Quote:
Originally Posted by mental4astro
Hi Don,
Not only coma alone that is seen in Hyperions. Coma is present, as it is in all eyepieces with Newts, that are not corrected to some extent. I mentioned pincushion (particularly in the 24mm & 8mm), field curvature (worse in some than in others of the line up - which is why some people find those focal lengths with milder field curvature a better EP. I think most people find the 17mm this way) and yes, a little astigmatic - one key aberration denoting an optical mismatch.
Not all Hyperions show the same aberrations, nor to the same extent - again why some people don't mind specific individual focal lengths. All but the 5mm and 24mm use a Smythe lens field grouping, which is the primary hint for the optical mismatch with Newts. The 24mm has no such field grouping, and if I remember correctly is a modified Erfle design, which again is not an optical match with Newts. The scope I used them on was an f/4.5 Newt, which will test most EPs, and like Don said, the Hyperions will perform better in slower Newts. Again, those people who don't mind their Hyperions are also using slower Newts.
Don also points out that there are better options than the Hyperions, albeit with smaller AFOV. Thanks Don for listing some of these,  . A staggeringly big AFOV should not be your only guiding factor in EP selection.
PLEASE remember, I am not dismissing the Hyperion line up. My argument is purely on of horses for courses and giving a well meaning reply to a given question. I TOTALLY agree that the perception of aberrations is a subjective one. And I have only noted what these eyepieces show so you know what to expect. You can then decide if some field curvature, or a little astigmatism is acceptable or not, and in my books, that is completely acceptable and logical. Most of my EPs some some aberration or other, and some EPs are absolutely not for my Newts, just my SCT and refractors. I am not an 'aberration free zombie', 
There is another great thing about the Hyperion line up - every focal length, other than the 24mm, makes use of a removable field group in the 1.25" barrel fixture. Removing this field group alters the focal length of the resulting new lens combination to a longer one. The Baader info sheet on the Hyperion line up mentions this, but for some reason this is little known in the wider astro community. I have a 13mm Hyperion in my clutch of EPs, and have removed its 1.25" field grouping. In this configuration (roughly being 23mm) this EP is the one I most use in my Daystar Quark solar filter. Blooming brilliant this way in my refractor, 
I've mentioned the Smythe lens field grouping of these. It occurred to me some time ago to see what happens if I replace the Smythe lens with a Barlow lens of my 13mm. And bugger me! The performance of this new configuration markedly improved the image quality in fast Newts! The main aberration I noticed was coma (a given), but field curvature was greatly reduced as was pincushion and astigmatism. Hey, not perfect, but a vast improvement on the straight 13mm Hyperion. But not everyone is as willing to experiment with their EPs this way, 
The Hyperion Zoom Mk III is a different beast altogether again, and is great in Newts. |
The other eyepieces I mentioned first have the same approximate apparent field as the Hyperions.
As with many eyepieces in this price range, the Hyperion's primary downfall is the edge of field astigmatism. I used to think this was only due to their inability to handle a short f/ratio, but I also found the same astigmatism at f/8 and longer, so they simply aren't corrected there.
And that astigmatism seems worse when they are used as 2" eyepieces without the bottom Smythe lens.
But even with that said, and you could add EOFB in some focal lengths and spherical aberration in others (which clears up with use of a barlow instead of the Smythe lens as you noted), they still represent better eyepieces than many of the inexpensive Chinese eyepieces out there, and if you add a pair of Fine Tuning Rings, the 21mm becomes a 21/17.8/15.5/14mm set, and also can be used with a 2X barlow for 10.5/8.9/7.8/7mm. Someone on a tight budget could buy one eyepiece, one Barlow, and a set of rings and suddenly have 8 focal lengths.
If it's one of those Barlows with a separate lens that can be unthreaded and added to the eyepiece separately, then 16 focal lengths!
And, used in a refractor or SCT, 32 focal lengths (counting lens on front of star diagonal and complete barlow in front of star diagonal).
And the 21mm is the best in the entire series to my eye.
Of course, not everyone is on that tight of a budget, but I didn't even count the eyepiece sans Smythe lens, which makes it one more.
I do agree with John that the 13 may be the weakest in the series.