Until recently, high power eyepieces usually meant cramming your eye against a tiny eye lens and viewing a dim narrow field. Then optical designers got the idea of combining eyepieces and Barlow lenses into a single package which gave high magnification, long eye relief, and bright, easy-to-see images. Such eyepieces have been quite expensive until Orion® recently announced their new Expanse series of eyepieces, available in four focal lengths: 20mm, 15mm, 9mm, and 6mm. The 20mm and 15mm are normal wide-field designs, while the 9mm and 6mm versions combine a fairly low power eyepiece with a built-in Barlow lens. That's why, if you look at the picture above, the 9mm and 6mm versions are nearly twice as large as their low-power brethren, the opposite of what we see with most eyepieces. So, while the 20mm and 15mm are nice to have, the real interest is in the 9mm and 6mm versions, which offer a totally different viewing experience from typical short-focus Plössl eyepieces.
I tested the 15mm and 6mm Expanse eyepieces which came with the StarBlast EQ as standard equipment. Both offer wide 66° fields of view, as compared with the 52° fields of most Plössls. This gives you a 61% increase in the area you can see at a given magnification, which in itself is impressive. Secondly the lens you put your eye to is much larger than we're used to in eyepieces of this size. The eye lens on the 15mm measures 20mm in diameter, and that of the 6mm measures a whopping 22mm. The effect is like looking through a picture window rather than straining to peep through a tiny pinhole. This contributes to the large eye relief of these eyepieces, 13mm to 17mm, depending on the focal length. Eyeglass wearers will be able to see all or most of the field of view, and for non-eyeglass wearers, it will be just as comfortable to look through the 6mm as through the 20mm.
These eyepieces come with all the deluxe features you expect from a high quality eyepiece:
- Fold-down rubber eyecups
- Undercut barrels to keep them securely in place in your eyepiece holder;
- High quality coatings on all optical surfaces;
- But the most remarkable feature is the price: these eyepieces cost just about what we are used to paying for Plössl eyepieces with narrower fields of view and much shorter eye relief.
In addition to testing these eyepieces in the StarBlast EQ, I also tried them in my Starmaster 11" f/4.3 Newtonian. As I always do with this telescope, I used a Tele Vue Paracorr to correct for the aberrations of its short focal ratio mirror; this results in an effective focal ratio of f/5, a pretty stringent test for any eyepiece. The Expanses delivered really sharp images almost to the very edge of their wide field, challenging eyepieces costing many times more. Once again, the Double Cluster in Perseus was my acid test, and both eyepieces delivered pleasingly sharp images all the way across their 66° field of view.
With eyepieces like these Expanses available now at such a reasonable price, why even consider Plössls with their narrow fields of view and short eye relief?