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I will however grant the concession that the reporting of an eyepieces RFOV in no way means that the eyepieces AFOV should not also be openly reported. In fact with both numbers reported, loads of detail can be calculated regarding the inherent distortions present in the design, so both must be reported.

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Good point! I thought of this, but OTOH there is (I would hope at least) likely no one who is not by now aware of the highly non orthoscopic character of the Nagler's and Panoptics, and this certainly has not stigmatized their very well deserved reputations. I therefore rapidly concluded that there is simply no valid stigma associable with being either orthoscopic or not orthoscopic, so I completely discounted this potential.

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The field stop diameter very well *should* be the standard for eyepieces, although the apparent field of view is something people might like to look at to see how much of a span they will see when they look in the eyepiece. I see little reason to "calculate" some odd-ball "real apparent field of view" that pulls in the field stop number to the equation, when the simple field stop formula gives great results to begin with. Introducing some "fudged" apparent field is just unnecessary and potentially confusing. Clear skies to you.

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OK, I get what you're saying here. Not all eyepieces have a directly measurable field stop, however. So what the drift test gives me is a kind of "virtual field stop" number that I can use to predict true field of view for a given eyepiece in any telescope.

That seems rather arbitrary too, but at least I see how it can be tied to something physical -- even if it's just imaginarily so.

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"Since the process of science generates more mysteries than it solves, I predict that we'll never learn everything: and we'll continue to generate new ignorance at the speed of knowledge."



"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror

Under Construction: The "Eye of Sauron" Observatory!

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I don't think it's an artificial concept at all. It's a real characteristic of a specific eyepiece. When all distortions and aberrations are taken into account, the Hyperion 8mm will perform as a 76 degree eyepiece, and the UO Orthoscopic will perform as a 39 degree eyepiece.

Once that measurement is done, I can insert the eyepiece into a scope of a different focal length and predict what the true field will be.

Is that incorrect?

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The way I look at it, true field is one thing, apparent field is another. And there's no easy way to derive one from the other.

DonP

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Don, have you read my posts in this thread? The RFOV is a quite easy way to bridge the gap between AFOV and TFOV. All we need is to encourage its adaptation by the manufacturers, distributors, and retailers. In fact, TeleVue has given us all of the requisite tools required already.

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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.

Ayn Rand (in the appendix to 'Atlas Shrugged')

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The field stop diameter very well *should* be the standard for eyepieces, although the apparent field of view is something people might like to look at to see how much of a span they will see when they look in the eyepiece. I see little reason to "calculate" some odd-ball "real apparent field of view" that pulls in the field stop number to the equation, when the simple field stop formula gives great results to begin with. Introducing some "fudged" apparent field is just unnecessary and potentially confusing. Clear skies to you.

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OK, I get what you're saying here. Not all eyepieces have a directly measurable field stop, however. So what the drift test gives me is a kind of "virtual field stop" number that I can use to predict true field of view for a given eyepiece in any telescope.

That seems rather arbitrary too, but at least I see how it can be tied to something physical -- even if it's just imaginarily so.

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For one eyepiece in particular, I have had to use this approach: the 5-8mm Speers Waler. Its variable effective focal length due to the changing location of the Smyth "field flattener" lens out in front meant that there was no way to actually measure any field stop. The only thing I could do was to measure the true fields at the ends of the focal length numbers and then work "backwards" to arrive at two "effective field stop" figures for the shortest and longest focal lengths of the eyepiece. However, this was far better than what the *company* actually did. They worked backward using the AFOV/Mag formula and got apparent fields that varied and were just plain silly to begin with (well beyond what the eyepiece actually provides). The apparent field of view of that eyepiece was actually fixed at a constant 79 degrees no matter how the eyepiece was set in focal length, so the company's claims were just plain ridiculous. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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I do, however see the utility of the "RFOV" concept. it allows you find an equivalent field stop and predict the true field of view of an eyepiece in any Telescope, even if you don't have access to the written specifications.

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I do, however see the utility of the "RFOV" concept. it allows you find an equivalent field stop and predict the true field of view of an eyepiece in any Telescope, even if you don't have access to the written specifications.

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Here are calculated "RFOVs" for the various Nagler eyepieces: 77.6, 77.1, 81.0, 78.5, 81.9, 79.1, 77.6, 81.6, 77.6, 78.9, 79.4, 80.2, 78.6, 77.9. How are these numbers easier to use in the field or elsewhere than the effective field stop diameters that Tele Vue quotes?

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As far as I know, they are not easier. Maybe it is because people almost always see the old TFOV = AFOV/Mag formula and then get hung up on using that particular equation rather than the somewhat lesser-known but more accurate TFOV = 57.3*EFSD/Fl formula (EFSD is eyepiece field stop diameter and Fl is telescope focal length). Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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David, I haven't looked thru a lot of eyepieces, but found interesting that the Hyperion 8mm might have a wider view than advertised--

I have a 38mm Q70, supposedly 70 degree AFOV, but every time I look thru that thing, the AFOV seems huge. It gives the subjective impression of being much wider than some cheapie 80 degree AFOV UWA's I have, and that 38mm Q70 feels drastically wider than a 66 degree AFOV Expanse.

I might buy a 26mm Q70 to see if it gives the same 'huge' subjective impression as the 38mm Q70. Maybe the 38mm Q70 just subjectively seems wide because of the huge eyelens, but it is a very repeatable subjective impression. Every time I look thru the 38mm Q70, it just slaps me in the forehead, "Boy, that's an awfully wide view!"

Is the large eyelens the likely explanation? Or maybe the big exit pupil?

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Hi there. Well, visual appearance does have a lot to view with the impression one gets about the apparent field. I think the only way to know for certain whether the eyepiece apparent field of view figures are accurate is to just go ahead and measure the apparent field of view of the eyepiece. Here is how it is done:

MEASURING THE APPROXIMATE
APPARENT FIELD OF VIEW OF AN EYEPIECE

MATERIALS: 1. A Meterstick, Yardstick, or other linear device whose length is accurately known, which can be hung vertically on a wall, and whose exact middle or center is accurately marked. This could also be a narrow strip of paper of known length with its exact middle and ends marked clearly. This object will be known as the observing "target".

2. A method of holding and properly supporting an eyepiece rigidly in a horzontal position (like a bracket attached to a camera tripod), but which can be manually moved towards or away from a measuring target.

3. A tape measure.

STEP #1: Mount the vertical "target" (ie: the Yardstick or its substitute) on the wall so that its exact middle is will be about same height above the floor as the center of the eyepiece. For a meter stick, the midpoint will be the 50cm mark, and for a yardstick, it will be the 18 inch mark. Mark this midpoint with a visible marking like a small piece of tape or a black felt tip marker, so the middle can be easily seen from a distance.

STEP #2: Mount the eyepiece at a height above the floor which is exactly the same as the mid-point of the target, so that the observer can look into the eye lens with the eyepiece optic axis or barrel horizontal and parallel to the floor. Make certain the eyepiece is as horizontal as possible, and that it can be easily moved towards or away from a nearby wall from as little as two feet from the wall to as much as six feet away.

STEP #3: place the eyepiece straight out from the wall from where the observing "target" is located. Look into the eyepiece with *both* eyes open and merge the images of the eyepiece field of view and the target. Make the center of the superimposed eyepiece field centered on the mid-point mark of the observing target as closely as possible, and keep your head level with the floor (ie: keep your eyes at the same height above the floor).

STEP #4: Look at the top and bottom of the target, again with both eyes open. Try to make the top and bottom edges of the eyepiece field match the top and bottom edges of the target on the wall by carefully moving the eyepiece towards or away from the wall. Make certain when moving the eyepiece that it remains pointed exactly towards the center of the observing target, and that its height above the floor does not change. Once the edges of the eyepiece field match the top and bottom of the target, take the tape measure and measure the distance from the back of the eyepiece just beyond the eye lens (ie: where your eye was sitting when you were looking through the eyepiece) to the middle of the target on the wall. If the target has a length of "2Y" and the distance to the wall you measured is "D", then the apparent field of view of the eyepiece is then AFOV = 2*ATAN (Y/D), where Y is *half* the total length of the target and ATAN the arc-tangent (or inverse tangent) function. For example, if you were using a yardstick (36 inches in length, or Y = 18.0 inches) and your eyepiece field matched its length at a distance of 37.0 inches from the center of the target, the apparent field of view of the eyepiece would be about 51.9 degrees. Measure the apparent field several times and take an average of the measurements for the most accurate results.

Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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Here are calculated "RFOVs" for the various Nagler eyepieces: 77.6, 77.1, 81.0, 78.5, 81.9, 79.1, 77.6, 81.6, 77.6, 78.9, 79.4, 80.2, 78.6, 77.9. How are these numbers easier to use in the field or elsewhere than the effective field stop diameters that Tele Vue quotes?

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They do have a significant advantage in that they speak directly to the observer, and they are directly valid and true in the equation:

TFOV = RFOV/Mag

The simplicity and intuitiveness of this trumps calculations and comparisons involving field stop numbers. Mike, you and I are in professions where complex calculations are a daily happening. Most others are not. We must attempt to see the world though their glasses.

Back to how the RFOV speaks to the observer directly and intuitively. I offer an example using the findings of the first poster to this thread. He observed that the RFOV of the 68 degree AFOV Hyperion and the RFOV of the 82 degree Nagler were virtually one and the same. He thereby intuitively knows that despite the markings on the eyepieces, they will in fact show him the exact same amount of the night sky, assuming only that their FL's are the same. Is this not immediately intuitive and simple?

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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.

Ayn Rand (in the appendix to 'Atlas Shrugged')

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Is this not immediately intuitive and simple?

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Hi, I have a 31 nagler and and just received a Ethos 13mm.
with a laserpointer you can easy measure the FOV by pointing the laser through the eyepiece on to a board,
After some playing with the laser you will notice the edge of the field on the board, mark this edge of the fieldstop (of the eyepiece) ON the board,
slide the laser to the opposite of the fiedstop and mark it also.

Measure the distance between the eyepiece focal point and the board.
Measure also the distance between the markings.

With basic mathematics (and a calculator with tangent) I calculate for the 31 Nagler (first measure): 84 degree
(second measure): 83 degree.
For the Ethos (first measure): 99.96 degree
(second measure): 100.2 degree

Simply a 31 Nagler HAS a 82 degree field, and a Ethos HAS a 100 degree field.

Holland from the Netherlands

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I have edited things here. This is a *very* good method, but does not require a laser. All you need is a well-collimated flashlight of sufficient size and a fairly parallel output. "Lambo" and I did this tonight. We set things up and moved the flashlight around a bit until we could actually see the edges of the field stop projected on the wall. Usually, you can't see both edges at the same time, but for a number of eyepieces, the whole disk from field stop edge to field stop edge was visible. Particularly amusing was his little reticle eyepiece, where we saw the reticle projected as well. To get the precise location of the focal point, we put the flaslight several meters away from the eyepiece so as to approximate a distant source. The fields of view we got were nearly identical to that using my method, but for the wider field eyepieces, they ended up closer to the figures given by the manufacturer. Indeed, I suspect that because of the difficulty in viewing both side of the field stop at the same time in the eyepiece, my "both eyes open" method may occasionally be somewhat less accurate than this nice projection method. When we measured the 24mm Panoptic, we got *exactly* 68.0 degrees rather than the 69.5 degrees I had with the "both eyes open" technique. The only eyepiece which gave us a few headaches was my old Meade 14mm Ultrawide, as the flashlight had to be held very steady to get the field edges to be projected with any repeatability. My 40mm Mk-70 Konig ended up having an apparent field of 68.8 degrees rather than the nearly 70 degrees I had gotten earlier. Johns old 32mm Burgess was 70.9 degrees, while his 20mm Type 5 Nagler was 80.5 degrees. This is a simple yet quite elegant way of measuring the apparent field of view, so again, thanks a bunch for bringing it to light. Clear skies to you.

--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org


Edited by David Knisely (05/21/08 05:00 AM)

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