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Introduction

There are lots of spectroscope designs on the web but most involve dissecting a CD or DVD to get a transmission grating. I wanted to build one into which one could just drop a standard DVD and get it to work. I also wanted (ideally) a direct-viewing instrument, so that the light path enters and leaves the instrument in the same direction. This makes it much easier to point at a light source during use.

The design shown comprises four parts, all printed in black PLA with 0.2 mm layer height and 20% infill:

Main body
Central baffle (but see below)
Eye tube
Eye cup

If you want the direct viewing option you will need to print the following component instead of the Central baffle:

Central baffle with slit

and you will also need a 12.5 x 12.5 x 2 mm mirror (rear-surface silvering works fine). These are available very cheaply from craft shops.

Indirect viewing instrument

For the basic (indirect view) instrument, print the main body and ensure that the slit doesn't contain any printing debris. Print the central baffle and press it into the shallow receptacle on the main body, with the locating disc uppermost. Print the eye tube and clean out any debris. Then push it onto the tapered end of the main body (orientation is hopefully obvious). It should click into place. Finally print and fit the eye-cup on top, which is a snug fit (deburring might be needed).

Take a clean DVD and fit it to the main body with the data side facing the same. You will need to bend the DVD so that it snaps under the clips at either end of the main body. Then release the DVD so that the central hole locates on the raised disc in the central baffle. Light enters the instrument almost parallel to the DVD.

Direct viewing instrument

Strictly speaking the light path in this configuration follows a dog-leg, but the input and output rays are parallel. The mirror pushes into the oblique slot in the main body, blocking the indirect light path. Print the central baffle with the slit assembly. This clips onto the main body as in the indirect viewing instrument, but the oblique tube with the slit at the end should be parallel to the eye tube as shown. This makes it a bit trickier to install the DVD but it's not too bad. The direct viewing version works quite well, but there is possibly a bit of vignetting by the narrow aperture which passes through the hole in the centre of the DVD. It's hard to avoid this.

General observations

Whichever version you print, you can improve the resolution by reducing the slit width. This is easily done by putting a small piece of aluminium tape along the length of the slit. The price is a reduction in transmitted light. The DVD has 1350 lines / mm and can resolve the mercury line at 577 and 579. The Fraunhofer lines in daylight are easily visible (but DON'T view the sun through this instrument - it wasn't designed for it, there's no protection and you may damage your eyes). The first diffraction order is typically just off normal (and doesn't change much with respect to incident angle), so when there's a lot of ambient light you might see an annoying reflection from your eyeball at the red end of the spectrum.

I've attached a picture of a fluorescent tube spectrum taken through the direct viewing variant, although it's not optimised for photography so the lines are a little blurred and overexposed. It gives you some idea of what you can expect from this device.

When you get bored with it, put your DVD back in its box on the shelf!

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