How far apart are the lenses placed from each other in a refracting telescope?

12 Comments

1. al_moskowitz says:

   May 8, 2010 at 7:27 pm

   There is no standard answer to that question. It really depends on the lens size, focal length, type of refractor… ETC…
2. david_bowman_sc says:

   May 8, 2010 at 7:53 pm

   It depends upon the focal length of the primary lense. 60 mm telescopes generally have focal lengths of 900 mm (f/15), so the objective and eyepiece are set just under 900 mm with the eyepiece traveling toward and away from the objective to allow focusing. Larger refractors are going to be much longer. The tube for the 40″ Yerks refractor is 63 feet long.

   http://astro.uchicago.edu/vtour/40inch/
3. David A says:

   May 8, 2010 at 8:38 pm

   It all starts with the objective lens (that’s the large one out in front). If it has a focal length of 1,000 mm. then the light it receives comes to a focus (you guessed it) 1,000 mm. away, in the tube of the telescope. So, the ocular (eyepiece) is placed at that point, and sharp focusing is done by moving the ocular back and forth with some sort of device (rack and pinion, sliding, etc.).

   Of course, quality refractors are built with multiple-element objectives, (usually made of different materials with different refractive qualities) in order to help focus more sharply. You see, different colors focus at very slightly different distances, so a 1,000 mm focal length lens will focus red light at a different distance than blue light, creating a blurring effect called chromatic aberration. The combination of two lenses at the objective decreases that effect.

   But, the focal length of the combination determines the distance from objective lens to ocular, or from ocular element to ocular element (most oculars have at least two elements).

   The very same thing determines the distance between the objective mirror of a reflecting telescope and the oculars (taking into account the distance to secondary mirrors, and in the case of cassegrains, the distance to the secondary and back again to the ocular). In the case of cassegrains they have what is termed “folded optics”, meaning the light bounces off the main mirror, goes to a secondary mirror placed about halfway down the focal distance, and back through a hole in the main mirror to the ocular. In that way a 1000 mm focal length mirror can be placed in a telescope tube only about half that length.
4. OrionA says:

   May 8, 2010 at 9:21 pm

   The secondary should be at the focal plane of the primary mirror, so all photos collected at the primary can be redirected to detectors. And that dictates how far apart they should be.
5. Helpful person says:

   May 8, 2010 at 9:32 pm

   The distance apart is the sum of the two focal lengths.
6. gn says:

   May 8, 2010 at 9:38 pm

   The answers here do indicate that “it depends on the design” but they’re not really answering the question. Refractor lenses have all kinds of potential positions.

   There are doublets, where one lens is placed behind the other; and there are triplets, where you have three lenses in series.

   How far? Not far at all. Sometimes they are directly in contact with only a thin one-molecule-thick layer of oil between them. “Oil spaced triplets” are a popular design: If you looked you’d say they were touching. . There are also “air spaced triplets” and “air spaced doublets” with more space between lenses. You might have two of the lenses in physical contact and the third one set back a ways.

   Finally, there are Petzval and other designs, which actually consist of four lenses, two up front and two anywhere from half way back to all the way at the end just in front of the eyepiece. So now you have four lenses with considerable space between the two groups of two.

   So without further specification: the answer is: the lenses can be physically touching each other or they can be widely spaced depending on the telescope design. More precision than that is not possible without a more precise question.

   If you want to know WHY there are these different designs, it has to do with many detailed aspects of telescope performance. Flat, undistorted fields are preferred by imagers. False color is a problem in refractor designs and many of the very expensive high end triplets etc. are designed to control that. *Wide fields of view* are highly prized by refractor owners and causes them to desire short focal lengths which then add considerable design difficulty and expense for the telescope maker, because they require exotic glasses. Long telescopes are cheaper to make because the glass can be less expensive and it is easier to figure, but the field of view will be less, and long refractors are difficult to handle as observing instruments.

   Hope that helps,

   GN