[Humboldt Jim]

It seems like there should be a defined inverse relationship between the focal ratio of a scope and the maximum practical field of view. I have not been able to find any chart or formula to make this determination.

There is so much focus (pardon the pun) on power, we forget how useful a wide FOV can be. However, a too-wide eyepiece for a given scope can cause blurred images.

For example: If I want to obtain a standard 1.25 inch eyepiece for scope with an specific focal ratio, how would I figure out what focal length eyepiece to get for maximum field of view?

Of course there are variables from scope to scope, but a standard optical formula would be a good place to start.

Thanks.

[AlanFrench]

Jim,

Field of view is determined by two things - the focal length of the telescope, and the diameter of the field stop in the eyepiece. Focal length determines image scale, and the shorter the focal length the smaller the object is in the image plane. The field stop of the eyepiece defines the area of the image plane that is examined by the eyepiece.

The formula for true field is:

True field, in degrees = 57.3 times (field stop diameter divided by telescope focal length)

Often the focal length of the telescope is not specified, nor is the diameter of the field stop given. You can get an approximation of the true field simply by dividing the apparent field of the eyepiece by the magnification. Most companies supply the apparent field of the eyepiece (apparent field is simply the angle your eye would move through if you looked at one edge of the field and then scanned across to the other edge).

TeleVue provides detailed information on all their eyepieces, so let's look at there little 60mm APO and see what happens. The TV 60 has a focal length of 360mm (f/6).

Here are some 1.25" TV eyepieces with large field stops. In any eyepiece the field stop diameter is limited by the eyepiece o.d. and the need to have some thickness to the wall.

Eyepiece Focal Length/Field Stop/ Power/True field/Apparent field/Approx field
16mm (Nagler 5)/22.1mm/22.5x/3.52/82/3.64
24mm (Panoptic)/27.0mm/15.0x/4.30/68/4.53
32mm (Plossl)/27.0mm/11.25x/4.30/50/4.44

The true field and apparent field are given in degrees. The "Approx field" is the approximation obtained by apparent field and manification.

The typical "wide field" spotting scope eyepiece has an apparent field of about 67 degrees. Most companies provide the true field of the eyepieces supplied with each spotting scope.

I hope this helps.

Clear skies, Alan

[Humboldt Jim]

Thanks Alan. That does help. I had not considered the field stop.

I also relaized that my question was not as clear as it could have been. To boil it down, what I really should have asked is how to determine the maximum possible field of view obtainable, with any eyepiece, with a given scope. I am assuming an inherant optical limitation of the angle of view that is imposed by the geometry of the scope, regardless of the eyepiece configuration. Is my assumption reasonable?

Clear skies in Humboldt County California are not as rare as the Black Skimmer recently seen here, but are greatly appreciated.

JIm

Originally Posted by AlanFrench

Jim,

Field of view is determined by two things - the focal length of the telescope, and the diameter of the field stop in the eyepiece. Focal length determines image scale, and the shorter the focal length the smaller the object is in the image plane. The field stop of the eyepiece defines the area of the image plane that is examined by the eyepiece.

The formula for true field is:

True field, in degrees = 57.3 times (field stop diameter divided by telescope focal length)

Often the focal length of the telescope is not specified, nor is the diameter of the field stop given. You can get an approximation of the true field simply by dividing the apparent field of the eyepiece by the magnification. Most companies supply the apparent field of the eyepiece (apparent field is simply the angle your eye would move through if you looked at one edge of the field and then scanned across to the other edge).

TeleVue provides detailed information on all their eyepieces, so let's look at there little 60mm APO and see what happens. The TV 60 has a focal length of 360mm (f/6).

Here are some 1.25" TV eyepieces with large field stops. In any eyepiece the field stop diameter is limited by the eyepiece o.d. and the need to have some thickness to the wall.

Eyepiece Focal Length/Field Stop/ Power/True field/Apparent field/Approx field
16mm (Nagler 5)/22.1mm/22.5x/3.52/82/3.64
24mm (Panoptic)/27.0mm/15.0x/4.30/68/4.53
32mm (Plossl)/27.0mm/11.25x/4.30/50/4.44

The true field and apparent field are given in degrees. The "Approx field" is the approximation obtained by apparent field and manification.

The typical "wide field" spotting scope eyepiece has an apparent field of about 67 degrees. Most companies provide the true field of the eyepieces supplied with each spotting scope.

I hope this helps.

Clear skies, Alan

[AlanFrench]

Jim,

The main factor for the telescope itself is its focal length. A bird has a certain size in the focal plane of the telescope, and that size is determined solely by the focal length. In a longer focal length instrument, objects are larger in the focal plane, and this reduces the maximum true field. The other issue is illumination and vignetting. You want the entire field to be bright and fully illuminated, although often the designer will allow some vignetting at the edge of the field. This reduces the size of the optics required, and the cost and weight.

The ultimate limit on any particular focal length telescope is the size of the field stop in the eyepiece, and this is limited by the size eyepiece barrel the telescope uses. If you know the focal length, just assume the biggest possible field stop is about 27mm, and that should give you a rough idea.

Clear skies, Alan