advice on building a science fair telescope

sravet

I am helping my son design and build a refractor telescope for a science fair project (8th grade). He has read some on lens types and focal lengths. We also read that Galileo's most powerful telescope was 33x, and was able to view Saturn's rings and Jupiter's planets. We want to be able to do at least that with purchased lenses and a home made tube. Our first step will be to mount the lenses with clay on a yardstick so we can easily measure lens position.

I'm hoping to get there with something like United Scientific lenses which run $5 to $15. We aren't looking for professional grade results but we don't want to be disappointed either. Depending on cost we may get several lenses so we can experiment and note what works best. So I have some basic questions, and am looking for some rules of thumb:

1) glass or acrylic? After a quick search it looks like plastic is surprisingly more expensive than glass. What's the difference?

2) Lots of suggestions for achromatic lenses. Given that this is a science fair project are single elements lenses suitable?

3) focal lengths. We can get there (Galileo 33x) with something like 300mm/10mm, or 900/30, or lots of other values. It's my understanding that there is less distortion with longer focal lengths, but they require longer tubes. Are there any general guide lines on focal lengths? longer generally better?

4) Anything else I may not have thought to ask?

thanks in advance for any help or suggestions.

--steve

Drakkith

Typically refractors have 2-3 lenses of differing material to counter what is called "chromatic aberration". This means that the different wavelengths of light, aka the different colors, are refracted by different amounts when traveling through the lens, leading to a big colored blurry mess when you view anything through it. I don't know the properties of glass and acrylic offhand, but acrylic may be better than glass and more expensive to produce, leading to higher cost.

I'm sure it would be fine for a science fair, especially if you can explain WHY the light doesn't all focus in the same spot. If you make a color filter you could simply hold it in front of the lens and show people the effect, as the filter will only let a small range of wavelengths through, greatly reducing the chromatic aberration while you hold it up.

The longer the focal length, the more zoom you will have and the less the different aberrations will show. With just a single lens you will need an absurdly long focal length to reduce the chromatic aberration to acceptable levels. however if you intend to show and explain it, then a shorter one may work fine. If you want to have a scope that will show a decent unfiltered view, then you will either need a very very long focal length or an achromatic setup. (2 lenses)

You will need an eyepiece. You can make one, but it may be much easier and simpler to simply buy a cheap one, as eyepieces are not that easy to make. Cloudynights.com has a classified section on the website where people sell used equipment, so you might be able to find one there for a good price.

Do you know much about telescope optics? For example, the eyepiece is what determines your "zoom" when you use a telescope. If you didn't know that then I suggest searching for various books or guides online that explain the optical system of a telescope. If you have any specific questions feel free to send me a private message or reply here.

jnorman

just assembling a telescope from parts is not much of a science fair project. what is your thesis? what are you trying to learn, or to be able to teach others? try to select a particular aspect of telescopy and explain it with your work, eg, how do lenses work? how do lenses magnify an object? how do various lens configurations affect what you see?

sravet

Hey that's a great idea, explaining why it doesn't focus as well and using the colored foil.

We aren't planning on any zoom, just two fixed lenses. The experiment in his book showed two convex lenses mounted with clay to a yardstick, with the smaller eyepiece lens placed just behind the focal point of the objective. We want to buy a handful of lenses to try different focal lengths and positions to see what happens. What do you consider very very long? The longest I've been able to find is a 1000mm that I was planning to get for the objective.

I studied optics a long time ago on my way to an engineering degree, but basically he has read a couple youth science books on microscopes and telescopes, and I've spent a couple nights reading up on this forum and others on the topic, and that's about the extent of our knowledge. We'll keep reading though. Any pointers you might have to good introductory WWW pages would be great.

thanks,
--steve

sravet

It's a science report, not really a science fair, so lighter weight. I didn't know the above were prerequisites for the questions I posted, but to answer them, the thesis is to design and build a telescope that equals or betters what Galileo used to observe the solar system. Together we are doing the research that will enable him to create the design, then we will buy the parts to implement it. We will experiment with different focal lengths and lens types to see what's better or worse. Along the way as we discover effects like chromatic aberration we will note and explain them.

thanks,
--steve

Drakkith

What is the diameter of the objective? If it's fairly small 1000 mm might be ok. It really depends on how much abberation you can take and still be satisfied with it. Also, you will have zoom with any setup, otherwise it's not working correctly.

A quick google search for "building a telescope" revealed multiple sites that seem to be good. Here's a link to one.
http://www.funsci.com/fun3_en/tele/tele.htm

Also, you might be able to find some objectives and various other parts at this site for a good price: http://www.surplusshed.com/new.html

vociferous

You can see the moons of Jupiter just fine with a cheap pair of binoculars.

If you are just trying to replicate Galileo's telescope, the quality was very poor by today's standard, so even some cheap lenses mounted in a cardboard tube would probably give you better quality than what he had (although the chromatic distortion will be horrible compared to "professional" refractors and modern reflectors).

If you remember back to the Newtonian optics you learned as an engineer, you should have enough knowledge to construct the thing to suit your purpose (the lensmaker's equation is where you want to start).

http://galileo.rice.edu/lib/student_...pe_design.html


I would suggest using the cheapest lenses you can find and not worrying at all about color.

Once you have done that, and if you are ready for more of a challenge, building a Newtonian reflector is a great summer project. Unlike Galileo's telescope, you can build a pretty inexpensive Newtonian which is nearly as capable as a professionally manufactured telescopes costing thousands of dollars.