OK i just HAVE to ask.

[LukeSkywatcher]

I dont know why it has taken me this long to ask but better late then never.

We all know (well i hope we all do.......but if not thats ok) what is meant by FOV "field of view".

WHAT exactly is AFOV "APPARENT FOV"?.

I mean either you have a certain FOV or you dont.

My new 4mm Omni EP has a 50 degree "apparent" FOV.

It also has blackened lens edges to decrease internal reflactions and improve contrast (which is good).

Thanks in advance.

[Ant]

Is this any good?

http://astropedia.stargazerslounge.com/index.php?title=Apparent_FOV_(AFOV)/

Cheers

Ant

[ajg]

The forum seems to have messed your link up.

Apparent FOV (AFOV) - AstroPedia

In short, it's the angular size, in degrees, of the circle you see through the eyepiece.

[John]

I've nicked these definitions from the Tele Vue website:

apparent field: perceived span of sky seen through eyepiece without the telescope.

true field: span of sky seen through telescope/eyepiece combination.

[LukeSkywatcher]

I've nicked these definitions from the Tele Vue website:

apparent field: perceived span of sky seen through eyepiece without the telescope.

true field: span of sky seen through telescope/eyepiece combination.

Thanks John that just confuses me even more. Who looks through an eyepiece without a telescope?

I have tried it..................cant see a thing.

[LukeSkywatcher]

The forum seems to have messed your link up.

Apparent FOV (AFOV) - AstroPedia

In short, it's the angular size, in degrees, of the circle you see through the eyepiece.

Thanks Andrew. That makes more sense.

So AFOV is the perceived FOV through an EP against the actual FOV.

For example: My 32mm Celestron Plossl had an Apparent field of view of 46 degrees

True Field of view was 1.49 Degrees (that’s about 3 Moon diameters)

Yet when i observed through it i felt like i was totally immersed amongst the stars.

[brantuk]

I allways thought it was apparent field of view as given by the geometry of the eyepeice and its aperture, as opposed to what you can truly see with your eye and it's capabilities. But I'm probably only half way there lol

[ajg]

Thanks Andrew. That makes more sense.

So AFOV is the perceived FOV through an EP against the actual FOV.

For example: My 32mm Celestron Plossl had an Apparent field of view of 46 degrees

You got it.

True Field of view was 1.49 Degrees (that’s about 3 Moon diameters)

... and that will vary from telescope to telescope with the same eyepiece.

True FoV = Apparent FoV / ( Telescope FL / Eyepiece FL )

Yet when i observed through it i felt like i was totally immersed amongst the stars.

Hah! Wait until you get a glimpse through a Nagler (80°) or an Ethos (100°)

[LukeSkywatcher]

I live in hope of one day owning even a single top of the range EP.

Ha with 80-100 degrees FOV................do you also get a space suit included?

LOL.

I have a Vixen 30mm EP on the way.....................50 degrees apparent FOV. I'm pretty damn excited about that because i know it is gonna run rings around the 32mm i had. I also have a 40mm Elux on the way which should also impress????. Both bought from SGL members here.

Thanks guys.....................you really have been helpful in explaining something so simple that i really could not grasp.

[BlueAstra]

Here's a thread discussing FOV some time ago:

http://stargazerslounge.com/equipment-discussion/91808-how-calculate-maximum-fov-scope.html

The FOV of the primary lens (or mirror) without an EP is ill defined since vignetting will cause the image in the focal plane to have uniform intensity out to some radius, then it will gradually fall off to zero as the light cone is clipped. This can be seen sometimes in large uncalibrated CCD images, and in flats.

When an EP is used, if it has a circular field stop at its front focal plane (at the primary focal plane) then the FOV is a well defined circle within the un-vignetted capabilities of the EP. The more complex the EP design, the greater angle (FOV) it can handle without vignetting. If there is no field stop the FOV will be less well defined.

[LukeSkywatcher]

Thanks BA. FOV i understand. I just didnt get AFOV.

I do now.

[John]

Thanks John that just confuses me even more. Who looks through an eyepiece without a telescope?

I have tried it..................cant see a thing.

If you hold an eyepiece up to the light you will see an illuminated circle of light - that is the AFoV of the eyepiece. It's a good way of comparing eyepiece AFoV's - hold one eyepiece up to each eye and, with a bit of practice, you will be able to directly compare the AFoV's of the eyepieces - you see not all eyepieces actually give you the AFoV they claim

The boundary of the illiminated field is defined by the field stop and it's the diameter of that which determines the AFoV of the eyepiece AND the TRUE field of view of the eyepiece / telescope combination - the formula for which is:

True field of view = eyepiece field stop diameter ÷ telescope focal length x 57.3

Again the realility does not always match the published specs ......

With certain eyepice designs, the field stop is not in a convenient place to be measured, ie: it's between the lens elements rather than below the field lens (as it is in the plossl design for example).

So there is method in this madness

[John]

I have a Vixen 30mm EP on the way.....................50 degrees apparent FOV. I'm pretty damn excited about that because i know it is gonna run rings around the 32mm i had. I also have a 40mm Elux on the way which should also impress????. Both bought from SGL members here.

Interestingly the published AFoV of the Vixen 30mm NPL is 50 degrees wheras the Celestron 32mm Omni plossl is published as 52 degrees. If those figures are correct the Celestron should show a wee bit more sky than the the Vixen. The latter may well be better figured and coated though.

If the E-Lux plossl is the 1.25" model then it's AFoV (published) is 43 degrees and it will show almost exactly the same amount of sky as the 32mm plossl.

The 40mm will work fine in your 90mm refractor but the exit pupil it creates when used with your 130mm F/5 Heritage will be 8mm which is on the large side. You may be able to see the shadow of your secondary mirror when using it.

[michael.h.f.wilkinson]

Just to put it simply: Assuming no distortions in the EP (pincushioning)

FOV = AFOV / magnification

(Someone posted something equivalent earlier). To see e.g. 1.64 deg in the sky at 50x magnification, the apparent field of view must be 50x1.64 deg = 82 deg (what a coincidence, a Nagler ), simply because 1.64 deg diameter TRUE circle in the sky is stretched 50x.

[LukeSkywatcher]

Thanks all. I'm really starting to understand this more.

[great_bear]

Paul,

Whilst you've no doubt seen dozens of links to "eyepiece calculators" that show the kind of stuff discussed here, personally, I like the following calculator the best, since it shows graphically how all this stuff interacts.

<click here>

[Ant]

Paul,

Whilst you've no doubt seen dozens of links to "eyepiece calculators" that show the kind of stuff discussed here, personally, I like the following calculator the best, since it shows graphically how all this stuff interacts.

<click here>

That really is very clever and perfectly explains what the terms mean in a "real world" setting - if you know what I mean.

Cheers for posting that!

Ant

[LukeSkywatcher]

I like that one great bear. Even though it doesnt have my exact scope or EP...........i can get a rough guess going by my apeture/FL/EP size.

Cheers.