Degree of Confusion

[Herb]

Hello All

First-time poster, long-time lurker.

I'm hoping some-one can assist me, I'm having trouble getting my head around the terms "apparent field of view" and "true field of view".

I have looked up the definitions and formulae for both - what puzzles me is the use of the word "apparent".

For instance, in a previous thread there is a discussion regarding 100 degree (AFOV) eyepieces. Now I know I'm not going to see from the horizon to zenith with one of these eyepieces, so in what sense is the word "apparent" being used?

Regards

Herb

[dannat (Daniel)]

apparent field of view is the eyepiece stated spec of 100deg.

true is what you can actually see for the entire system

eg 100mm 1000mm focal length scope & 20mm 100deg ep

1000/20 = 50x magnification. 100/50 = 2 deg true field of view when looking thru this scope & eyepiece combo

[Herb]

Hi Daniel,

Thanks for the reply.

Unfortunately, I don't seem to have asked my question clearly.

I understand the formula - what I don't get is the use of the word apparent.

"apparent field of view is the eyepiece stated spec of 100deg".

I realise the field of view can't possibly be 100 Degrees, so where does the "apparent" come into it?

For instance, your example shows a true field of view to be 2 degrees in that particular scope, why then is that eyepiece "apparently" 100 degrees?

Hope I've explained myself better

Regards

Herb

[Astro_Bot]

The FOV visible to the human eye at the eyepiece is 100 degrees (for that eyepiece). What you see within that field of view depends on the magnification, as Daniel explained.

Apparent FOV = what you see at the eyepiece. It does not change with magnification or what scope you put it in. It is an inherent characteristic of the eyepiece design. A small AFOV is like looking down a narrow tube, while a wide AFOV is possibly wider than the human eye can take in at once.

True FOV = what appears in your FOV as determined by the magnification and AFOV of the eyepiece. At high magnification the TFOV will be much smaller.

[dannat (Daniel)]

The apparent field of view of the eyepiece is 100deg, it relates to the light cone coming in compared to what i svisible at the eyeglass. eyepieces started at 40-50 deg fov & keep getting larger. There is apparently a 120 deg fov ep in the pipeline but this is tarting to push the laws of physics

[Steffen]

The image produced by an eyepiece is a virtual image (unlike the one projected onto a CCD sensor or screen). The "apparent field of view" refers to the apparent diameter of the field stop (the black border surrounding the image in the eyepiece) compared to an actual circle drawn on a wall with a given diameter viewed from a given distance.

A Super-Plössl with an apparent field of view of about 52˚ shows a field stop that looks like a circle with 1m diameter viewed from 2m distance. A 100˚ eyepiece has a field stop that looks like a 1.2m diameter circle viewed from 1m distance.

The actual amount of sky shown depends on the magnification of the telescope/eyepiece combo.

The true field of view on the other hand is a circle projected onto the sky. If two stars have an angular separation of one degree, and they sit on opposites edges of your field of view in the eyepiece then the telescope/eyepiece combo has a true field of view of one degree.

Cheers
Steffen.

[Allan_L (Allan)]

Hi Herb,

How about this:
Apparent field of view is the circle of light you would see when looking through the EP on its own.

TRUE FOV is what happens to that circle of light, when you introduce Magnification to that eyepiece.

That is why TFOV changes if you stick the same eyepiece into two different telescopes (of different magnifications).

This is because, the more you magnify the image, the less you can fit into the same circle that reaches your eye.

Hope that makes sense.

[Herb]

Excellent, thanks all for the replies, I get it now, I was getting hung up on the word apparent.
Allan_L wrote

"Hope that makes sense."It does now, sort of.Perhaps I can refer the treasurer to this thread when I try to explain to her that the 30 mm 82 degree eyepiece I really, really need is acually only 1.6 degrees in my GSO 12" F5!

Also ....(forgive me for being a PITA, I'm new to all this)

"That is why TFOV changes if you stick the same eyepiece into two different telescopes (of different magnifications).

This is because, the more you magnify the image, the less you can fit into the same circle that reaches your eye."

I found a handy calculator online whilst I was trying to understand all this (http://www.davidpaulgreen.com/tec.html) which gave me the above calculation, however if I input 30mm with a 2x barlow the TFOV remains the same and yet for a 15mm without the barlow the TFOV is halved. How can this be if I have increased the magnification of the image with a barlow? Am I missing something again?
Regards

Herb

[barx1963 (Malcolm)]

Herb
I think that calculator only changes the power when you input a barlow value. The TFOV definitely changes in real life.
I know I found AFOV a confusing concept. If you think about it, if you close one eye and without moving your open eye try to get a feel for how wide your field of view is, that can be thought of as the AFOV of you eye without any eyepice and/or scope. It is probably about 100deg. So if you look through a plossl you see a smaller circle that is the FOV of that EP. If that is half the FOV of your eye, voila! 50deg AFOV.

Malcolm

[Geoff45 (Geoff)]

Look at an object at the bottom of the eyepiece, then look at an object at the top. How many degrees has your eye moved? That's the apparent FOV.

[Don Pensack]

Apparent field of view is the angle change from one side of the field in the eyepiece to the other. Your eye subtends this angle when moving from edge to edge.
True Field is the actual field of view on the sky that the eyepiece sees.
Keep the magnification the same, and you can see that as the apparent field gets larger, so does the True Field on the sky.
A 100 degree apparent field eyepiece will see a True Field twice as wide as a 50 degree apparent field eyepiece of the same focal length.