Monday, February 25, 2008

Binoviewing I

From today, I'll start a new series of article on binoviewing. This is not a new one from scratch, but it's based on the one which appeared in my website several years ago. I'm going to renew the content, and to make it here in my observation blog, and also to make it into smaller pieces so that it's hopefully easier to read and understand.

Binoculars and Binoviewer

Observation with two eyes has always been the most natural way to observe, but as the aperture size increases, it is expensive to keep since you need two sets of optics, and they're required to be well aligned.

Binoviewing and wide field eyepieces

To me, going for binoviewing and going for ultrawide eyepieces (80+ degree) are two surprisingly similar things. They are similar in a sense that it is not a must to go with either, and both of them have some downsides on paper, like more glasses, a more complicated optical chain, and also the price, etc.

Most people believe that to squeeze the most out of your equipments, binoviewers and ultrawide eyepieces should NOT be used at all. The reason is that most glasses in the optical chain is not very desirable, unless absolutely necessary, so you see why there are some simple eyepiece that costs a lot. While I agree with the facts behind, but I disagree with the causal relation derived since it's an oversimplification. Visual observation is NOT to squeeze all the detail from our equipments. If I am going to get the most out of my equipments, I should simply resort to photography and look no further.

No one can see more than a stacked and processed planet/lunar image produced by a webcam, and no one can see more than a long exposure deep sky photography produced by a camera. So, why we still use our eyes to observe? To me, visual observation has a strong emotional reason associated with it. Finally, to be precise, we are here to squeeze most detail or visual impact from our equipment visually. Other than emotional reason, our eyes are very good detector as well. For example, when you first see a larger globular cluster, you saw a faint fuzzy at first, but if the aperture is enough, after awhile, you will begin to see pinpoints of star popping into the view. Amazing experience! No photograph can really reproduce this effect.

Visual observation

It is a fantastic experience to have photons from so far aways to go through your eyeball and then hit your retina directly, those photons travel so long both in terms of time and distance in order to hit your eye! Good equipments bring a lot of visual impact and enjoyment and I believe it one of the fundamental reasons why we have so many amateur astronomers around the world.

Therefore, except what is inside those images formed by the eyepiece, the feeling is important at the same time. Feeling is something which is extremely hard to judge, or I should better say, we should not judge. Theoretically, going higher magnification than the resolution of the aperture would allow, does not make sense at all on paper, but practically, we usually push a bit higher since larger image is simply easier to see. So, it depends on the observer, the quality of the whole optical chain, plus the atmospheric condition at the time of observation.

Having said that, feeling is not the only reason why we go for wide field eyepieces and/or binoviewing, let's read on. Binoviewing does help to bring out more detail otherwise very hard to see with one eye.

Some wide field eyepieces

Wide field eyepieces

Wide field eyepieces improve contrast by giving more magnification, thus darkening the background sky, and at the same time keeping the same true field size of an eyepiece with a smaller apparent field at longer focal length.

For example, a 24mm Panoptics delivers the same true field as a 32mm Plossl, but at a higher magnification. To me, going lower power only mean two things, a wider field and a larger exit pupil. When the exit pupil is large enough, going lower power means only the need for a wider field. By this logic, you immediately know why we go for wield field design, for what we want is field size but not the lower power itself, for most of the time.

Other than the improved contrast, the feeling of openness provided by these eyepieces is great. Tele Vue names such feeling as "space walk". You feel like you're seeing things outside, and you are part of the scene instead of looking through a telescope, you feel like you're out in the universe, instead of standing on Earth, peeping through a small hole with a tube filled with a complex set of optical components.

The wider field can frame your target better, and it is particularly good for deep sky objects. You would love to use one in a telescope on an non-driven mount so that your object stays in the field for longer.

Having said that, I found the view provided by a cheap high quality orthoscopic to be better than my wide field eyepieces at high power especially for the moon and planets. So, there is no absolute answer to what eyepieces one should get. Maybe the only absolute is, we want more different eyepieces for different situations.


Entering binoviewers, which is why I write this article.

I believe most people started with a pair of binoculars in astronomy. Binoculars are handy, portable, and they provide a very wide true field of view, they are very comfortable to use, and your eyes are relaxed during the observation.

Somehow when we graduated with a telescope, we learn to use (and close) one eye to observe, we see a lot more than a pair of binoculars, but we missed the goodness of binoculars at the same time.

Can we get the best of both world? Yes, go ahead and invest in two telescopes, mount it together to form a pair of binoculars? Portability asides, collimation and price make it hard to realize. Or, invest in those giant binoculars? Again, price is a concern, also portability.

Binoviewer is the viable ticket.

(to be continued)

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