Pirich's Full Review: Celestron Binocular Viewer (93690)
Unlike cameras, binoculars, or telescopes, this device is something most people just haven't seen. The Celestron stereo binocular viewer is a telescope accessory which takes the single image coming through the telescope and splits it into two images- one for each eye. It does work- and quite well. No only that, but it turns out it has some undocumented design features which let it come to focus with high end refractors like the superb Celestron 80ED when other binocular viewers designed to drop into a diagonal will not.
Background
Anyone who has spent hours looking through a telescope with one eye has wished they could see with both. It is possible to hook up two telescopes and make giant binoculars. It is also possible to split the image with a binocular viewer. The last option is less expensive, but is also makes many people feel this somehow has to short change you.
The binocular viewer is a difficult accessory to understand in some ways. First, it is splitting up one image into two apparent copies. The second is it appears logical everything must be half the brightness or dimmer. Third, there is the question of being able to integrate the two images so you can make sense of them.
Strangely enough, just about everyone who has used a binocular viewer with a fair sized telescope reports the images are somehow much more satisfying, and don't actually appear to be dimmed by the viewer. It sounds absurd. It did to me when I first ran across it (obviously if you split the light in half, the best you could do is half the brightness, right?). However, experience has made the binocular viewer become my favorite optical accessory.
Most people think of the image through a telescope as being flat like a photograph. However, it really isn't. Photographs are flat because the image was brought to a single averaged focus against the plane of a piece of film or a CCD or CMOS chip. The image data available through the optical image has been flattened as a result.
If you look through a telescope with a diameter equal to or larger than the distance between your eyes, it is possible to prove there is more information in the optical image. Something 4" in diameter or larger will show this fairly easily: if you look at an object on the ground (a tree, street sign, or other small object against a background further away) at a range of about 300 ft. away at low power, you will notice objects have a sort of "Sharper than real" quality. What is actually happening is you are viewing an image with better depth data than the small diameter of your eye can take in. For example, a sign with nearby objects behind it will appear super crisply outlined compared to the more distant objects than it does to the unaided eye. Now, if you move your head from side to side so you are looking at the image at the extremes of what is coming out of the eyepiece without touching the telescope, you would see the object in front move with respect to the object behind it. This is even more apparent if you do this when looking at the pole holding the sign up, where you can completely see around it with a stationary telescope by moving your eye left and right over the eyepiece.
The image actually has information on what is around the edge. In short, because the telescope is larger in diameter than the width of the signpost, the left side of it is seeing past the left edge of the object and the right side of the aperture is seeing what is behind on the right side. The center of the telescope is seeing the center of the image. With our own vision, we get this effect looking at a window screen- the wires in the screen are smaller than the diameter of our pupil, so our eyes see right around them and you don't have a grid shaped blank cut out of the image you see, even though the image isn't as good as what you can see with no window screen in the way.
So, even a single telescope tube does have three dimensional data present in its image. The most famous version of depth perception is the way our brains usually gets this information in everyday life by comparing images between two eyes, so in a sense we get vision equivalent to a bigger eye without having eyes the size of basketballs. This parallax method only works well within the length of a football field, but it is apparent the human brain is using other tricks for depth perception as well.
Now, if you projected the 3-D image from the telescope on a surface (as in a camera), the focus would have to be an average of these images spanning the front of the telescope. Many observers have found photos are very difficult to make as sharp as the visual image was. Well into the photographic era, visual observation was recognized as the sharpest observation. As far as I can tell, it was Percival Lowell and the widespread publishing of his bizarre and far-fetched interpretations of Mars observations (canals, dying civilizations, and so on) which led to photography taking the lead because you could pass the photos around to different people and discuss them objectively. And, of course, high sensitivity long exposures are possible with photographs, so they are a primary means of observing dim deep space objects.
Visual observations still come with a vivid sense of depth and range of brightness photos lack. A photograph can only display what the image you see can show. So, you are between white and black on paper, or full brightness to black on your computer display. They eye has a dynamic range somewhere around 400,000 to one. This is much more sensitive than 256 gray levels.
So, what this is all leading up to is getting everything out of the raw information isn't easy. The binocular viewer helps since it lets your brain use the mechanisms it has for dealing with this in everyday life take over. In short, the use of two eyes results in a whole greater than the sum of its parts.
The Celestron Binocular Viewer
I looked around a bit before getting this accessory. To put it simply, it costs as much as many small telescopes. It is small. It requires you to get two copies of every eyepiece you use with it. But, I heard enough from people who had been using these and took the leap of faith.
The Celestron binocular viewer is made by Baader Planetarium, a German optics company which has taken over making binocular viewers from Zeiss-Jena. This is one of their more compact viewers. The next step up in them is about twice the price.
This viewer is known for having high contrast views, and I decided to go ahead and give it a shot and see what it did.
A purchase, an Adventure!
As it turned out, it was leap a bit further than I thought. I ordered one from Adorama, www.adorama.com, who I had always had good results with in the past. UPS intervened by destroying the box mid-transit. The box showed up as damaged in transit and I let Adorama know- thinking it should be on its way back It seems UPS isn't called "Ups!" for nothing since they delivered the binocular viewer loose in an empty box with the shredded remains of the previous packaging taped to the outside with my address to my doorstep while I was at work. No documentation was with it, and the adapter to attach it to the telescope was missing.
I was, to put it bluntly, floored they had deliver it at all. This was a piece of equipment more expensive than an SLR camera and more dependent on perfect alignment of its many internal prisms. I was wondering if the device would even work, so I talked to Celestron. They went ahead and shipped me a nose piece (what adapts it to your telescope- the front has threads different from a telescope or the common threads used to attach to it). Without it, I could not tell if the item was damaged or not. If it had been damaged, it would be impossible to align the two images. When, I talked to Adorama and they told me to go ahead and ship it back and they would replace it. So, I boxed it up and sent it back.
A few days later, a package came from Adorama. Strangely enough, it was the same binocular viewer (I had written down the serial number). I talked to them again, returned it again, and finally got a viewer that hadn't been in a calamity. The people at Adorama were apologetic and gave me free shipping on another item, so I really don't have a complaint with them.
As it turned out, the new one still didn't have the nose piece, but the one Celestron had shipped me a week before had arrived and I had gone ahead and blackened the interior of the tube. The sleeve of any eyepiece or other accessory which slides in the back of a telescope will be blackened to reduce glare. You can reduce this over what most manufacturers do if you remove the sleeve (in this case it was a separate part, anyway) and paint its interior with a very dark flat black. The darkest I have found in one-on-one tests is Tamiya XF-1 flat black acrylic model paint. This reduces internal reflections off of side walls which reduce contrast in images. It's a cheap tune-up to any telescope.
I put the two items together and attached them to a C5, dropped in two 25 mm eyepieces, pointed it at the top of a telephone pole, and took a peek. It was an image, and I was able to combine it.
Using The Binocular Viewer
The binocular viewer looks like a small triangular block with two eyepiece collars on a black plate on the top and an opening to attach to the telescope at a 60° angle to them on the front. The unit is surprisingly dense and heavy for its size. Essentially, it is a casing filled with prisms. the eyepiece holders can slide toward and away from each other to accommodate the distance between just about anyone's eyes. The eyepiece collars each are machined with reliefs in the sides and an extra hole for a set screw on the bottom, so you can tighten the eyepieces in from the top or you can move the screws to the other side if you prefer.
The viewer has a nose piece you thread into the front, so it goes into the back of a telescope like it was an eyepiece, itself. The item is actually heavier than many cameras, so your telescope's balance may be very different with this item attached.
In use, the device is actually very intuitive. Drop in two identical eyepieces (for example, two 12.5 mm Plossls), adjust the width to yourself, and go. The images are very freeing compared to using one eye. The only example I can give is it is like going from peeking through a hole in a fence to looking over it. The images don't give an obvious impression of being dimmer. And, I know I am saying what others have said before me when I say I know it must be, but it just doesn't look like it in practice. The moon becomes extremely three dimensional, and subtle details are much easier to see on planets and in nebulas.
In short, it appears the brain is able to make a lot more of the information it is getting if it can see with both eyes instead of with just one. This comparison between two eyes may be why the image doesn't appear significantly darker. I really don't have a provable explanation for that; just the observation.
There are a few limitations on eyepieces, and the strange thing is I find myself motivated to live within them rather than find ways around them. Because of the internal path, the largest eyepiece this can handle is around 25 mm in size as a Plossl. So, the only way you are going to get to very wide angle views will be if you are using a refractor with lots of focuser travel and a short focal length.
The other restriction is going up in magnification is not so easy. I have been using a pair of 12.5 mm eyepieces, which are usually one of my favorite sizes as a combination of power and image brightness. But here they are much more difficult to use because the exit pupil is much smaller as is the projected eye relief, so you have to get both of your eyes aligned and the width adjustment perfect to use them. Higher power eyepieces, such as 6 mm eyepieces appear to be a nightmare waiting to happen. I rarely use a barlow with most viewing, but with the binocular viewer, the preferred way to get to high magnification is to use 20 mm eyepieces and a 2X barlow so they act like 10 mm eyepieces, but still have the eye relief which makes using the viewer easier.
A few people appear to be unable to combine the images in a useful way. Out of my family and friends, I have found one person who can't combine the images and reported he saw nothing he didn't see with one eye. He also reports he can't resolve those dot-based 3-D pictures, so there may be a linkage there. Comments would be welcome on that. A large part of this may just be the reduction in eye strain, but it looks better than that in practice.
One other thing I have noticed in the summer is the friction on the turning ring changes when it is warm. On a summer night in New Orleans, the viewer will slide down to point down if you get it far out of alignment where the eyepieces are pointed straight up. The grease appears to be getting thin from heat. The friction was about perfect in winter, so keep this in mind and keep your eyepieces well secured so they don't fall out if it turns over.
One more trick is the binoviewer actually will go into a 2" focuser as-is! The way I discovered this is I wanted to adapt it to the Celestron 80ED, which at f/7.5, would offer significantly briighter images than the Schmidt Cassegrains at f/10. So, I had tried attaching it and discovered with the nose piece in the 1.25" adapter, I could almost get to focus. Since the adapter is T-thread, I took the nose piece outant threaded in the 2" adapter directly. Then, thinking I was being clever, I removed the locking ring on the telescope's focuser (this means I can't have the set screws oriented any way I like, but the assembly can move in about 3.5 mm further). Still I was only close to infinity focus. So, despairing over the set of adapters I had, I removed the 2" adapter and resolved to just see if it could focus, or if we would be talking new back end to the telescope, and this is just a non event. So, I held it up to the focuser and waited for the round end to tap against the telescope, but instead, it fit into the collar...exactly. I pulled it out and looked at the binoviewer and with a start realized the end of it was designed to fit in 2" focusers- and they had moved the optics forward into this section. So, I put it in, tigntened the set screws, and did an experimental focus- it snapped to focus with 20mm of focuser travel left over. And, for the night sky, this combination has proved to be outstanding- stars are brilliant pinpoints, and the wider field of view means the Plieades and other objects easily fit in the eyepieces. All I can do is wish I had noticed this sooner.
Usage Tips
(1) When you move the telescope and adjust the binocular viewer, remember to take hold of it by the gray body and not by the eyepiece collars. That will move them so you need to readjust the distance between them when you get to a new object.
(2) When using a barlow lens on the front of the binocular viewer to double the magnification of both eyepieces, the viewer will sometimes produce false images. There are some points near to a bright object where a line of a dimmer false image of it can show up, even when you aren't pointed at it. I've had this happen with Mars, which although it is at its largest right now, it is small enough to fit in this window. The effect is a sort of dim mirror image will show up suddenly in one small spot. I haven't had a problem with this on Jupiter and Saturn, largely because I haven't been using a barlow with them. I have seen it on the moon, where a thin strip of a reflected image will show up while approaching it. Here it is obvious what it is since the edges are rectangular. If you see this, look around for the real object and the image will return to what it should be.
(3) The nose piece is threaded into the smooth rotating liquid bearing collar on the binocular viewer. As a result, if you rotate it to the left, you can start the nose piece unscrewing. So, make sure whenever you are adjusting the pointing direction, you always rotate it to the right or clockwise. Everything else on the back of the telescope will be threaded this way as well, so this really is the way to turn it (three guesses on how I learned this). Make sure the visual back you have inserted it into is good and tight (set screws or compression ring).
(4) When you put eyepieces in, leave them both about 1/16" (1.5 mm) out from being fully into the collars. This gives you some relief for balancing them. Point the telescope at a convenient object, close one eye, and look through the binocular viewer and focus the telescope. Now, open the other eye and adjust the height on that eyepiece to get the image focused in both. Even if both eyepieces are the identical make, your eyes are not perfectly identical, so what you are doing here is the equivalent of the diopter adjustment on binoculars. The reason you leave them both out a little bit is you won't know if the true focus is inside or outside of the first eyepiece.
(5) One of the largest problems I have had is the binocular viewer is extremely popular with lots of people, but setting it to different eye widths over and over is problematic. A way around this is to use as long a focal length eyepiece as possible- say 20 mm or 25 mm, which will have large exit pupils and nice eye relief so most people can use them without having to adjust the width. If you readjust the width, you have to refocus, so this helps.
(6) Although it will focus, the view with a 90 mm scope appeared dim at best. The binocular viewer appeared to work quite well with both 5" and 8" diameter scopes. The device will not work with Newtonians (where you look in through the side. However, it can be used with Schmidt Cassegrains (SCTs) and refractors if they have enough focus travel. To use it with a refractor you may need to add a barlow to get the focal point out far enough to resolve an image.
Summary
The binocular viewer is one of the more enabling accessories I have found. The views through it are so good that the restrictions it comes with just don't seem to matter that much in comparison. The image of just about any object appears to break out in depth and contrast. On the moon and planets, it really is breathtaking. On nebulas, star clusters and galaxies, they are easier to see and have depth, too. The only real problem is it has limited flexibility for image scale and magnification compared to just using an eyepiece and a diagonal. It says a lot for its capability that this doesn't seem to matter so much when you are looking through it. This started as an experiment when I got it, and I thought it was expensive at the time. It has become a mainstay and worth every penny.
In a follow-up note, the rise of the Euro against the dollar has caused this component, which comes from Germany, to go from being available for $450 to $700. At that price, it is a bit more difficult choice. However, I still find the binocular viewer works very well, and is one of my favorite accessories. It seems these are no longer available directly from Celestron, which is not a happy story since it is one of the best binocular viewers ever made.
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