Field of View

[brantuk]

Hi All

I've come back to the beginners section cos there's a rather relevant bit I skipped over when I first started - "field of view". Also I read Andrews primer and good as it was, I'm still a tad confused. How does FOV/AFOV work?

I was using a 430mm focal length, F6 scope, with a 72mm aperture and Stellarium tonight. Zoming into the object in Stellarium indicated a 10 deg field of view to discern a vague but recognisable shape.

Which eyepeice would I need to view my object? I tried a 40mm wide field swan, and also a 33mm, and a 25mm but still no joy.

I'm confused by "apparent" FOV, FOV, and what FOV Stellarium is telling me. Can anyone please explain? (feel free to chuck in a bit of maths if neccessary).

Cheers

[AndyH]

There's apparentFOV and trueFOV.

Every eyepiece has a AFOV, say 40, 52, 68, 82 degrees etc.

To get the trueFOV for each eyepiece on each scope you have, use the following formula........TrueFOV = Eyepiece-AFOV / Magnification

The Swans, I think, are as follows

40mm=70deg AFOV

33mm=72deg AFOV

25mm=72deg AFOV

Therefore with your 72mm F6 430FL scope, the

40mm=11x mag & 6.5deg TFOV

33mm=13x mag & 5.53deg TFOV

25mm=17x mag & 4.19deg TFOV

As can be seen above those eyepieces give low powers and very wide fields.

Most deep space objects like low to medium power, sometimes up to 100x. Some objects like planetaries and globular clusters like higher power, say up to 150x.

Stellarium is great but the eyepiece thing just doesn't work properly with me. I've given up on it.

When out with the scope, I usually use a low power eyepiece as a kind of finder to make sure I'm in the right place, star placement wise, then go up in magnification till the view seems best.

If I haven't grasped your question correctly, I apologise, but it's late and cloudy and I'm on the beer.

Cheers,

Andy.

[brantuk]

Thanks Andy

That clears it up very nicely. I can see where my confusion lies now and you hit the point very precisely despite the beer lol.

Stellarium obviously got me confused and I must admit I didn't realise the ep fov was the apparent fov.

I'll tell you now - I was trying to see the two very close white dots of M51 (one a good deal larger).

I must've been looking in the wrong area, or using too low a magnification - my star hopping skills have suffered since using goto for a year or two now - and I'm back on a manual eq mount lol.

Anyway - very helpfull buddy and thanks again.

Cheers

Kim

[great_bear]

Stellarium shows both the Apparent and the True field-of-view. That's if you're using the Eyepiece feature (which Andy was referring to). Is that what you were using? - Or were you simply using "Disc Viewpoint" to get a circular view?

[brantuk]

Hi Great Bear

I was just reading off the FOV from the horizontal bar at the bottom of the screen (next to the frame rate). Is that the "eyepiece feature" or is it an add on?

Cheers

[great_bear]

Ah - no, that's just the whole-screen TRUE field of view.

When you switch on "Disc Viewpoint" in Configuration(dialog)->Tools(tab), that figure doesn't even change, so it's not that great, but it gets you in the right ballpark.

No, the eyepieces feature is something quite different. Technically it's an add-on, but it comes pre-installed in version 0.10.4 (and presumably later versions too). To activate it, you press Ctrl+"o" (for "ocular")

Then you use Ctrl+"[" and "]" to switch between eyepieces, and Shift+"[" and "]" to switch between telescopes.

You use Alt+"o" to customize the list of eyepieces and telescopes.

Note: For this feature to work properly, you must exit Stellarium and re-start it after customizing the list of eyepieces and telescopes. Also, I find that the "Scale Image Circle" in Oculars(dialog)->General(tab) doesn't do at all what you'd expect and is best turned off.

Here's what the setup dialog looks like, and also how the information appears in the top right of the main screen when this feature is turned on.

[brantuk]

Oh now that looks really useful - I had no idea it existed. I'll give that a go straight away and let you know how I get on.

Thanks Bear

[great_bear]

Note that you don't need to enter "Field Stop" for the eyepieces.

I don't know why that's there - it isn't used for anything yet as far as I can see.

[brantuk]

Yup - I've got 10.4 and the ocular works - brilliant!! I'll look forward to using that when observing. Might even try hooking up the CPC cos it appears to do telescope control as well - one happy bunny with a new carrot here lol

Cheers

[Macavity]

There is the slight caveat that TFoV can be alternatively (more accurately?) defined by:

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

Maybe the above "always" works? From personal experience, very accurate predictions for wide field eyepieces may be a tad hit or miss tho'! It's often hard to measure the field stop diameter, which may be inaccessible... or even "virtual"? Sometimes eyepieces distort the image quite a bit, so trivial calculations may not be exact? But I do sense there will always be some uncertainty...

[brantuk]

One thing I'm allways sure of in life is that there will allways be some uncertainty lol. But thanks for that McCavity - will be especially useful to know when I start imaging

[AndyH]

There is the slight caveat that TFoV can be alternatively (more accurately?) defined by:

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

Maybe the above "always" works? From personal experience, very accurate predictions for wide field eyepieces may be a tad hit or miss tho'! It's often hard to measure the field stop diameter, which may be inaccessible... or even "virtual"? Sometimes eyepieces distort the image quite a bit, so trivial calculations may not be exact? But I do sense there will always be some uncertainty...

Well said.

A case in point is the not very common 2" Tal 24mm UWA. Listed as 80 degrees AFOV, it is actually has a measured AFOV of around 92 degrees, thanks to the field stop method and star timing done by some folks. So manufacturers listed AFOV can be a bit hit or miss.

Andy.

[great_bear]

Yes, that's why field-stop measurement is often termed effective field stop, because in some wide-angle eyepieces, the actual, metal, field-stop ring is between lenses and may be bigger than the 1.25" barrel and such a measurement you're not interested in.

What you want answering is "How big would a field-stop have to be, when placed across the focal plane, to mask out an area providing a field-of-view of [x] degrees?"

Your true-field-of-view calculation is mathmatically sound though, i.e:

TFOV (in radians) = field-stop-size/telescope-focal-length

[great_bear]

Tal 24mm UWA. Listed as 80 degrees AFOV, it is actually has a measured AFOV of around 92 degrees, thanks to the field stop method and star timing done by some folks. So manufacturers listed AFOV can be a bit hit or miss.

More likely that people's calculations are hit-and-miss!

I've seen a lot of false assumptions bandied about when people talk about these issues. You cannot calculate apparent field-of-view from field-stops and star-timing unless you know that the eyepiece focal length is stated correctly.

What we do know, is that it normally isn't

You can calculate focal length by measuring the exit pupil (which Chris Lord does), but I don't know how that is accurately performed. You could also probably make some jig to optically measure apparent field-of-view too. Either way, one of these measurements must be accurate before you can calculate the other one from it.

An easy way to understand this, is to write the two independent equations in their simplest form, where the lengths are measured in any unit (provided that you're consistent!) and the angles are in radians:

telescope_fov = field_stop/telescope_focal_length

and

eyepiece_fov = field_stop/eyepiece_focal_length

Where telescope_fov means "True field-of-view" and eyepiece_fov means "Apparent field-of-view"

Note that when performing drift-measurement/star-timing to work out the first equation, many people automatically assume that their telescope focal length is a fixed, known quantity. However, in the case of an SCT or Mak it is neither (although Chris Lord again, has some formula for working it out). This is because the focal length of a moving-mirror telescope with a non-planar secondary will vary dramatically depending on the location of the eyepiece focal plane, due to the change in distance between the primary and secondary.