Telescope Reviews: Strange Results in Eyepiece FOV Comparisons

For two weeks in a row, I've done drift tests on two different stars that are almost exactly on the celestial equator. I used a stopwatch to time the interval between the appearance of the star on one side of the eyepiece field, and its disappearance on the other. I was careful to only count the tests where the stars actually drifted across the center of the FOV, and my individual tests were consequently pretty consistent.

The two stars I used were Struve 1757 in Virgo (a pretty double of 2 arcseconds separation, listed in Burnham as magnitudes 7.5 and 8.5) and a 6.5 magnitude star slightly south of Lambda Ophiuchi (a.k.a Marfik, another pretty pair with a 1.5" separation, curiously not mentioned by Burnham).

I tested 6 eyepieces, 5 of them also using a barlow lens. Part of the reason for the test was to get a more accurate calculation of the power of the barlow -- it's listed as 2x, I ended up measuring it as 2.1x.

The eyepieces were:

32mm TV Plossl
25mm No-name Kellner
12.5mm UO Orthoscopic, the "volcano top"
9mm "Astrola" Plossl
8mm Hyperion
2.5mm Nagler Type 6

I was planning on doing a fairly general review of my subjective impressions of each eyepiece, but instead, I got some unexpected results measuring the fields of view.

Using the drift test to measure the actual field of view, I then multiplied by the magnification to get the apparent field of view. This is simply the inverse of what is often recommended as a means of finding the actual field of view from the listed apparent one.

The Plossls, Kellner and Orthoscopic had measured apparent FOVs that were not too far off from those commonly listed. In fact, they measured slightly narrower.

The TV 32mm Plossl showed an AFOV of about 47.5 degrees, the Kellner about 38.2 degrees, the Orthoscopic about 39.7, and the Astrola plossl came in at 49.9 degrees.

However, the Hyperion in all tests far exceeded its rated 68 degree AFOV. In fact, my measurements clustered tightly at an average of 76 degrees! And optical aberrations were far smaller than they were in any of the narrower-field eyepieces.

When I first did the test last week, I simply didn't believe it. I waited until tonight and redid it, thinking the first one was a fluke. However, the measurements repeated. My 8mm Hyperion seems to be a 76-degree eyepiece.

And the Nagler? Strangely, it didn't make the advertised 82 degrees. In fact, I "only" got 78 degrees out of it! They were a very good 78 degrees -- I saw no change in the star image from edge to center. It simply didn't quite make the advertised field diameter. That was very surprising to me.

In fairness, the Nagler is a NEAF blem, I bought it at a huge discount, and it's more than worth what I paid for it. So I'm not complaining. However, I don't think the FOV difference is related to the blemish, which is nothing but a couple of cosmetic specks near the edge of the field stop.

I'm most surprised about the Hyperion. Not only was the field significantly larger than expected, it compares very well with the Orthoscopic for image quality and transmission. The TV eyepieces are, as has been noted by others, a bit on the "warm side", and the Astrola plossl, while sharp, also exhibits a slight haze. The Kellner is almost useless at my F-5.6 focal ratio, although barlowing improves its edge performance considerably.

I'm wondering if anyone else reports similar results, or if there are indeed big variations between individual eyepieces of a particular make and focal length?

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