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Field Stop... what is it?

Started by astroboy314, 12/13/2002 08:06AM
Posted 12/13/2002 08:06AM Opening Post
Okay, call me stupid here, but what is field stop and what is it useful for? I've seen the formula to calculate FOV with it (FieldStop/EPfocallength)*57.3... what else does it imply? Should I be looking into field stop specs when lookig for a new eyepiece?

Thanks as always... I love this forum...

Posted 12/13/2002 09:57AM #1
Not only does the field stop ring inside the eyepiece barrel give a clear clean edge around the field of view, which makes the view to the eyee more pleasing, but also it is the determining factor in how much a True field of view you get. It eliminates the need to rely on the stated Apparent fov rating of the eyepiece which may or may not be accurate.

The correct formula is 57.3(1 radian)/(scope F/ep field stop)= Tfov in degrees. Multiply by 60 to get Tfov in minutes.

example: for scope with F = 1200

I have two meade 9.7 eyepieces one is stamped SERIES 4000 SP, the other one is stamped SUPER PLOSSL. The series 4000 has a field stop of 7.5mm while the super plossl has a field stop that measures 8.2mm.

S4000 fs=7.5 57.3/(1200/7.5)*60 = Tfov = 21.5 arcmin.

SP fs= 8.2 57.3/(1200/8.2)*60 = Tfov = 23.5 arcmin.

They are both the same power but the SP has a wider field of view. The simpler Tfov calc based on mag would give the same result for both, 49/(1200/9.7)*60 = 23.8

Only about 8 out of 45 eyepieces that I have recorded data for have a field stop that produces the same field of view as that calculated by the simpler formula which is Afov published/mag. More than 30 of the eyepieces have a field stop that is smaller and results in a True fov slightly to substantially smaller than you would get based on the Afov/mag calculation.

I have verified more than of dozen of these by drift times to get the true field of view. All the drift times agree within 1% of the field stop calc. The field stop formula almost always produces a more accurate indication of what will be the True field of view than the Afov/mag formula.

Posted 12/13/2002 06:13PM #2

It looks like nobody has answered your question.

A field stop is a circular disk, with a circular hole in the middle. It sits on the field end of an eyepiece (towards the objective, and away from the eye). Light from the objective comes through the circular hole. All other light from the field end of the scope is stopped. Basically, it marks the outer edge of where light is allowed to come through the piece of glass.

On most eyepieces, you can see the field stop by looking from the field end of the eyepeice. It is the blackened disk. ON some eyepieces, however, the "field stop" is actually buried on the far side of some of the glass.

And yes, different eyepeices have different locations for the fieldstop.

Imagine a lens that is very sharp in the middle, but progressively less sharp as you move to the edge. (All lenses, no matter who makes them and whose name is on them, have this problem.) At some point, you find the sharpness at the edge is just unacceptable. That (or better yet, just before that) is where the designer puts the "Field stop" to keep any rays from further out getting into the view.

If you have a 60 degree eyepiece, you may be able to increase its apparent field of view by taking out the field stop--but this would result in an unacceptable edge. THis is a technique employed by some manufacturers. That is why you can take a 55 mm plossl, remove or enlarge the field stop, and sell it as a "Wide Angle 68 degree.) It does in fact have a wider field of view, but the extra 13 degrees (or so) are pretty ugly.