In urban environments, local lights shining on your telescope can be an even bigger problem than the washed-out skies. Any stray light that finds its way into your telescope tube (or more precisely, into your eyepiece) is a contrast killer, making it even harder to see anything but the brightest objects. While there is little you can do to block general light pollution, you can keep stray light from entering your telescope with a simple tube extension that serves as a light shield.
A tube extension light shield is simply a tube that extends the front of your telescope, making it harder for off-axis light to get into your telescope’s light path. These light shields are particularly effective on Newtonian telescopes because the focuser is too close to the end of the tube on many Newts (especially on many commercial models). Dew shields (such as those commonly used on Cassegrains) also make effective light shields. You can buy dew shields/light shields from various vendors. But you can also make one pretty easily for under $5. All you need is need is some black poster board paper, white glue, and packing tape. It may sound flimsy, but it only needs to stop photons and be stiff enough keep its shape. Here is how I made one for my 10 inch (250mm) Dob …
The basic idea is to make a cylinder of black poster board paper that fits snuggly onto the outside of the front end of the telescope tube. You make the cylinder by rolling up an appropriately sized rectangle of poster paper.
Make a rectangle of black poster board paper
Step one is to acquire or fabricate a rectangle of black poster board paper of the appropriate size. The end ring of my telescope tube has an outer circumference of 38.5 inches (~97.5 cm). Allowing some excess for gluing, I needed a rectangle of poster paper about 42 inches (~108 cm) wide. The common wisdom is that the front of the telescope tube should extend a full tube diameter beyond the focuser. So for my telescope that meant the rectangle should be at least 12 inches long.
At the local arts and crafts store, I found sheets of 22 x 28 inch poster board paper. I glued two of these together to make a 42 inch wide sheet. I used ordinary white glue to join the sheets. Don’t make the overlap too big and apply the glue evenly but sparingly. Otherwise the overlapped section will be rather stiff and hard to shape into an arc of a cylinder (I overlapped about 3 inches). Be sure to join the two sheets squarely. After the seam dried, I overlaid the each side of the seam with packing tape. That was probably unnecessary, but I wanted to make sure the seam didn’t peel apart when I shaped it into a tube.
Shape the rectangle into a cylinder
After gluing, I had a 42 x 22 inch sheet. I could have trimmed it down to 42 x 12 inches, but I left it 22 inches long—after all, the extra length doesn’t hurt. I then shaped the big sheet into a cylinder and glued it. To hold it while the glue set, I used clothes pins to clamp the top and bottom of the seam, added a little tape in the middle, and then laid the cylinder, seam side down, on the floor with a couple of liter bottles of water inside the cylinder on top of the seam to keep pressure on it. Once the seam was dry I again taped it on both sides. To keep the cylinder ends from fraying, I also taped the rims all around. It was almost finished—almost, but not quite.
Fit the tube to the telescope
The final step was to try it on for fit. My intention had been to have the extension tube fit snuggly around the top end ring of my telescope and have it rest against the four "nuts" that anchor the secondary vanes. Unfortunately, the fit was a bit too loose. To make it a snug fit, I originally planned to line the inside bottom area of the cylinder with foam. But then I realized that the fit was loose enough that I could actually wiggle the extension tube over the tops of the four spider vane nuts. That made for a really snug fit and locked the light shield in place very tightly. To keep the nuts from eating into the poster board, I put some tape strips on the inside of the cylinder at the four spots where the nuts rub against the light shield. The smooth tape surface also makes it easier to slide the light shield on and off.
Finally, I marked the outside of the cylinder with white and red crayon (borrowed from my daughter) so that a red flash-light will highlight the taped areas on the light shield. This way even in the dark I can easily align the taped areas with the four nuts. The crayon markings are clearly visible in the photo below showing the light shroud fully seated on the telescope.
Performance
The light shield works pretty well. It’s easy to put on and take off. It is stiff enough to keep its shape: the front end does deviate a bit from circular, but not enough to interfere with the light path. If it gives me problems in the future, I’ll add a reinforcing hoop of somewhat thicker card-stock paper. Most importantly, it is quite effective at keeping out stray light. It’s easy to compare views by simply putting on or taking off the light shield. Observing from my yard the difference is quite noticeable, although it does depend on where I point the telescope. For example, with the light shield I can look at objects relatively close to my nearest street light; without the shield I can’t point in that general area without a big increase in background brightness, as well as various ghosts and glare spikes.
A separate question is how well the paper light shield holds up to repeated use. So far it has survived nearly a year of regular use. On nights with heavy dewing, the light shield becomes noticeably damp and somewhat limp, but so far not limp enough to intrude on the light path. It seems fine after drying out, but I expect at some point it won’t recover, or it might even tear when I try to remove it. Whenever that happens, I won’t hesitate to throw it out and make a new one. At $5, the light shield is “expendable”.
If you really are a fanatic, you could also flock the inside of the light shield. But I don’t think it is worth the trouble and cost: the shield works primarily by blocking external light from entering the telescope tube. While black poster board isn’t the darkest black I’ve ever seen, as long as you flock any areas directly visible through the focuser tube you’re covered. Yes, that’s really all you need to flock to get essentially all the benefits of flocking. But that’s a topic for a future article.