ETX Telescope Sizes

The ETX telescopes currently come in 4 apertures: 80mm, 90mm, 105mm and 125mm. In the past there have been 60mm and 70mm ETXs, but these models have been discontinued.

The ETX-80, a short tube refractor

The ETX-80 is am 80mm (3.15"), short tube refractor style scope with a focal length of 400mm (f/5). This scopes as well as its predecessors, the ETX-60 and the ETX-70 seem to be an exercise of how inexpensive can a portable Computerized scope be produced. Because of its short focal length is will be difficult to get high magnifications for lunar and planetary viewing. You will probably need a barlow lens to get into planetary magnifications. This scope will provide wide panoramic views of the sky. Specifications for the ETX-80 can be found here.

The ETX-90, 105 and 125, Maksutov-Cassegrains

The three main scopes in the ETX line are the ETX-90, EXT-105 and the ETX-125. All three of these scopes are Maksutov-Cassegrain (MAK) designs. A MAK usually has about an f/14 focal ratio and achieves a long focal length in a small package by folding the optics internally using mirrors. Scopes that fold their optics are know as "catadioptric".

These three telescopes are pretty much identical in the controls and their mounts; the only real difference is their size. The aperture of any telescope controls the amount of light collected by the optics. The larger the telescope the more light and because the light gathering power is a function of the area of the main lens (pi X radius²) the abilty to gather light, known as "lightgrasp", goes up in squares. So small jumps in aperture are significant.

The following tables Compares the ETXs

*Lightgrasp is defined by how much brighter would the view be compared to the unaided eye. (for example: the ETX-125 would be 319 times brighter than the unaided eye.)