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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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