June 2006 Archives

Saturn and moons

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Saturn has always been the most spectacular planet in the solar system, and it looks like every time you look you find a new satellite orbiting it; I think the total number is up to 47 already. Actually, it's hard to give a precise number, and the problem is similar to that of deciding whether Pluto is a planet or not: technically, all of the small chunks of ice and rock that orbit Saturn, both independently and as part of its rings, are satellites, and there's not a clear distinction between "small moon" and "large rock".

In any case, the Cassini Imaging Lab has released a series of beautiful animations depicting the movement of Saturn and some of its moons. Definitely worth taking a look. The third one in particular, "Staying with Epimetheus", shows quite clearly the amazing range of sizes the Saturnian moons come in.

And, admit it: you had no idea Saturn had a moon called Epimetheus, did you?

(on an unrelated note, Pluto's new moons very officially named this week: they're Nix and Hydra, joining the already known Charon; and the IAU should rule in September on what's the official definition of a planet, so we may end this year with anything between 8 and 10 planets in our solar system)

The sky tonight

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If you are in a location from which you have a reasonably dark sky and a good view to the west, go outside today just after sunset and look up. Saturn and Mars will be very close together somewhat low in the northwest sky, and this should be visible with your naked eyes from almost anywhere. The image you see here (generated with Stellarium) shows their relative positions as they will be around 18:30 tonight, in Melbourne; the view will be similar from other locations at similar local times. From Melbourne, Mars will set at 20:25 and Saturn right after at 20:41.

Also, if you go out a little earlier, Mercury will be making a very good appearance, much closer to the horizon; it will set at 18:41, so you will have to look for it against a sky that will still be a little bright.

The cluster of stars you see behind Mars in the image is the Beehive cluster (NGC 2632), but it will offer a much better view with binoculars than with your naked eyes.

If you can't go out tonight, don't despair: the two outer planets will be even closer at the same time tomorrow, but Mars will have almost finished clearing the Beehive by then. If you want to catch Mars on top of the cluster, this is the day.

Choosing binoculars - part 2

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In part 1, I covered three important variables to watch for when choosing a binocular: magnification, aperture and the size of the exit pupil. Now, I'll go through a few more.


Gathering light is important, but not losing it after it's gathered is just as important. Every time light crosses from air into glass or vice-versa, not all light actually gets through: some of it is reflected or scattered away. The loss of light can be of up to 5% on each transition, and a binocular may have as many as 16 air-to-glass or glass-to-air transitions inside it. This could cause as much as 55% of the light to be lost before it reaches your eyes. And that's why lenses and prisms need to be coated.

The function of the coatings is to prevent light from being reflected. High-quality, multi-layer coatings can reduce the reflections to 0.25% of the incoming light, or even less; this will significantly improve the quality and brightness of the image. You want binoculars that are described as "fully multi-coated", that is, all glass surfaces are coated with multiple layers. "Fully coated" means that all surfaces are coated, but not all (or any) have multiple layers, while "multicoated" means that some surfaces have multiple layers, but some have nothing.

A quick way to check for coatings if you have the binoculars in your hands: look at the reflections from the ceiling lights on the objectives. You should see several faint reflections, but none of them should be white. White reflections indicate the presence of non-coated surfaces somewhere in the optics; avoid instruments with this. Also, avoid "ruby coated" binoculars; the reflect large amounts of light and distort the colour of the light that does get through (the image will look greenish).


In short, you need binoculars that allow you to adjust the focus differently for each eye; this can take the form of individual adjustment selectors for each eyepiece or a single central focusing selector with a separate adjustment knob for the right eyepiece. Either of these is acceptable, and mostly a matter of preference (but make sure that you can adjust for eye differences; this is important). Binoculars with individual adjustments for each side tend to be more resistant to water infiltration, but that will depend mostly on the quality of the instrument.

Field of view

This is the amount of sky you can see through your binoculars at a time. For astronomy, the wider, the better, up to a point: if the field of view is too large, you will get distortion near the edges of the image.

The field of view can be expressed in degrees or in "feet at 1,000 yards" (which refers to the width of the image you see). In metric countries, sometimes you also see "meters at 1,000 meters". You want something in the range of 5 to 10 degrees, which is the same as 260 to 520 feet at 1,000 yards, or 90 to 180 meters at 1,000 meters.

This measure is strongly related to magnification; the larger the magnification, the smaller the field of view.

If you wear eyeglasses

Some binocular features are interesting for people with eyeglasses; one of them is called "long eye relief". Eye relief is the distance from the eyepieces at which the image is in focus; that is, it tells you how far away you can be from the binocular while still being able to see the image. For eyeglass wearers who intend to use the binoculars without taking their glasses off, this needs to be long, in the vicinity of 20mm.

Also, ensure that the instrument has retractable or fold-down eye cups, so that you can get the eyeglasses close to the eyepiece without the cups getting in the way.

That said, I personally prefer to take the glasses off and use the binocular with the eye cups up. It is more comfortable for me, and it makes it easier to keep the image forming in the right place (with the glasses on I found it too easy to move my pupils out of the image). Unless you are very near- of far-sighted, you will see everything in focus with or without glasses.


To sum things up, you will want binoculars with:

  • magnification not lower than 8x and not much higher than 10x (and definitely not higher than 12x if you don't want to use tripods)
  • aperture not smaller than 40mm and not much larger than 50mm
  • exit pupil between 5 and 7mm
  • fully multi-coated optics
  • independent focus adjustment (either one control per eyepiece, or a central one with an extra adjustment for the right pupil)
  • field of view of 5 to 10 degrees
  • and, if you wear eyeglasses, long eye relief and retractable eye cups

And, finally, you want to test the binoculars before you buy them. Hold it in your hands, make sure that it is solidly built, make sure that it is not too heavy to hold for long periods of time and so on. Repeat this with several models, see how they feel in your hands, see if you can perceive differences in the images. Prices can vary a lot; try to test both cheap and expensive models. Give preference to stores that cater to amateur astronomers instead of retail camera or general stores (that is, do not buy binoculars from Walmart unless you know what you're doing). Also, read this page and do everything it tells you to. Good luck, and clear skies!

(for the record, I bought an 11x56 binocular from Aquila Optical, in Sydney, and I'm quite satisfied with it)

Choosing binoculars - part 1

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For a novice amateur astronomer, binoculars may be very useful as a first instrument for several reasons: they are relatively inexpensive, have a large field of view and show images "right side up" (which makes finding things in the sky easier), are easily portable and require little to no setup. For more experienced amateurs, they may still be useful for most of the reasons mentioned above, even for those who have (or have access to) more powerful instruments. So, how do you pick good binoculars for amateur astronomy?

Binoculars are basically two small telescopes mounted side-by-side, with a set of internal mirrors and prisms that cause the images to be seen right-side-up and non-mirrored. There are two "models" of internal prisms: "roof" prisms are those used in "straight" binoculars, in which the objectives are exactly aligned with the eyepieces and the optical tube is straight, while "Porro" prisms cause the objectives to be more separated than the eyepieces and the optical tube not to be straight (that is the format that most people think of when thinking of binoculars). Functionally there is almost no difference between the two models, although Porro models tend to give you a wider field of view, while roof models can be more compact than Porro models (and tend to be more expensive).

Binoculars are primarily identified by two numbers; for example, 7x50 or 10x40. The first number denotes the magnification achieved by the binoculars, while the second is the size, in millimetres, of the objective (front) lenses. Both numbers are important, and are a good starting point to pick the best possible instrument.

First, magnification, or power. That indicates how much larger an image will appear than if you were just using your eyes. It may seem that the larger this number is, the better, but that is not necessarily so. For astronomy use, unless you intend to use a tripod, magnifications between 7 and 10 are the most indicated. Anything over 12 will cause the image to shake way too much, unless the instrument is stabilized (by a tripod, for example).

Now, lens size, or aperture. The reason this number is important is that it tells you how much light the instrument gathers. Binoculars not only magnify the objects you're seeing, they also act as larger light receptors than your eyes. Your pupils, when you're in a dark location and fully adapted to the light conditions, will have a diameter of between 5 and 7 millimetres (the younger you are - as an adult -, the closer to 7mm it is; as you get older, it gets smaller). Since the light gathering ability is defined by the area of the objective, a 50mm binocular will gather over 50 times more light than your naked eyes. This will increase your limiting magnitude by around 4, under dark conditions (from 6 to 10, for example).

As a general rule, any aperture smaller than 40 millimetres in useless for astronomy; you simply will not get enough light into your eyes. 50 is a good size; anything much larger will be, well, too large and too heavy for comfortable use. It will be significantly more expensive, as well.

Power and aperture together define another interesting measure: the exit pupil. That is the size of the image that is formed on the eyepiece and that, ultimately, is delivered to your eyes; you can get this size by dividing the aperture by the magnification power (for a 7x50 binocular, the exit pupil will be 50/7 = 7.14mm). Ideally, this should be exactly the same size as your dark-adapted pupil or very slightly smaller, so that your whole pupil is used in collecting light and no light is wasted. Since pupils are not all the same size, this is clearly impossible; you should shoot for an exit pupil in the range of 5 to 7mm.

In the next article, I'll talk about lens coatings and quality, focusing, field of view and considerations for people who wear eyeglasses (such as myself), closing with some tips on what to do once you are at the store.

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This page is an archive of entries from June 2006 listed from newest to oldest.

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