Monday, March 26, 2012

"I see no ships" Maybe you need a new telescope then Horatio?

After a good pair of binoculars a telescope is the next most important tool for the birdwatcher, offering higher magnifications for detailed and long-range viewing.
Out in the field.
Scope, tripod and Mule Pack ready for action.

Here I will be focusing on telescopes for birdwatching which are almost exclusively of the refracting type with an objective lens at the end and use prisms to fit a long focal length in a compact body in the same way as binoculars do. The other kind of telescope often seen is the reflecting type which uses a two mirrors, a large one at the back and a smaller secondary mirror at the front to focus the light into an eyepiece. This type of scope is favoured for astronomy because it is easier to make very large high-quality mirrors for good light gathering than it is to make large lenses of the same quality. Amateur astronomer will use scopes with mirrors of 8 inches (200mm) or bigger, something that would be extremely expensive if not nearly impossible with a refracting scope. Astronomical scopes are typically set up semi-permanently in one place, birdwatchers however require scopes to be rugged, weather resistant or waterproof and portable, making the refracting type a much better option.
Reflecting and refracting astronomical telescopes.

A typical birdwatching scope.
This also shows the two types of body available with
straight and angled eyepieces
So what do telescopes do that binoculars can't?
Binoculars are mostly between seven and twelve times magnification with eight and ten times being the most common and versatile. Scopes however, typically have magnification between 15 and 75 times depending on the eyepiece. As with binoculars the brightness of the image is determined by the magnification and the diameter of the front (objective) lens. Our video on binocular specifications is worth a look because the same principles apply to telescopes. Basically the larger the objective lens, the brighter the image will be, but the higher the magnification, the darker the image becomes.

Larger scopes, typically with a 77mm-100mm objective lens, are the best for a bright image and higher magnifications, but are heavier and less portable than mid-sized scopes, typically with 60mm-70mm objective lenses. The smallest and lightest are scopes like the Nikon ED50 and the Opticron GS52 ED, which with objective lenses of 50mm and 52mm respectively making them ideal for travel and carrying long distances, but compromised in poor light. Whilst the brightest image and highest magnifications may be appealing, I have known people with large heavy scopes that they often leave at home or in the car because of the inconvenience carrying them.

After you have settled on a size of scope suitable for your needs the next decision is between a straight or angled eyepiece mount on the scope body. This is a matter of personal preference, but it is usually easier for two people to share an angled scope because straight scopes need to be positioned at exactly the right height. the eyepiece.
Then we come to the eyepiece itself  Most cheaper scopes come supplied with a zoom eyepiece usually covering the range 15-45x magnification on 60-65mm scopes and 20-60x on 80-85mm scopes. Higher up the price range there is often a choice between a zoom and a range of fixed magnification eyepieces (for example 20x, 30x and 45x). These are more restricted than a zoom in that you cannot change magnification without changing eyepiece, but they are still popular with some birders because you get a wider field of view and often a slightly brighter and sharper image compared with a zoom at the same magnification. At the top of the range Leica and Swarovski make wide angle zooms covering the 25-50x range which offer a field of view similar to the fixed magnification eyepieces.

As with binoculars the main improvement as you go up the price range is the quality of the glass. The build quality does increase too, but this is not a criticism of the construction of the budget and mid-range scopes, which are designed very well to cope with use in field.

The same terminology referring to glass types and quality in binoculars applies here too, there is a short glossary of commonly used terms and acronyms at the end of this blog.

Another feature that varies between manufactures and models is the focusing control. There are basically two main types: a wheel mounted on the prism housing and a ring around the whole body like you would find on a camera lens. Some scopes with the wheel type have two wheels, one high geared and one low geared, for fast and precise focusing. The Zeiss Diascopes are unique in having this feature built into one control that responds to how fast you turn the wheel. Which one of these is right for you is a mater of personal preference and both systems are quickly adapted to.

Hawke Naturetrek showing wheel type focusing
Nikon ED82 showing ring type focuing

While most scopes are waterproof and fairly rugged, another popular accessory is a stay-on-case. These add an extra layer of protection to the scope and eyepiece keeping the outside in better condition and, as the name suggest, stay on the scope when in use.

With the high magnifications offered by telescopes a suitable tripod is essential to get a steady view. All telescopes and cameras use a standardised screw thread size to fit them to a tripod. Most modern tripods have a quick release system that allows you to remove and refit the scope without having to unscrew it. These work by having a small metal plate that screws into the foot of the scope which then clips into the tripod head. Some tripods come as a complete kit of legs and head, others you can choose the best combination for you requirements. The size of your scope influences the tripod you need, smaller scopes can use lighter tripods than larger scopes and still be steady. Another factor that effects the weight of a tripod is the material it is made from. Most low to mid range tripods are made from light-weight aluminium tubing, but the top of the range are carbon fibre. The same qualities that make this material ideal for sports cars and aircraft make for strong, ridged and light tripods.

So, that's a round up of what telescopes are all about, have a look at our website or visit us at Glandford or Cley for our full range of telescopes and tripod.

Cley Spy


APO. Abbreviation of apochromatic. A completely apochromatic lens system corrects all chromatic aberration (colour fringing). Leica's APO-Televid scopes also have fluorite lens elements.

BK-7 and BAK-4 prisms. These are two grades of glass, boroscilicate BK-7 (generally in cheaper optics) and barium crown BAK-4 (delivering better sharpness)

ED, HD, HR, Prominar. Terms used to denote higher-quality glass models, HD standing for High Definition, ED usually standing for Extra-low Dispersion, and HR standing for High Resolution. These terms are not standardised, one companies standard glass can sometimes be as good as another's HD. With Swarovski HD denotes models with fluorite lenses. Prominar is a term unique to Kowa and again denotes the higher quality glass versions, including the impressive fluorite-lensed TSN-883 with its 88mm objectives.

Eye relief. This is the distance that your eye should be from the eyepiece lens to get the optimum image. Spectacle wearers often need a longer eye relief when using a scope or binoculars with there glasses on.

Nitrogen filled. Waterproof scopes and binoculars are often filled with a dry, inert gas (most commonly nitrogen or argon) to prevent internal fogging.

SOC. Stay-on-case.

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