[ngcles]

Hi Darryl, (his mate) and all,

First principles.

The magnification an eyepiece provides is a function of the focal length of the telescope divided by the focal length of the eyepiece. Eg 200mm aperture 'scope with 1000mm fl and 10mm fl eyepiece gives x100 magnification.

The exit pupil is a function of the aperture in mm, divided by the magnification in use. Eg 200mm aperture / x100 = 2mm exit pupil. Alternately, it is also approximated by diviiding the fl of the eyepiece by the f/ ratio of the telescope

The Apparent Field Of View (AFOV) is the angle (expressed in degrees) your eye must turn through to shift your gaze from one side of the field stop to the other. By and large, it will be determined by the diameter of the field stop inside the barrel of the eyepiece. Lets assume for the sake of the exercise it is a 50 deg AFOV eyepiece like many popular Plossl eyepieces are.

The True Field Of View (TFOV) is the angular diameter of the actual field (slice of sky) visible within the eyepiece when used in the telescope. It is approximated (in degrees) by dividing the AFOV by the magnification the eyepiece provides with a telescope. For eg with the above 'scope/eyepiece 50 deg AFOV / x100 = 0.5 degrees diameter true field.

Eye relief is the maximum distance the cornea of the eye can be from the top glass element of the eyepiece (the eye-lens) while you can still see the whole of the field-stop within the eyepiece.

You asked for specific comment on these two paragraphs:

"He wants to maintain the largest TFOV he can as he bumps up the magnifications on his scope when he changes to a shorter FL ep. He has told me he presumes that using his wide angle 32mm 2" ep with a 2" barlow (2x) would give him the same magnification as just using a 16mm ep but with a wider FOV than the 16mm by itself.

Not having used a 2" (2x) barlow I am technically (and literally) in the dark on that one but intuitively "feel" there is still going to be losses - from an inch & a quarter perspective, and without bringing barlows into the equation, I believe that (imho) you can't raise the magnification and retain the same TFOV - it just can't be done; although if the ep with the shorter FL is of a different type (ie types with a wider FOV) and the increase in magnification isn't a large jump; then it is possible to maintain the FOV and have (somewhat) greater magnification."

Re the first paragraph:

In the other thread I think you referred to, Dennis wrote (partial extract):


16mm 82 degrees.
FOV 31.5 min
Magnification 156.2
Dawes limit 0.5”
Magnitude 15.6

30mm 43 degrees.
FOV 31 min
Magnification 83.3
Dawes limit 0.5”
Magnitude 15.3

And I added immediately below (partial extract):

"The view through the 43 deg AFOV eyepiece while about the same size will be much less aesthetically pleasing, will have a slightly brighter background sky, the view will seem much more "compressed" and because of the narrowness of the AFOV, will seem a bit like looking down a sewer pipe. (Emphasis added)

The 82 deg AFOV ep while showing essentially the same slice of sky will be much more like looking out of a porthole, show a more contrasty view and more detail will be more easily discerned due to the higher magnification -- assuming it is a high quality occular -- which an 82 deg AFOV usually implies! All round much nicer!" (Emphasis added)

And I add specifically now, No, the TFOV will be almost exactly the same as I said (see figures by Dennis above). He (your mate) wants to maintain the highest TFOV while bumping up the magnification -- well the only answer is to get a shorter focal-length but wide-field design eyepiece of some sort with a larger AFOV. The TFOV is determined by the AFOV/magnification. To maximise TFOV, get a bigger AFOV -- there is no free lunch! Your comment in the second par: "I believe that (imho) you can't raise the magnification and retain the same TFOV - it just can't be done" is succinct and correct. You cannot "buy" TFOV while also raising magnification unless you use a different eyepiece with a larger AFOV

Re the Barlow. If it is a 2x Barlow for example, in effect it doubles the magnification the eyepiece produces with the 'scope. As the TFOV is a function of AFOV/magnification, if you double the magnification, you halve the TFOV.

As the exit pupil is aperture/magnification, if you double the magnification by adding a 2x barlow, then you must also, axiomatically, halve the exit pupil.

The eye relief is a different matter. The eye relief is an in-built characteristic of the eyepiece itself and is not affected by the design, aperture or focal length of the 'scope. If say a 30mm ep has an eye-relief of 20mm and is then used in a 2x barlow (and in effect acts like a 15mm ep which would normally have less eye-relief), the eye relief of the eyepiece remains at 20mm because that is built into the eyepiece. In the same way, an eyepiece with a 20mm eye-relief will have exactly the same eye-relief characteristic in a 200mm f/5 Newtonian as it will in a 400mm f/10 Schmidt/Cassegrainian, though the magnification and accordingly TFOV will be dramatically different.

Hope this is of some help.


Best,

Les D
Contributing Ediitor
AS&T