Thursday, 16 April 2009

Binoculars - A good start

As I have mentioned before a great place to begin your astronomical hobby is with binoculars, but do you actually know what it is you are looking at when the nice salesman is telling you about them being nitrogen filled and that they have a zoom ya de ya de ya, total discombobulation (thanks to K for the word of the day).

So lets start with a nice basic thing that you'll want to know, what is the magnification of the binoculars that I'm looking at, what does it all mean? Binoculars are commonly described by using a pair of numbers that just look like a maths sum on the side, it's the number you look at and go "oh yes", like you know. You'll see it represented like this; 7x50 or 25x50 or any other kind s of numbers. This is really incredibly simple, the first number represents the amount of magnification that the binoculars offer so in the case of the 7x50's the 7 means that whatever you look at through the eyepieces will appear 7 times closer than they would to the naked eye. See told you it was simple, the next bit is just as easy to understand so don't worry.
The 50 in the 7x50's relates to the size of the lens, or the big bit of the binoculars, the objective lens as it's commonly called, so the 50 stands for 50mm,

The objective lens is very important to you if you want to use the binoculars astronomically, as the bigger the lens at the end, the more light that you can catch, the better you will see in dim light. One important thing to remember is that everything through binoculars is magnified, which includes your own movements, so any shakes or wobbles will also be very noticable, so the higher the power the harder it is to hold them steady, and if you are using 50mm or above the binoculars will also be slightly heavier than smaller ones, this again adds to the problems of holding them steady.

So now you know the magnification and the objective lens diameter you can do a a calculation that will tell you the exit pupil, why is this important you ask, well it's a bit to do with how old you are, no I have not gone totally nuts, as you get older your pupils shrink and so getting a pair of binos with a huge exit pupil may not be worth it to you if you cant take advantage of them, with this in mind the formula is this. Divide the objective lens size by the magnification and this will tell you the exit pupil size so with our 7x50's you get approx 7.1mm of exit pupil, so 7.1mm of light is transmitted to you pupil. A bit techy but maybe important to you.

The next thing that you will probably come up against is the lens's themselves, you turn the len's towards you and are greeted by a lovely coloured lens and you think "ooooooogh, pretty", but beware, amber and red coatings are an astronomical no-no, they seem to be a bit of a gimmick to hide poor optics, but are said to be to reduce light glare.

The prism will be your next area of attention, the prism is what lets you see a correctly oriented image when you look through the binoculars. Typically there are two types of prism, Porro prisms and roof prisms, what's the difference though?

Porro prism can be identified by their offset tubes, so the objective lens is not in line with the ocular lens, or eye piece. The eyepieces are usually closer together than the rear lens's, but the reverse can be true in compact models. Porro prism are usually much better quality than roof prism another benefit is that they have a single pivot between the two halves of the binocular, making then easier to adjust to your eyes. Porro prism are also described as Bak4 or Bak7, with Bak4 actually being the better quality, and therefore the ones to go for if possible.

Roof prisms are essentially in line inside the optical tubes, and therefore can make for a more compact set. Roof prisms usually have two pivot pints between the two halves of the binoculars, and are more difficult to adjust to your face. Roof prisms can give an optical image equal to Porro prisms.

Another thing to think about is dewing of your lens's, when your outside and the night is cold, and you bring out your lovely warm binoculars from their brand new bag, the difference in temperature between the outside air temp, and the binocular temp is likely to cause one thing, dewing, this will end your observations very quickly, so consider getting nitrogen filled binos as they will not suffer from this problem at all, my own binoculars (15 x 50 in case your wondering) are nitrogen filled and I have never known them to mist, they are also waterproof.

So what to do to choose, well heres a few things you can do to test them in the shop;
  • Steer away from zoom binos, there has not been a decent zoom made EVER (yet)
  • Steer away from Red or amber coatings
  • Give them a bloody good shake, if they rattle, don't buy them
  • Check the focussing through its entire range, reject any with loose spots or that bind whilst focussing
  • Do the same check with the focussing for the right eyepiece adjustment
  • and check the hinge is also smooth
  • If you re going to use your new binoculars with spectacles then make sure you can see properly through them with the eyecups folded down and the binocular at spectacle distance, so about 25mm
  • If you have a big nose make sure it fits between the binoculars
  • Check what prism you are buying, remember Bak-4 are best in my opinion
To see just what can be achieved with a monster set of binos, the look no further than Space Jocky he uses a set of Strathspey 25 x 100's (see pic at top of post)

Now these are some big binos, and will only be used on some form of tripod or support, but the results that you can get with them speak for themselves, check Space Jockys site for cracking images.

Follow Space Jockey on his blog or on Twitter, he really is an expert when it comes to using binoculars.

So I hope that helps a bit and good luck with any purchase you may make, and if you do, tell us about it.


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