Introduction: Telescope Spectroscope, or How to Know What Is a Star Composed Of

In this first instructable I show you how to make a spectroscope that can be adapted to your telescope. But the first question is : what is a spectroscope, and what is it useful for? Good question.

The basic principle is very simple : when a star burns its fuel, it emits a certain color of light, and when it emits a certain number of wavelength, the light seems to become white. But with a spectroscope, you can divide this light. And so you'll be able to know what is a star composed of! Personally, I think it brings your observations to a next level.

Let's make it!

PS: every picture used in this instructables are my own

Step 1: Which Method for Light Splitting?

To divide light, you have two options :

-First, you can use a prism. It uses the principe of refraction, as different colors are not refracted the same angle, when white light is passed through a prism, it is split into its basic colors, and the final image can be seen on a screen. But the major problem with this type of method is that it requires a certain amount of light, otherwise the light on the screen will not be enough powerful to be seen.

-The method we will use is the method of grating. When a light passes through such component, it is diffracted, and divided into its basic colors. You can find diffracting grading on the internet, but you can find something which is more conventional, because it can be taken in photo more easily : it simply... a slice of DVD. We will make our own grating from a DVD.

Step 2: Cut Your Dvd

-First, take and old DVD. You can check if its diffracting is fine by exposing it into light, if you see different colors appearing into the DVD, it will be fine.

-Then you have to cut a chunk into it, but according to a certain pattern, it to be able to diffract correctly, you can see it on the picture, you have to cut on the red line. You should have a result like the second image. BE VERY CAREFUL NOT TO SCRATCH IT OR DAMAGE THE SILVER PLATING, OTHERWISE IT WILL NOT WORK.

-It is very important to cut the DVD according to its radius, because it is the very thin lines on the DVD that will allow it to diffract the light, and they have to be perpendicular to the direction of the light. In the end, you should get a rectangle with dimensions of 15 mm wide and 35 mm long.

Step 3: Build Your Box

The box is the where the chunk of DVD will be put. It is important to have a great build to get nice images. You can choose to make it different ways, the most simple and the one which gives best results is to 3D print it, but you can also make it using thin wood, but you have to be enough precise, not any light to enter it. You can see the dimension of the box on the picture, and you can download the .STL file. You can also find the PDF file with all the dimension.

I chose personally to make it using thin wood.

Step 4: Adjust the DVD Slide in the Box

It is important to adjust well the slide in the box, with the right angle. According to my experiment, the best angle is 40°, and the beam of the telescope light must be a the very beginning of the slide. You can check on the picture how it must be adjusted.

You can put a long screw into one of the two bottom holes, to get maximum precision.

Step 5: Test the Angle of the Slide

In this part we will test if the slide can separate well the light, and if the angle of this slide is the right one.

To do so, there is a simple experiment : you simply light the hole of the box with a LED light, like shown on the picture.

Simply check that when your eyes are at the vertical of the box, you get something which looks like the second picture.

The light mustn't be too powerful, otherwise you'll not be able to see the different colors of the LED light.

If you don't see any colored light, simply change the angle of the slide until it works.

Step 6: Cut the Box Adapter

According to my experiment, the easiest way to connect the box and the eyepiece is to use soda can metal. As it's easy to shape this metal, we will use it to make a sort of tube which will adapt onto the eyepiece of the telescope. Measurements shown in the picture will only work with 1"25 inch eyepiece.

The easiest way to cut it is to print the pattern (pdf file) and then stick it into a flat metal piece, it to be convenient to cut.

You have to cut through every line, and once it is done, ply it to get a result like the first picture.

Once it's done, use tape to secure the adapter into its final shape, like shown on the third picture.

Step 7: Set the Telescope Adapter Into the Box

in this step you will glue the metal piece you've made in the previous step into the top of the box.

The easiest way to do it is to use hot glue.


To glue it easily, the best way is to put hot glue in places as shown ont the second picture.

Once it's done, you should have a gap between the bottom of the box and the adapte. To solve it, just cut another piece of metal and glue it using hot glue.

After making all this, you should get a piece like shown on the picture. You can then adapt it onto your telescope eyepiece, it should just go onto it. If the adapter is too big, just put small chunk of paper between the inside of the adapter and the outside of the eyepiece.

Step 8: How to Use It

Well done, you've made this spectroscope, but of you don't know how to use it...

I think the best way to use it is to use your phone camera. You can secure it into the box using a strong rubber band. You will have to lock the auto-focus of you camera, and take it into long exposure (you have plenty of tutorials on the internet of how to do it). It's not a problem if the photo is very blurry, what matters is the color.

To make a spectroscopy of a star(if it's very bright it will work very well), point the star with your telescope like if you were making an observation, then plug the adapter onto the eyepiece, then adjust the focus of the lens until you get the colors split into the DVD slice.

(The illustrating image is the spectroscopy of a lighter flame 20 meters away, which lightness is comparable to a magnitude of 0, I can't make the spectroscopy of a real star because there's too much light pollution in my town).

Step 9: Analyse the Image

By taking the previous picture, you can analyse it using a simple software such as and excel. For the example, we'll take the spectroscopy of a LED, it is the most simple way to analyse it.

-First, take the tool which allow you to know to color of a pixel

-Secondly, open the color tab, then click on the beginning of the spectrum to collect its color, then write down on a paper its RGB code.

-Then, go into this website : . On it, you'll find a label, write down the code of your color : rgb("Rvalue", "Gvalue", "Bvalue"). Then choose on the color picker the color which is the nearest from the original color.

-Each chunk of color has a number, as shown on the second picture, it goes greater an greater from the left to the right and from the top to the bottom. Then download the excel file, and once you've found a color, replace the 0 beside the color number by a 1, and you should see the graph evolution. Once it's completely done, congrats, you've got your spectrum!

Step 10: Analyse the Chemical Elements of a Star

You will get different areas on your graph: when the intensity is high, take the color and search on the internet its wavelength.

You can easily find on the internet emission spectrum of different elements. You can compare them with wavelengths you found, and so you'll be able to know what elements the star's composed of.

Normally, you should find that it is composed of a lot of hydrogen and helium.

I hope you enjoyed this instructables, I think spectroscopy brings your observations to a next level, more scientific one. Spectroscopy can also be used to know what a mixture is composed of, but at this time, it's not astronomy any more, it's chemistry. Of course, this type of adapter can also be used with self-made astronomical observation devices, you can find plenty of them on the instructables website.

PS : the picture is the spectrum of emission of helium.

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