "authority". I'm certainly no poet either. I'm still learning every day -- like most people here. I know a bit about telescopes and astronomy but I'm no authority.

I'll take a look at the collimation thread and might add something.

Best,

Les D

[OneOfOne (Trevor)]

Originally Posted by ngcles

...
The view through the 43 deg AFOV eyepiece while about the same size will be much less aesthetically pleasing, will have a slightly brighter background sky, the view will seem much more "compressed" and because of the narrowness of the AFOV, will seem a bit like looking down a sewer pipe.

The 82 deg AFOV ep while showing essentially the same slice of sky will be much more like looking out of a porthole, show a more contrasty view and more detail will be more easily discerned due to the higher magnification -- assuming it is a high quality occular -- which an 82 deg AFOV usually implies!
...

I like to think of the one with the wider AFOV of the same "amount" of sky as "stretching the light out". The wider one takes the same view and stretches it out over a larger area. The background sky is stretched making it darker whilst the stars, being of zero size, remain the same size and brightness, making the contrast greater. It may not be totally technically correct, but I makes it easier to picture what is happening.