Hyperion Zoom: What I am seeing is not what's advertised
As I have written recently, I love the Hyperion eyepieces and feel the 8-24 zoom is my all-purpose eyepiece of first choice. But the cold truth of what I am seeing is either the AFOV is way off, or the focal length is way off. We're not talking a little distortion here, nor splitting hairs on advertised focal length - and we are talking about a major selling point for the Hyperion over other zooms which is its wide field of view. Many differences from eyepiece to eyepiece are subjective, such as the contrast - but the FOV is objective and easy for anyone to see.
Most competing zooms offer a relatively narrow apparent field of view – 40 degrees – at the lowest power setting. Hyperion says they deliver 50 degrees at this setting. That would be nice. Puts it in the same league as the typical Plossl, an eyepiece I also like very much. What the difference between 40 and 50 in terms of real field of view. Well, the ballpark way to arrive at a number is to divide the power obtained into the AFOV. So, with my 8-inch LX90 operating at F10, the power of a 24mm eyepiece is 83X. A true field of view then, would be 40/83, or about 29 minutes. That's typical of zooms on the market. For the Hyperion , you would divide 50 by 83 to get 36 minutes.
Figure the area of a circle with those diameters and you can see this is a substantial increase. In the first case the area 660, and in the second, 1017. So with the 50 degree you should be looking at quite a bit more sky.
(Click the image abovw for a larger image. ) This image is from Starry Nights software and shows the predicted true field of view. (I suspect it calculates by the same formular as I do, as our answers were the same. ) The outer green circle is for the 25mm Sirius Plossl. Next (in red) is the predicted FOV for the Hyperion - not what I saw, but what the numbers predict. The yellow circle is the predicted field for the 20mm Televue Plossl - a little larger than I saw. The inner most red circle represents the predicted FOV for the Vixen and Celestron zooms when set at their lowest power.
Since the Moon is typically about 30 minutes in diameter – it varies a little by where it is in its orbit – an eyepiece giving a 36 minute field would fit the entire moon with some comfort. The Hyperion doesn't. well, I should say, the specific Hyperion zoom I own does not.
But forget about the numbers for the moment. The 7-day moon was in and out of clouds last night, so I could not do the type of observing I like, so I devoted an hour to carefully comparing the true FOV of five eyepieces. They were:
25mm Sirius Plossl, 52-degree AFOV, 39' calculated true FOV
20mm Televue Plossl, 50-degree AFOV, 30' calculated true FOV
8-24 Hyperion Zoom at 24mm, 50-degree AFOV, 36' calculated true FOV
8-24 Vixen Zoom at 24mm, 40-degree AFOV, 29' calculated true FOV
8-24mm Celestron Zoom at 24mm, 40-degree AFOV, 29' calculated true FOV.
To test theory against reality, I placed the moon so its south end was just at the center of the bottom edge of the eyepiece. I then noted what Lunar feature was at the top edge of the eyepiece. It happened that the distinctive crater pair of Aristoteles and Eudoxus were near enough to the terminator to provide a convenient mark. In all cases I was using the same 8-inch LX90 at F10 with 1.25-inch diagonal.
Here are the results:
Celestron Zoom seemed a bit below the advertised 40 degree AFOV - but at $63 I feel forgiving. (Truth is that many time when I've tried to evaluate the FOV of eyepieces they've seemed a little shy of their advertised value.)
The Vixen Zoom seemed to be very close to the advertised 40-degree AFOV.
The 20mm TV Plossl was giving slightly larger FOV than the Vixen Zoom, but this to was pretty close to its specs - remember, it is a 20mm eyepiece, not 24.
The 25mm Sirius Plossl, with an advertised 52 degree AFOV seemed to be close to its specs, though it was difficult to measure since the field extended well beyond the edges of the moon. I didn't have a handy mark, just a guestimate.
The Hyperion 8-24 set at 24 barely edged out the Vixen. In terms of the Moon it got me across Mare Frigoris as it appears above Aristoteles.
If I had to assign a number to this, it performed as if the AFOV was 42 degrees Or you can assume the advertised FOV of 50 degrees is correct, in which case it performed like perhaps 20.2 mm rather than the 24mm it claims to be. Either way, the numbers seem way off. (This, btw, pretty much matches my experience with the click stops which seems to have only a rough relationship to reality as I wrote earlier.)
Perhaps this is a matter of quality control. Maybe my Hyperion isn't the same as others. BTW, I also put it at the 20mm click stop and compared the view to the 20mm Plosll. My judgment is it was magnifying more than the 20mm Plossl. I would have guessed it was at least at 18mm at that point. BUT this is very much a judgment and was especially tricky since the two views I was comparing showed different sized fields.
But again, if I had to put numbers to it, I would say that on the low end the Hyperion Zoom is behaving more like a 20mm Plossl and that the click stop markings are pure fiction. They may be consistent from one eyepiece to the next making them suitable for binoviewing, but they have little to do with numbers painted next to them.
But in the final analysis you take your pick - my guess is the Hyperion is behaving like an 8- 20 mm zoom - that is, assuming the 8mm part isn't way off. I haven't even attempted to check that - maybe I will tonight. As I said, this is not the way I like to spend my observing time - but it is important to me to know what the telescope is delivering so I can make reasonable estimates about the true size - and/or distance - of what I am seeing.
I still really like the Hyperion eyepieces and it hurst to find such discrepancies in what seems like a well-designed, well-made product with a reasonable price tag. But my level of trust in their specs is way, way down. I guess I'll have to go back now and look closely at the FOV delivered by the 3.5 and 5 to see if they come close to their specs.
I wrote the folks at Alpine Astronomical, giving them a heads up about this post,
Hi Bob:
I continue to love the Hyperion's - excellent eyepieces - see:
http://www.giveyoujoy.net/awe/blog/archives/2007/04/hyperion_defini.html
But, I also continue to challenge the specifications on the zoom. I don't
have the knowledge or ability to do preise optical tests and I know there
can be all sorts of problems with my approach, but for the record, I could
fit abou 7/8ths to 9/10ths of the moon in my LX90 at the 24mm setting. Even
if the moon is quite close right now, all of it should have fit by the specs
and the accepted rule of thumb.That is, I'm using a 200mm F10 - so fl is 2000mm. At 24 mm it should have
delivered 83X. Divide that in to the 50 degree AFOV and you get a true fov
of 36 minutes. But the moon is closer to 32 arc minutes (on the date and
time I was looking) and yet I wasn't getting it all. Go figure. Seems to me
some number in this group has to be off.
What I was seeing was closer to an AFOV of between 40 and 45 degrees.If I'm right this is too bad. Misstatements about specs can be quite
damaging because it feels like a solid piece of measurable information.
Don't get me wrong - I don;t play by the numbers. I like the eyepiece,
period. But if I';m right I can imagine a serious reviewer doing a real
dance on this.I don't have the patience to do real testing, I want to focus on observing.
I'm just passing this along as a friend.
I got this response:
Most eyepieces (Hyperion Zoom included) have some amount of distortion -
change in focal length over the field. The apparent field of view cannot
therefore be so easily computed from the focal length and field stop size.Some eyepieces are orthoscopic - such as the Abbe Orthoscopic. They do have
low enough distortion that you can relate apparent vs true field of view.
Best Regards,
BobAlpine Astronomical, LLC
www.alpineastro.com
-----Original Message-----
From: Gregory Stone [mailto:gregstone@verizon.net]
Sent: Monday, April 23, 2007 3:13 AM
To: Bob Luffel
Subject: Hyperion5 - zoom
----On May 25, Clay Cooper, a new owner of a Hyperion Zoom, made this largely postive report:
Hi Greg,I had clear skies after all last night, so I was able to get first light with the new zoom in my C80ED (I have beaten the "new equipment curse" more often than not).
How did I get by without one of these for so many years?! It is so neat to just dial up the power instead of changing eyepieces - very convenient indeed. It may not quite have the sharpness or light transmission of a high quality EP, but I don't feel that I'm giving up much in terms of image quality. The darkening of the sky background is quite noticable as you crank up the power.
I wish that the eye relief were better - I kind of have to press my glasses into the EP to see most of the FOV. I know from experience that 20mm of ER is my comfort zone for ER with my glasses lenses, which have a good bit of curvature to them. The moon looked great thru the zoom EP, and I even 2X barlowed it with pretty good success, but the moon does take high powers very well. Saturn & Jupiter looked nice too, but Jupiter was very low to the horizon when I was observing it (I have to get up at 4 AM every morning to go to work, so staying up late is generally not an option for me unless I'm able to come home & get a nap in before nightfall). I kept wishing that it would keep going beyond 8mm to about 4mm. I'm not going to be too concerned about the statistics of the EP, I just plan to enjoy the views thru it for now. I do need more sessions to evaluate the EP more thoroughly though, I realize that.
Regards,Clay
On June 4, Larry Stedman, having seen this post, had some interesting comments and additional thoughts on the subject:
Greg,
Liked the material on the zoom. Very helpful. Another way of testing the fov--and likely even more accurate--is to use star fields. Position a bright star at one edge of the fov and sketch what's at the opposite side-- then check out Starry Night and see what the actual fov is.
Alternatively, one can do the old yardstick at the other side of the yard and gauge fovs that way.
I share your dismay over the advertised fovs on these zooms. Televue and Orion should be ashamed of themselves. And this has been going on for years. There was a consumer class action suit over computer monitors and screens because the advertised dimensions pertained to the hidden tube or screen not the actual visible area. Manufacturers were forced to restate their specs (or add the visible area info). Same principle here, though I'm not suggesting anyone sue. But we should start a letter writing campaign. The limited fov at 24mm (40 degrees) of the 8-24mm zooms has been well known for many years and yet the sellers persist in misleading advertising about them (claiming 50 degrees). There has also been a concern at the 8mm end, too. The fov doesn't measure up to 60 degrees, but is more like 53-54. Overstated fovs are a problem with certain wide field eyepieces, too!
By the way, how do you like the Celestron zoom? How does it compare to the Vixen 8-24mm?
Nice blog!
Larry
I wrote back, in part:
Thanks for the kind words. The Celestron zoom is acceptable at the price, but really can't hold it's own when compared to the Vixen or Hyperion. The biggest problem I see with it is kidney beaning.
I have a question s for you if you have a moment . . .
You're right about the star drift test being the best way to test fov - just the other day I picked out a good star near the celestial equator to focus on for such a test and I'm just waiting for the skies to clear to try it - I really do like the Hyperion, but it's important for me to know the fov at the various click stops. But. . . I was wondering - do you know an easy way to test magnification?and here are the relevant parts of Larry's response:
Posted by Greg Stone at April 24, 2007 01:15 PM Comments? Please email me: gstone@umassd.eduGreg,
. . . Actually, I didn't mean a star drift test. I find that very hard to do! One can never get the star lined up properly for a dead center transit and one has to wait many minutes-- I ran out of patience after I tried it once. No, what I meant was a star field sketching approach. Park Deneb, e.g., at one edge of the fov and see what is just entering exactly opposite it. Sketch that star pattern there and then check Starry Night to see what the actual fov is. I found a 21mm Pentax XL to be just about exactly the advertised 65 degrees.
I subsequently did the backyard, yardstick test, with the 21mm as the benchmark of 65 degrees. I checked out the fov of a 8-24mm Vixen and a Celestron spotter scope zoom and found that the Vixen went from 41 to 53 degrees, while the Celestron spotter zoom went from 39 to a nice 63! It takes a while before the extra fov kicks in, too. The Vixen was only 43 degrees at 16mm, while the Celestron was only 44.
I use the Vixen mostly for barlow planetary work--one can dial in the magnification for the seeing. But I just re-read about dialing in the right magnification for contrast, too, so I'm going to revisit that in my next observing sessions . . .
The tricky part about all this is that it depends on assuming that the stated focal lengths are correct. Off-hand, I don't know of an easy way of measuring absolute magnification. I'm sure one of the Cloudy Night or Astromart gurus will. One can certainly use the yardstick approach and see that the inches shown varies greatly... but this is like determining the scale of the solar system... for decades (centuries?), the planets' distances were known in# of a.u. (astronomical unit, Earth-Sun distance), but we didn't know that distance.