One of the best upgrades you can make to your telescope observing system is the purchase of a pair of binoviewers. Binoviewers allow you to observe through any telescope (adapters may be needed depending on the type of telescope) with both eyes instead of just one. This allows you to observe more comfortably with less eye fatigue, and for objects like the moon and planets, generally see more detail. Deep sky objects also can look more impressive with both eyes, since even though the image remains two dimensional, your brain often tries to force a three-dimensional appearance to the image.
While binoviewers can improve your observing, there are some downsides. A big one is that many of the units available are quite expensive, plus you need to buy two eyepieces instead of one for different magnification options. Also, they add another 1-2 pounds of weight (up to another kilogram) of weight to the back of your telescope, which may cause balance or vibration problems depending on the robustness of your telescope mount. Finally, refractors and reflectors may require the use of a focal extender or barlow lens of some type to reach focus, which in turn forces you to only use them for medium to higher power observing.
The William Optics binoviewers are not the best you can buy as more expensive binoviewers on the market will give you a wider field of view and slightly sharper image, and possibly offer more features and accessories. However, they are a great option in the budget range and my favorite of the ones on the market. Further, you get a great set of eyepieces with the binoviewer package, as well as a 1.6x multiplier lens that can be attached to the nose of the unit. The multiplier will allow the binoviewers to reach focus in most refractors, and give you two magnification options when using them in an SCT, as all SCT telescopes like the Celestron C6 I currently use, should be able to reach focus without the 1.6x multiplier lens. Additionally, the fit and finish of the William Optics unit are quite nice, something that William Optics takes pride in with all their products. While performance is more important than looks, having a nice, well-functioning unit certainly adds to the appeal of these binoviewers.
One of the benefits of the William Optics binoviewers over other units, including the more expensive options on the market, is that they are smaller and lighter weight than most other options. This is important when considering balance and stability issues with your mount, as well as the ability to reach focus, as a shorter light path through the binoviewer, in turn, allows it to achieve focus in more telescopes without the use of a higher power barlow lens or multiplier. While the William Optics unit comes with a 1.6x multiplier lens to help it reach focus in most telescopes, some other units on the market require a 2x barlow lens to achieve focus.
The William Optics binoviewers measure roughly 12x12 cm as measured from the top of the eyepiece holder to the bottom of the nose piece, and about 17 cm tall with eyepieces. The weight of the unit was 18.4 oz bare, and 26.2 oz with eyepieces and barlow lens attached. Most other units on the market weigh more and are a little larger in overall size. In some cases that allows them to produce a wider field of view since they have larger prisms inside the unit, but if weight is a primary concern, these are a good option.
If you are interested in maximizing your viewing of planets and the moon, I highly recommend picking up a binoviewer, and the William Optics unit is a great performing, low-cost option, available for less than $300 including the pair of eyepieces. There are some cheaper binoviewers on the market, but they are usually bare units, so you need to supply your own eyepieces and barlow lens.
While having an extra set of eyepieces can give you a lot more magnification options, the best way to start is to buy the binoviewer along with the optional 2x nosepiece attachment that William Optics makes. Not only can you use that lens instead of the included 1.6x lens to achieve a higher magnification (it will double the magnification of the eyepieces), but you can stack the 2x and 1.6x lenses to boost magnification even further. So, you may not even need an extra set of eyepieces. You need to be careful stacking the lenses though as that will raise your binoviewer higher off the diagonal in your telescope (unless using a reflector that doesn't have one) and may cause some balance or stability issues depending on your diagonal, especially if a single thumbscrew is all that attaches it. Since both the 1.6x or 2x lenses have smooth sides, they can be more prone to slipping out of your diagonal if your diagonal happens to rotate around in the telescope. It is essential to make sure your diagonal is clamped down firmly in your telescope, as you do not want the weight binoviewers to cause the diagonal to rotate, and possibly allow the binoviewers to fall out and hit the ground.
In summary, I highly recommend the William Optics binoviewers, especially if you like to observe planets and the moon. They will help you see more detail, are fun to use, and usually let you observe longer with more comfort than when using a single eyepiece.
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William Optics Binoviewer: https://amzn.to/2CsoRbr
2x nosepiece:https://amzn.to/2PhyIUt
Agena Dual ED 12mm Eyepiece: https://amzn.to/2M7BtFf
Celestron Zoom Eyepiece: https://amzn.to/2PeYys0
Baader zooms: https://amzn.to/2O6Sa9A
The above links are Amazon affiliate links. I receive a small commission if a purchase is made using one of them, which helps to support this site. You can watch my video review of the units below.