is there a formula for FOV

[Rob wright]

As title says realy I am trying to fine the fov through a Orion starblast 6 with different eye pieces and wondered if there is a formula to find out?

say a 25mm ep and the scope is a 150 mm with fl of 750mm

Its a great scope I want to be able to set stellerium to roughly the same fov

[Ben Ritchie]

I've always used true field of view = apparent field of view / magnification, where magnification = telescope focal length / eyepiece focal length.

So for a 13mm Ethos, 100-degree AFOV, 1040mm Focal length you'd get 80x magnification (1040/13) and TFOV = 100/80 = 1.25 degrees. Can't answer your example, as I don't know the AFOV of the 25mm - magnification would be 750/25 = 30x

[acey]

True field of view can be measured directly using the drift method: Measuring field-of-view

Or it can be calculated from the apparent field of view of the eyepiece divided by magnification (as Ben says). The following is from Luginbuhl and Skiff's Observing Handbook:

"The apparent field is sometimes given by the manufacturer, but it can quite easily determined as follows. Holding the eyepiece to one eye, position yourself such that the circle of light you can see through the eyepiece is the same size as something viewed simultaneously with your other eye: a window frame for example, or a standard 11-inch piece of paper on a table. After measuring the size of the object, and the distance from it to your eye, the apparent field (in arcminutes) can be computed as:

apparent field = 120 arctan[d/(2r)]

where

d = size of object

r = distance from object to eye (in same units as used for d)."

[Ben Ritchie]

It's quite interesting too to measure TFOV and work backwards to figure out magnification - doesn't always match what you might expect

[Altair40]

Again, thankyou forum - this is just the sort of hard info I need in starting to observe again. Didn't know enough to ask that question. You have me thinking now!

[Rob wright]

hmmm thanks very much, just what I am after

the net says my 25 mm has a afov of 52

so 52/(750/25)=1.73

does that sound correct?

[brantuk]

Andrews primer explains all this perfectly and then some

http://stargazerslounge.com/primers-tutorials/63184-primer-understanding-choosing-eyepieces.html

[Rob wright]

ahh thats brilliant thanks

I have looked all over but missed that some how.

so many things to learn and so little patience rofl

only joking I love this i'm learning so much and loving it.

[acey]

Another method: TFOV = (FSD / Fo) X 180/Pi

Where: TFOV= True field of view, Fo = Focal Length of scope, FSD = Field Stop Diameter of eyepiece, Pi = 3.14.

Methods summarised here (bottom of page): 40mm Sirius Plossl and 40mm UO MK-70 - Review

[Glen]

Hi, this site may help

http://www.12dstring.me.uk/fov.htm

Good luck

Glen