Dec

2

Planisphere

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Before you even purchase your first telescope, the very first thing that you need to buy is a plastic planisphere, or a star map, so that you can learn the constellations to be able to navigate your way through the night sky.

You need to purchase a planisphere for the correct hemisphere that you will be observing from, and also for the correct latitude that you will be observing from. For the northern United States, the 40-50 degree latitude star map will be sufficient, for other regions, determine the latitude that you will be observing the night sky from, and purchase the appropriate planisphere.

The number one reason for owning a planisphere is to be able to know what you are looking at, and to be able to navigate your way from object to object and from constellation to constellation without having to guess or rely on the auto-tracking computer system on your telescope.

Planispheres, or star maps, are around $10 and should not need to be replaced for many years, if ever.

The planisphere is available from Amazon.com.

Nov

27

Eye Relief and Why it Matters

Filed Under Accessories, Telescopes | 2 Comments

When buying a telescope eyepiece or binoculars, an important consideration is eye relief, which is the distance between the exit pupil of the optical system and the entrance pupil of the eye; in other words, how much space there is between last element (the farthest back piece) of the the eyepiece or binoculars and the place where your eye goes to clearly see the image.

The greater the eye relief, generally, the more comfortable the binocular or telescope eyepiece usage will be, since your eye will not be pressed right up against the last piece of glass. Also, the greater the eye relief, the easier it is for people who wear glasses to use the eyepiece or binoculars, as the extra eye relief accommodates the space that glasses take up.

Ideally, you want what is known as the exit pupil of the eyepiece or binoculars and the entrance pupil of the eye to be the same size so that you are able to see a bright and crisp image. However, to have a large amount of eye relief, the exit pupil of the eyepiece or binoculars must be larger than the entrance pupil of the eye so that you have space to move back from the rear optical element; the result is that you are wasting some of the light that your telescope eyepiece or binoculars are gathering, but you are gaining a more comfortable viewing experience.

This post was written to answer a reader’s question, if you would like to have your questions answered, email questions@telescopes.org or comment on the site.

Nov

7

Telescope eyepieces

Filed Under Accessories, Astronomy, Telescopes | 5 Comments

Eyepieces are the part of the telescope that makes the image for your eye to see. The first half of the telescope is the lens or mirror, located at the front of back of the telescope’s tube, at the eyepiece is located at telescope eyepiecesthe side or back of the telescope’s tube.

On a refractor telescope, the eyepiece will sit in a focuser at the back of the telescope, which is a unit that is able to be moved in and out to focus the image that the telescope is looking at. The focuser unit comes in two standard sizes, 1.25 inches and 2 inches, before you buy your eyepieces, it is important to know which diameter eyepiece your focuser unit utilizes.

Calculating the power of the eyepiece that you will be using on your telescope requires some simple math. The primary lens or mirror in your telescope has a focal length, which is the distance that it takes for the light to be focused into a point; this is called FL. The eyepiece that you will look through also has a focal length, which is called fl. You need to convert the focal length of the primary lens or mirror and the focal length of the lens piece to millimeters, and then divide the focal length of the telescope by the focal length of the eyepiece (FL/fl). As an example, if your telescope had a 2000mm focal length (FL) and your eyepiece had a 20mm focal length (fl), the magnification would be 100x.

There are two main types of telescope eyepieces, Kellner eyepieces and Plossl eyepieces.

Kellner eyepieces utilize a three lens design, are fairly inexpensive and have good image quality from low to medium magnification.

Plossl eyepieces utilize at least a four lens design, where high quality eyepieces can become rather expensive. Plossl eyepieces are some of the most common on the market today and offer a relatively large field of view, however, also suffer from a very small amount of eye relief, which can lead to discomfort with excessive use.