Quote:
Originally Posted by nightshade  Just out of curiosity (I do not fully understand the criteria for
astronomy Binos) |
Hi Nightshade...
A few of the important things to consider when buying binocs for astronomy...
Don't get carried away with magnification - more is not neccesarily better. When you get above 12x magnification, you need to mount the binocs, tiny movements when hand-holding may not be a big deal in daylight - but when you are near the limit of your eyes' light sensitivity, trying to see the maximum amount of dim stars or dim deep sky objects, those tiny movements will mean that you fail to see A LOT that you would see if the binocs were steadied. And at higher powers, you also need the aperture to match, otherwise you get dim images.
Exit pupil
This is the diameter of the cone of light that leaves the eyepieces, and enters your eye.
It is easy to work out, just divide the aperture, by the magnification...
A 10x50 will give a 50mm /10 = 5mm exit pupil
8x56 gives 56 / 8 = 7mm exit pupil
Again - during the day, exit pupil isn't a big deal - a daylight scene will usually give plenty of light throughput, even with a small aperture, and also a small exit pupil.
At night it's another matter - most people's pupils will dilate to somewhere between 5mm and 7mm wide - so the wider the exit pupil the binocular has, the more light it can supply to your eyes. As a general rule of thumb, for astronomy you want binocs that give AT LEAST a 4mm exit pupil - preferably 5mm - 7mm (more than 7mm is a waste - most people's pupils won't go any wider than 7mm.
Aperture
The size of the front lenses - Bigger is better, because the surface area of the main lens dictates how much light the binocs can draw in. A 70mm binoc will draw in about twice as much light as a 50mm binoc (you square the diameter when comparing surface area)
Coatings
Lens coatings reduce the amount of light that the glass reflects - no glass lets through 100% of the light that strikes it, some light is always reflected away - and anti-reflection coatings can mean the difference between 94% throughput, and 98% throughput (PER LENS ELEMENT) - 4% doesn't seem much, but when you consider that the light has to pass through usually about 5 or 6 lenses, and 2 prisms, it all adds up. Coated optics are OK - Multi-coated is better - Fully coated is better still - and Fully Multicoated is best.
The colour of the coating is also a big clue, the best coatings usually have either a blue, purple, or green tinge.
Red (Ruby) and Amber coatings are to be avoided.
Prisms
Porro-prism design is best - Roof prisms are often OK, but are more difficult to manufacture well aligned - so cheap roof-prisms are to be avoided.
Prism material - BaK-4 is best, avoid BK-7 (BaK-4 has a much higher light throughput)
If two binocs are of similar spec, quality, and price - go for the one with the widest field of view - the wider your view, the better for navigating your way around.
And as mentioned above - for astronomy, zoom binocs are the Devil incarnate.
Best binocs for astronomy for under £100 are probably Celestron's SkyMaster 15x70
Other good sizes... 7x50 - 8x42 - 8x56 - 9x63 - 10x50 - 10x70 - 12x60 - 20x80 - 22x100 (HUGE)
Personally, I'd avoid 16x50s, 20x50s mainly because of the small exit-pupil - I've owned a 20x50, images were too dim for astronomy.
(I also have a pair of 4x28 opera glasses - great for low-power sweeping round when at a dark site - Lovely bright images, I much prefer those to a 20x50)
I think that's about it - any questions just ask and I'll try to clarify
