Choosing eyepieces for your new telescope

We usually recommend two eyepieces and a 2-times Barlow lens to start with.

For most F6, F8 telescopes (in the 8" or 10" diameter range) a 25mm eyepiece will give a reasonable field of view with a magnification of about 48 times (for the 8" F6) and 60 times for the 10" F6. With a two-times Barlow lens this will allow double the magnification. (96x and 120x for the 8" and 10" telescopes respectively.)

The Barlow lens is a "negative" lens which effectively increases the focal length of the telescope's main mirror. The regular eyepiece is fitted into the top of the Barlow and then the whole assembly is inserted into the focus mount. The two together give double the magnification of the normal eyepiece but you still have the large "output lens" of the regular eyepiece to look through. This gives better "eye-relief" (That is - you can see through it more easily than you could through a 12-1/2mm eyepiece which would be needed to give the same magnification and would have a smaller "out-put lens")

When selecting a second eyepiece you should choose a size (Focal length in mm's) which is not exactly half the focal length of your first one, as you can (with the Barlow) effectively halve the focal length of the one you already have.

So we would suggest something like a 17 or 15mm eyepiece as the second choice. Then you will be able (by using the Barlow with either eyepiece) to obtain four different magnifications:

For example - (using a 25mm and a 15mm eyepiece with the 2x Barlow - with the 8" F6 'scope,) you would get the following four different magnifications: 48.7 times, 81.3 times, 97.5 times and 162.5 times. ( You can round these off to say approximately 48x, 81x, 97x, 162x.)

To calculate the magnification you divide the focal length of the telescope by the focal length of the eyepiece you are using -- but eyepiece's focal lengths are usually given as so many millimeters(mm) so you need to figure out your telescope focal length in mm's - simply multiply the Focus in inches by 25.4 So in the above example the focal length of the 'scope is 48" x 25.4 = 1219mm's.

If you should then decide to buy a third eyepiece - you can again pick a value that does not become a half-multiple of the ones you already have. Let's say you pick a 32mm - which will give a lower magnification of (1219/32, using the above example) which is 38 times, for a wider field of view, this with the Barlow would give 76 times. This number falls within the 48x and 81x which you have with the other eyepiece/Barlow combinations. Then you will have six different values of magnification.

At first reading - this may seem a little complicated, but you should be able to select eyepieces and an appropraite Barlow lens which will make the maximum use of the eyepieces you have bought if you choose them with these ideas in mind.

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