Telescope Reviews: Ultra low-power observing

That's a longer story

In 2006, I visited Yellowstone National Park in the USA. The Milky Way in dark skies at 7700 feet altitude is breathtaking beautiful with numerous star-clouds, dark rifts and knots visible to the naked eye. In a 10x50 binocular a lot of new details could be seen with dark lanes cutting through a luminous background in Ophiuchus and Sagittarius.

But I did have a (small) problem. The naked eye view and the view through 10x50 binocular were like looking at two different skies. Some large objects visible naked eye was not visible or (more correctly) recognizable as such in the binoculars. The 10x50 did not enhance them.

The gain in limiting magnitude from using the naked eyes (a 1x7 binocular!) to a 7x50 or 10x50 binocular is 4.2-4.6 magnitudes. What I needed was something that could bridge the gap. A hypothetical binocular with a gain of 2.1 to 2.3 magnitudes and an exit pupil of 5-7mm should be capable to do just that.

Binoculars with those specifications are rare, but I have managed to obtain two.

Bushnell Xtra-Wide 4x21 Binocular.

This is fixed-focus binocular with a very wide field for watching sport events. It uses (so I’m told) internal mirrors or a combination of mirrors and prisms for erect-image views. I have measured the magnification as 3.5x (6mm exit pupil) and the true field of view as 18 degrees. It should (in theory) give a 2.6 magnitude gain in limiting magnitude.

In daylight, it is actually a quite nice binocular. I like it. The entire surrounding context is there and the connection to the naked eye view is not broken like in a 10x50 binocular.

At night, though, several problems begin to emerge. The fix-focus makes it impossible to make adjustments. Faint star in the binocular do (with my eyes) need something akin to a slight twist on a focuser to make then pinpoint. But that can’t be done resulting in a loss in limiting magnitude.

Furthermore Milky Way star clouds do not get much easier to observe compared to the naked eye. They are bigger but surprisingly dull or dark. The light-transmission through the mirror system does not appear to be very high.

Despite the problems, I am using this binocular as a ‘pre’-finder making good use of the 18 degree wide field. It is a good match to the Pocket Sky Atlas.

2.3x40 Binocular.

The optics is of this binocular is of the Galilean type. It doesn’t need any prisms or mirrors for getting an erect image. Each barrel has only 4 multicoated lenses, resulting in maximum light transmission. The two eyepieces can be focused individually. The magnification is 2.3x (8mm exit-like-pupil). I get up to a 24 degree true field, if I press the eye close enough to the eyepiece. The gain in limiting magnitude, compared to the naked eye, is in theory 1.8 magnitudes.

On paper this binocular would be ideal to bridge to gap between the naked eye and the binocular view, despite the slightly lower gain than my requirement of 2.1 to 2.3 magnitudes.

I tried to get one in Russia, but was told that it no longer was in production. Later I got a Chinese version (clone?) from a dealer in Japan.

I must say, it has not disappointed me. The stars are sharp and Milky Way star clouds (or ordinary clouds lit by the city lights) are not only larger but conspicuously brighter than the naked eye. I know it is impossible for the surface brightness to increase, but subjectively it appears to do so.

Looking through the binocular is in essence like a slightly larger and boosted naked eye view with a 2 magnitude increase in star brightness. If only I had this binocular at Yellowstone ...

Camera lenses.

I have also experimented with using Camera lenses as mini-telescopes. But they are not really the solution to this particular problem. You lose the context in the sky because the image is upside-down - and looking with one eye only does the opposite than enhancing the view.

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