# Can I build a telescope with a lens from a CD-ROM as eyepiece?

I saw some article on the web about making an eyepiece of a telescope with the lens from a CD-ROM. Will this really work? I have an objective lens of 100 cm focal length. What will be the magnification if I could make a refractive telescope? Can I see the planets and moon with that? Can anyone suggest the arrangement of the lenses?

– user1569
Feb 2 '18 at 8:41
• You can make a telescope out of just about any 2 lenses, but the magnification, resolution, and ease of use will vary significantly. I'd recommend finding a much larger diameter, cheap, eyepiece lens. Feb 2 '18 at 14:17

This article gives a construction guide for how to make an eyepiece using a CD ROM player lens: How to make an eyepiece for a telescope.

As to what the focal length of your eyepiece will be, you can refer to this paper, in particular Figure 1 from which I derived the following relation:

$f=0.73r$

where $f$ is the focal length of your lens, and $r$ its radius (this is a rough approximation, of course). So given the fact that a typical CD drive lens is about 5mm in radius, its focal length will be only a few millimeters.

Now, assuming the focal length of your telescope is 1000mm, the magnification will be a few hundred times. What you will be able to see with that depends on the width of your objective lens and how well you will have built your eyepiece, but you should probably be able to see the Moon with your custom eyepiece. As for the planets, it will be hard to get something good given how much you are magnifying—not impossible, but be aware that magnifying several hundred times requires a good weather, a good telescope and a good eyepiece.

• Thanks for your replay Romain. But i am getting a lot of aberration while looking through this tiny lens. What is the minimum magnification required for seeing planets....
– Sans
Feb 2 '18 at 6:39
• It depends on what you want to see. For Jupiter and Saturn, you can start seeing a few details (the rings of Saturn, some stripes on Jupiter) with a magnification of only a few dozen times: a lens like yours from a CD drive is a bit "overkill" for that purpose. If you want to make your own eyepiece, I recommend buying a couple of cheap convex lenses, you'll get much better results than with your CD drive lens. There are a lot of guides on the subject, like dma1.org/~wagner/eyepiece.htm Feb 2 '18 at 7:15
• @Sans Simple lenses will always have aberrations. Those from the objective lens can be reduced by stopping it down (put a ring of paper in front to reduce the effective diameter). With the eyepiece you could try the same, but if the lens is already tiny then it won't work. Also, reducing the diameter doesn't eliminate all aberrations, only some. Avoid being fixated too much on magnification; more is not always better; this is the mistake that all beginners make. Instead, find the right amount of magnification that works in each case for your instrument. Feb 2 '18 at 19:41

Eyepieces are carefully designed. They try to achieve a sharp image with good contrast over a wide field of view. Even the simplest eyepieces of type Huygens consist of two lenses that match up. Good eyepieces can have 7 or more lenses.

In the end every eyepiece is a fancy magnifying glass, so your CD ROM lens will work. It will just not make for a good image. It'll be much worse than the 10/25mm Plössl eyepieces that so many sell for cheap or sometimes even give away. They sell them for cheap because even though they are much better than any single lens, they are still on the lower end of the quality spectrum.

Another problem is: the CD ROM lens has a tiny focal length which will result in very high magnification and a very dark image. A rule of thumb says: twice your mirror diameter in mm is about the useful upper limit for magnification. If you have an 8" reflector, that would be 200mm x 2 = 400x.

Nonetheless an interesting experiment. You can try to put a cardboard ring in front of your telescope to give it a longer focal ratio. It might help a little while costing a lot of light and resolution.

You can see 5 planets without any magnification. To see details, I'd say the fun starts at around 150-200x even though they appear as a disc at much lower magnification.