Mercury in the early morning sky

There will be a fine opportunity at the end of the month of September (always weather permitting) to see the planet Mercury in the early morning sky before dawn.

Dia 1: Looking east September 24 at 05 45 BST (Click the image for enlargement)

On the morning of the 24th September Mercury will rise at 05 10 BST, approximately fifty minutes before sunrise.

The diagram (1) shows Mercury in company with the crescent Moon and Jupiter at 05 45 BST. At this time Jupiter will be nearly five times as bright as Mercury. Never the less, Mercury will be brighter than the bright star Procyon which resembles the planet in colour .

Mercury moves swiftly eastward under Leo during the course of the next few days increasing substantially in brightness the while. It may be visible to the naked eye up to and around 12th October after which, despite its brilliance, twilight and sinking altitude will make observation progressively more difficult.

J V 18/09/03

Choosing a Binocular

Catering for the wide range of people likely to access this site we shall, from time to time, include features of general interest.

Choosing a binocular.

1) All-purpose work.

The main factors to consider are ease of use and effectiveness. Anything above x8 magnification will be difficult to hold steady for the majority of users. A light-weight binocular is not necessarily easier to hold still than a slightly heavier one (for observations of short duration - about a minute, maybe). The binocular should feel comfortable in the hands. Always use both hands where possible. There is little to beat the 8x30 combination of power and aperture for general use. Remember, the ease with which you can accommodate the emerging light from the eyepieces is determined by the size of the exit pupil. The smaller the exit pupil the more critical the eye separation distance and the more fussy the binocular is to use. (Anything under 3.0 mm falls into this category - see below.)

Most binoculars have a central focusing wheel with adjustment to the right eye at the eyepiece. This is to enable the differences between right and left eye to be accommodated and once set for a given user should not require adjusting too often. The individual eyepiece focusing design is usually to be found with 7x50s. This is of advantage where one is usually observing distant objects (astronomy) and the focusing setting remains relatively "fixed" for the same observer.

2) For use in poor light, 8x40 is strongly recommended. For very poor light, 8x 42 or 7x50. (For some reason manufacturers avoid 8x50 which is in many ways better than 7x50 since too wide an exit pupil - larger than the pupil itself - means wasted light).

3) For bird watching, and for detail at not too great a distance, try 10x40 or 10x42.

Testing a binocular

Test the quality of the image for colour - blue fringes to white objects are a bad sign. Your binocular should focus to give a clear, natural image with minimal distortion across the field of view. Also make sure you can focus on close-up objects. For really close work (birds, say) roof prisms, all else being equal, give the best results. "Double imaging" (see cautionary footnote) can sometimes occur even in new instruments. Always be on the lookout for this defect especially in second-hand binoculars. Inspect the lenses for scratches and abrasions. Look into each object lens from the front to ensure the internal prism system is free from discolouration - the un-cementing of prisms can occur in older instruments.

Binocular design.

The two designs most commonly encountered nowadays are the German classical prism system (the American system is similar), and the roof prism. In the latter the main lens (object lens) and the eyepiece lens are in-line. Such binoculars usually enclose all mechanical focusing parts within the body of the instrument and are therefore dust proof and sometimes waterproof as well; as a result, they are generally more expensive than other types. A definite advantage for bird watching etc. is that the roof prism binocular suffers less from parallax effects when observing objects at distances of less than 15 metres or so. Compactness is another plus point; but be WARNED, quality can vary enormously. Even quite expensive roof prism binoculars can be outclassed by lower priced older designs from good manufacturers. The old Eastern Germany Carl Zeiss firm produced fine binoculars known as the Binoctem and the Deltentum (Deltrentis for individual eyepiece focusing) ranges. These still come onto the second-hand market from time to time at reasonable prices.

Pocket, or compact binoculars. These are almost without exception of the roof prism design. Remember the earlier remarks on size and weight. The only way to decide on performance is to compare one binocular against another on the spot and therefore under identical conditions.

Because these instruments use small objective lenses (usually 20mm) the exit pupil will be correspondingly small and therefore the eye separation distance becomes more critical (getting both eyes lined up with the eyepiece). There are many inexpensively priced binoculars in this class. Most give relatively poor light and may have a blue cast. With compact binoculars, even more than with larger instruments, you tend to get what you pay for in terms of quality.

Astronomical use

A good binocular can provide very rewarding views of the night sky not accessible to a telescope. A binocular has a wider field of view, for one thing, and two eyes are better than one for wide-field work. The Pleiades and the Beehive Cluster, for example, show well in a small binocular and the Andromeda Galaxy can look impressive in a pair of 7x50s. For comets 10x42 is generally preferable to something with a larger aperture and lower power since magnification is important. Another factor to bear in mind when viewing objects in a twilight sky, or a sky badly affected by light pollution, is that so-called twilight value works to our disadvantage. (See explanation for "stellar enhancement".) For wide-field astronomical work the old Zeiss 7x50 Binoctem is a very useful tool. The same firm also produced a 10x60 glass of the German design. Anything larger than this would be difficult to use without a mounting.

For serious comet searching 15x80s would be a good compromise. Instruments of reasonable quality of this size may be had for around £260 - £300; 25x100 for a little under £500. Cost escalates once one moves up into larger binoculars of quality - you may expect to pay at least £1500 for a good 20x100 (or 30x100). A tripod is, of course, essential and will add to the above costs. The much larger, and therefore weightier, 30x150 instruments used by serious comet hunters will be beyond the aspirations of most people.

It is important to bear in mind that as soon as a power greater than x10 is used, whether in a binocular or small telescope, hand shake becomes significant. There are now binoculars available with compensating mechanisms for movement and shake. These instruments are more expensive. Although many people may find such binoculars handy for all-round use, conventional designs - tripod mounted - will be more useful for astronomical work.

Sample binocular specifications

Mag./Aperture   Twilight    Relative Stellar       Exit
'm' 'a'         Performance Light    Enhancement   Pupil 
               (mm)

6 X 20           8.16         40          49        3.3
8 X 20           7.07         40          56.5      2.5
10 X 25          7.9          62.5        79        2.5
8 X 30*          10.6         90          85        3.75
7 X 35           13.2         122         105       5.0

8 X 40**         14.1         160         113       5.0
10 X 40          12.65        160         126.5     4.0
7 X 42**         15.87        176         111       6.0
10 X 50          15.81        250         158       5.0
7 X 50**         18.9         250         132       7.1

8 X 56           19.8         313         158       7.0
15X 60           15.49        360         232       4.0
16X 70           17.5         490         280       4.4
20X 80           17.8         640         358       4.0

'a' = aperture (mm).
'm' = magnification.
In the first three columns, efficiency is proportional to numerical value.

* Recommended for general use.
** Recommended for terrestrial use in conditions of poor light.

Anything above x10 magnification will require a support system - tripod etc.

Notes:

Twilight Performance: This represents the field brightness. A higher magnification at the same aperture will have the effect for an extended object of 'spreading' the available light thereby producing a fainter image.

Relative light. This is the 'light collecting' capacity for a given instrument. For a point source (e.g. star) it is purely a function of the relative 'surface area' of the object lens. But for an extended object the same principle applies as for twilight etc. and magnification becomes a significant factor.

"Stellar Enhancement": When a point source (e.g. star) is observed against a continuous background, magnification has the effect spreading or 'diluting' the background light, thereby increasing the perceived contrast between the background and the point source. A star will appear brighter, therefore, the higher the magnification for a given aperture.

Exit Pupil: The diameter of the cone of light leaving the eyepiece. This can be demonstrated by holding the binocular at some distance. The small illuminated disc seen in the eyepiece is the exit pupil. The fully dilated pupil for the average human eye is 8mm.* This will be smaller, depending on the brightness of the light perceived. Consider half this value for the eye working under daylight conditions. There is little to be gained by having an exit pupil much in excess of this since the light cannot enter the eye and would tend to be 'wasted'. On the other hand, a too small exit pupil means the eye will have more difficulty locating it. (This usually arises when using compact instruments e.g.. 8 x 20, 10x25.)

Note: magnification may be calculated by dividing the diameter of the object lens by the aperture of the exit pupil.

* This decreases with age - of the user, not the instrument!

CAUTION: Always carry your binoculars with a strap or lanyard slung from the neck. It is all too easy to drop binoculars. Even if you are fortunate enough not to break the lenses the internal prisms will almost certainly suffer. Binoculars damaged in this way give "double images" and will have to be re-aligned by an expert.

J.Vetterlein. 1992/93 (updated 2003)