Telescope Lens Calculations for Refractor Telescopes

Check out this java applet that lets you play with lenses and mirrors, it might help you, but its fun to play with either way.

I don't know much about building a refactor telescope, most diy telescopes are reflectors such as the one I built and wrote about in my blog, although I bought my mirrors. I did consider making my own optics, and their is plenty of documentation on how to do that, I just decided it would be easier and possibly cheaper to buy the mirrors.

Not sure how you would spin cast a lens to optical quality, but good luck, its a fun project either way.

I too built my own reflector and You are absolutely right about the amount of work grinding the mirror. I started with a 10 inch piece of pyrex about 2 inches thick and a plate glass about 3/4 inch thick. I ground and polished off and on for about a year and still did not get that pefect parabola needed for good seeing.

oilcan13

1. Most important: Do not direct the telescope towards Sun. Never. You will loose your eye sight.

2. Making refractor is simple, but getting good objective with no chromatic aberration is difficult.

Magnification = Focal length of Objective / focal length of eye piece. You need to keep eye piece at a distance of addition of two focal lengths. Obviously, you need to keep eye piece movable on the axis joining two lenses.

Its so simple.

The focal length of a lens is the distance that parallel incoming rays will be refracted to a point. You can hold the lens in sunlight and measure the distance to the point where the rays converge. The magnification of a telescope is the ratio of the objective focal length to the eyepiece focal length. For example, if the objective focal length is 60 in. and the eyepiece focal length is 1 inch, the magnification would be 60X. To observe at infinity (astronomical objects) the distance between the lenses would be the sum of the focal lengths, in this case, 61 inches. The amount of light (brightness) of the image is proportional to the area of the objective lens.

I would think that there will be two difficulties here:

• A decent refractor will require at least an achromat for the objective, consisting of one convex lens and one concave lens, with the concave lens having a higher refractive index. Otherwise the chromatic aberration will be a serious problem. Designing an achromat (balancing the two elements) is a complex problem, fairly straight forward for a lens designer but not recommended for a novice.
• You could spin cast the concave lens - the centrifugal force will cause the material to flow outward resulting in a bowl shape. I have done this using epoxy and it works fairly well. Unfortunately the resulting concave surface will be parabolic rather than spherical. I don't know how you would spin a convex lens (perhaps spin two concave lenses and use them as a mold?). But even if you could do this, the surfaces would still be parabolic. While this is the perfect shape for a Newtonian reflector, it would create massive aberrations for a refractor.

Good quality achromats are available commercially for very reasonable prices. Anchor Optics in the US (www.achoroptics.com) offers one with a focal length of 335 mm and a diameter of 43 mm, which would be suitable for a telescope and costs $23.00. This would be less adventurous than making your own lens but would almost certainly work better. If you do want to make your own I can send you the formula to calculate focal length, but you will need to know (or measure) the surface radii and thickness of the lens, and the refractive index of the lens material.

PS: if the $23 is high for your budget, I have boxes full of used lenses and might be able to find one that would be suitable for a small telescope.

the idea you came up with of spincasting a dish and using it as the mold for the lens is what I was planing to do I as under the impresion that the lenses were suposed to be parabolic curves and that the scope only had 2 convex lesnses I understand this dosent make for a perfect scope I am just hoping for beter than what we have at the moment I have since found all the formulars I was looking for thanks all for the responces the next idea is how would I miror a resin cast parabolic bowl for a reflector scope?