View Full Version here: : True field of view

12-03-2007, 11:41 PM
As I understand it - regardless of whatever size eyepiece you use, the theoretical TFOV as claculated from eyepiece size, focal length of the telescope, and AFOV is still restricted by the field stop diameter.
So say my 4.5" f8 newt (1.25" holder) with a say 40mm plossel eyepiece gives
mag = 900/40 = X 22.5, (and an exit pupil (e=a/mag) of 114/22.5 = 5.07mm)
TFOV (calculated) = AFOV/mag = 50/22.5 = 2.22 degrees
TFOV (field stop) = field stop/focal length X 57(degrees), and the field stop max in a 1.25" eyepiece is 27mm (actual)
So its actual TFOV is 27/900X57 = 1.71 degress (102 minutes of arc).

It does not matter whether it was a super plossel or Nagler in this respect as the field stop limit greatly curtails the TFOV.

A 31mm plossel would give a calculated TFOV of 50/(900/31) = 1.71 deg.

Thus the 40mm and 31 mm give the same TFOV in my scope, but the 40mm has X 22.5 mag versus X 29 mag, and exit pupils of 5.07mm versus 3.93mm.

It also follows that a 25mm Super wide plossel (70 degree AFOV) has a calculated AFOV of 70/36 = 1.94 degrees - and would be utterly wasted.
There is no point for me buying Super wide angle eyepieces (approx 70 AFOV) in the F8 more than about 22mm if what I want is larger fields of view. I may as well get a good plossel. If I do anyway, I will see the same field ov view, merely at greater magnification (If that is what I want).

Did I get that right?

14-03-2007, 12:23 PM
Col what you've said is basically dead right - the actual theoretical limit of a 1.25" barrel ep (one in which the barrel diameter itself is the field stop) is approx 32mm I believe - anything over this is being vignetted by the barrel.

The precise calculation for True FOV using field stop diameter is :

TFOV = (180/Pi)*field stop / Scope focal length.

180/Pi converts the calculation from radians to degrees - approx 57.296

The problem can occur in actually measuring the field stop of a given ep, usually simple for standard plossl's although it gets more difficult as the field stop gets smaller or harder to access to measure accurately. In some more advanced design ep's it can be very difficult to measure the field stop, especially for those designs where the field stop itself lies between lens elements inside the ep and is not externally visible as such.

I have read that "calculating" true FOV by dividing magnification into apparent FOV gives an approximation only and may have up to 10% error, however it's still a fairly good guide and easily calculated in any ep where the apparent field of view is known.

IF you want a big field of view (and I very much enjoy such a view frequently) get a good 2" ep in a longer focal length. Generally if you want a 30, 35, etc mm ep then it should be manufactured as a 2" barrel version for pretty much the reasons have related. You just can't get a really good "true" wide view in a 1.25" barrel.

I hope I've helped clarify things and not muddied the waters further.


14-03-2007, 01:59 PM
Hey guys, just a real beginners question that I probably could Google the answer for, but it is probably quicker this way. I have a telescope which takes 1.25" EP's, if I wanted a 2" EP, is there such a thing as a converted or adapter that will permit the use of a 2" into a 1.25" hole?

14-03-2007, 04:03 PM
Many thanks. I needed to know I was not barking up the wrong tree. This is why longer tubes have wider eyepieces I suppose. Otherwise the problem gets worse. A 10" f5 reflector witha 1.25" eyepiece would only have a TFOV of 27/1250x57 = 1.23 degrees, even with a 2" focuser only gives 46/1250x57 = 2.1 degrees. If you really want wide views you need short tubes...

15-03-2007, 04:07 AM
"If you really want wide views you need short tubes..."

That's basically true, which is why people tend to use binoculars if they want really wide field views - remember telescopes are really about magnification instead of wide angle viewing.

Conversely if you really want more magnification you want a long f ratio tube, eg a 25 mm plossly in my old 1250mm fl dob gave 50x magnification, put it in a mak, etc, with an efffective focal length of 2500mm and you instantly have 100x mag. It all depends on what kind of results you are trying to achieve.

Ron regards your question if you put a 2" ep into an adaptor that then fits in a 1.25" hole the problem is still that all your incoming photons are still being blocked / restricted by that 1.25" hole - think 2 lanes of traffic into 1 lane during roadworks then back to 2 lanes again afterwards. You are still limited by the narrowest point in the light path, ie the 1.25" hole.

What you need to do is see if your focusor already has 2" capability, or alternately replace the focussor with one that can accomodate 2" ep's. Not knowing your scope I'm unable to tell you much about that sorry, but I'm sure someone else here will be able to. Try asking the person you got it from, he(she) may know.

15-03-2007, 08:32 AM
Thanks Steve, that is really descriptive, it illustrates the point precisely :thumbsup: