For views of big deep sky targets with slower f/8 to f/15 telescopes, the new Omni 56mm does the job at low cost.
Plus: Widest actual field; sharp off-axis star images; low cost
Minus: Narrow apparent field of view
Summary: The 56mm Omni Plössl provides ultra-low power and the widest field possible at a low price for a 2-inch eyepiece.
Who Is It For? Owners of Maksutov- and Schmidt-Cassegrain telescopes.
Most telescope owners wanting to expand or upgrade their eyepiece set think first about buying higher power eyepieces. But one of the most useful eyepieces to add to a personal collection is one that provides the lowest power and widest field of view. This class of eyepiece is ideal for viewing big deep sky targets such as the Andromeda Galaxy and the Pleiades, and for Milky Way starfields and nebulas.
Celestron’s new 56mm Plössl in their Omni series of economy eyepieces fits the bill. It provides the widest possible actual field of view in a 2-inch eyepiece. I tested a sample on loan from Celestron.
The apparent field of view of the Omni is only 47 degrees (°), narrow by today’s standards of wide-field 65° to mega-wide 100° eyepieces. But this is a simple four-element Plössl, and the field is typical of what that design of eyepiece can deliver. Building an eyepiece with a wider apparent field requires more lens elements with top-class coatings and exotic glass, which all equate to high cost.
However, premium “lowest-power” eyepieces with fields 65° or higher all have shorter focal lengths than the Omni 56mm, and therefore provide higher power. So despite their wider apparent fields, their true fields of view — how much sky they actually show — won’t be any greater than with the 56mm Omni, or any Plössl of that focal length.
For example, I compared the 56mm Omni to a top-of-the-line 41mm TeleVue Panoptic, with its 68° apparent field. Both eyepieces presented identical true fields of view, about 1.3° on my Celestron C8, the widest field possible. But the Panoptic costs $525. The Omni costs $100.
True, the Omni’s 47° apparent field does present a more tunnel-vision view compared to the picture-window field of a Panoptic, Nagler or other premium wide-field. But that’s the tradeoff for the money you save.
Also, the Omni 56mm weighs 440 grams (just under a pound), less than half the weight of the big Panoptic’s 945 grams (2 lbs). The Omni won’t pose the balance problems that hefty 2-inch eyepieces can.
All those plus points would be for naught if the optical quality of the Omni was poor. But in tests with my f/10 Celestron C8, star images appeared crisp right to the edge of the field, distorting only a little at the edge as I moved my eye around. There were no ghost images on bright stars and planets, and only a little flaring at the edge with the Moon just outside the field. I also saw little in the way of pincushion or barrel distortions elongating the Moon’s disk at the edge. The Omni presented a very flat field, with a crisply defined field stop.
I also tried the Omni on my 8-inch f/6 and 12.5-inch f/5 Dobsonian reflectors. The steeper light cones from the fast mirrors and the coma inherent in faster Newtonians should have produced aberrated stars at the edges, but in each telescope star images in the Omni still looked quite sharp to the edge. I was surprised and impressed.
That said, any eyepiece with a focal length as long as 56mm is not an ideal match for a faster Newtonian. The Omni provides an exit pupil 9mm wide on an f/6 scope and a whopping 11mm on an f/5 scope. Your eyes’ pupils can dilate to only 6mm or 7mm wide when fully dark adapted, so can’t take in the entire light cone exiting from the eyepiece.
Not only are you not seeing all the light the telescope’s mirror is gathering, but with Newtonians the dark shadow of the secondary mirror can appear floating in the field. In practice the shadow was obvious at night only when looking at the Moon.
Even so, an eyepiece with a focal length of 56mm is best reserved for f/8 to f/15 telescopes where it will provide an ideal exit pupil of 7mm to 4mm, respectively.
Typical f/6 to f/7 refractors are more forgiving. I tested the Omni with my 130mm f/6 apo refractor and it worked very well. When a refractor is used with an ultra-low power eyepiece like the Omni you don’t see any shadowing effect (after all, there is no secondary mirror), but you are effectively stopping down the aperture of the telescope.
The ideal match for the Omni 56mm is to use it with an f/10 Schmidt-Cassegrain (provided it is equipped with a 2-inch star diagonal) where the Omni provides a 5.6mm exit pupil, and 36 power on an 8-inch SCT. (To determine the longest focal length eyepiece suitable for your telescope, multiply the scope’s f-ratio by 6mm or 7mm.)
In conclusion, the new Celestron 56mm Omni provides great ultra-low power views for slow f-ratio telescopes in a lightweight and affordable 2-inch eyepiece.
MSRP: $99.95
Website: www.celestron.com
