Step by step

Step 1

Measure the external diameter of your tube and use compasses to draw the circle. Cut it out leaving 40mm tabs every 75mm or so. Cut strips of card, about 40mm wide – long enough to go around your tube at least twice (about 7x the diameter).

Step 2

Measure the minor axis of your secondary mirror and use the downloadable calculator (see page 5) to find the largest possible aperture for the mask hole that gives a clear unobstructed aperture. Carefully cut out this hole in the main disc.

Step 3

Use tape to fasten a single layer of strips around the tube. Add the main disc, fold and carefully glue down the tabs to the strip. Add more layers of strips and tape to form a stiff ring and secure the tabs. When the glue is dry, take the mask off the tube.

Step 4

Use card off-cuts to make additional reduced apertures to add to your mask. You can experiment with these by using the downloadable calculator (see Step 2). Paint card parts black if necessary. Label them with a white marker to indicate diameters.

Step 5

To make the card more rigid and provide some resistance to moisture, apply thin coats of lacquer or varnish. Don’t use too much at once or the card may get soggy and distort. Once dry, Velcro dots can be applied to hold additional masks in place.

Step 6

When fitting it to the scope, turn the mask so the incoming light misses any vanes. If your scope has not cooled down it may be advantageous to have the hole in the lowest position, as warmer, swirly air tends to gather at the top of the tube.