Telescope Reviews: ETX-80 vs ETX-90

Although the above response states things pretty well, sometimes it is good to “know the numbers” when comparing equipment. There are at least three (4) things to consider when looking at telescopes. These would be “construction style”, such as “Reflector/Refractor”, Aperture (the diameter of the tube), Focal Length (distance light travels from entry of the tube to the eyepiece), and “Field of View” of the telescope (developed with the other values…).

There is a saying in astronomy, that “Aperture is King”. Meaning, that more is better. Why? Because the bigger the opening of the tube, the more light that it collects. That makes seeing fainter objects easier. Four (4) inch telescopes are very popular, then 6”, 8”, 10”, 12”, and 14”. Some of the “Dobs” reach out to 36”. The difference between 80mm and 90mm (3.15” and 3.5”) is not a significant amount, although 3.5” (90mm) is better, as bigger is better.

Focal Length is a very important item. It measurers the distance between the beginning of “light handeling” of the telescope, and the “focus point” at the eyepiece. It is a value provided by the telescope maker, and is usually stamped on the telescope somewhere. A “refractor” (such as the ETX-80) is a “one-pass straight thru” type telescope. A “reflector”, (such as the ETX-90) makes multiple passes through the tube, using mirrors. You can see this by looking down the tube from the front. Focal Length is used to calculate “speed” (the “f” ratio), and multiplication factor. The longer the focal length, then the higher the “magnification value” for similar eyepieces. A standard eyepiece for Meade telescopes is 24mm. The ETX-80 has a focal length of 400mm. The ETX-90 has a focal length of 1250mm.

Magnification is determined by dividing the Focal Length by the size of the Eyepiece. For a constant eyepiece of 24mm, then the ETX-80 would be (400/24) about 16x. The ETX-90 would be (1250/24) about 52x.

Field of View is the calculated by dividing the “Apparent” field of view of the eyepiece (supplied by the Mfg…), by the magnification (as calculated above). Assuming a nominal field of view of 50 degrees for the constant 24mm eyepiece, the ETX-80 would have an operational field of view of (50/16) about 3 Degrees, and theETX-90 would be (50/52) about 1 Degree. From this we can learn two ways to express “field of view”. Higher magnification produces narrower field of view. Longer focal length produces narrower field of view. Another way to say the same thing would be these two comments. Lower magnification produce wider field of view. Shorter focal length produces wider field of view.

OK, that’s my 2 cents on the subject. Hope that helps. By the way, I have an ETX-70 (Refractor), and an TX-90, as well as an 8” LX200 and an 80 mm Series 5000 ED APO. Each scope has its own “best application”, and I use each of them for various opportunities. There is no “one is best”. It really is a function of what you can afford, and what you want to see.

--------------------
David M. Douglass
Tempe, Arizona