Astronomy Notes

Light gathering ability is the most major factor when considering a telescope. Large power, ie. high magnification, is not always encouraging. Moreover, over-magnifying the objects will cause diffraction as it’s over the diffraction limit of the telescope. Why? The term, light loss is always linked to this.

For any telescope entrance aperture, the brightness intensity seen at the eyepiece is inversely proportional to the square of the magnification. This is the natural “square law”.

When you double the magnification, you get one quarter the brightness. ie, 1/2 squared.

When you half the magnification, you the brightness increases four times. ie 2 squared.

You can work out the relative brightnesses between any pair of eyepieces by doing the simple arithmetic.

FAQ
>>> Is there a light loss if I use a barlow lens?

Same square law applies. A 2x Barlow will result in a quarter the brightness because you have 1/2 squared it.

>>> Does a eyepiece connected with a barlow lens offer the same brightness as the doubled magnification eyepiece above?

Nearly the same. There is a slight bit more loss because of the extra number of lenses in the Barlow that the light must pass through.