DIY Focusing Mask

This DIY is one of my easiest to make and is very rewarding, however if you find that you simply ‘don’t get it’, then fill out the form here, and you will be able to purchase a purpose built mask specifically for your telescope, for a very reasonable price. For those willing to try it yourselves, read on and enjoy.

Materials - Any kind of thin flat board including thick card, craftwood (MDF), plywood, Perspex, plastic or even aluminium.

Tools (For anything other than aluminium) - Pencil, ruler, compass, drill, jigsaw and sandpaper.

Click any image for a larger view

Step 1.

Measure your telescope's aperture. The mask will need to fit neatly inside the front of your telescope tube.

If you have a secondary mirror in the middle, you will need to measure this also.

If you have a corrector plate (Glass in the front) or an objective lens, this will stop the mask from sliding all the way down the tube. If you have an open tube at the front, you can install small tabs on the sides of the mask, or make a rim to fit over the outside of your tube.

Using a compass and pencil, mark a circle the same diameter as the inside of your tube.

Step 2.

Your triangles cannot go all the way to the outside of the mask, so mark a second circle inside the first. My telescope has a corrector plate held in place by an inner ring, so I used that as my measurement. If you don't have this then ten millimetres in should be enough.

Step 3.

Using the same radius that you used for the second ring, mark six arcs that intersect in the centre and on the end of each other where they cross the inner circle. This should form and even six-petal flower.

Step 4.

Using a ruler, mark six lines from the petal points, to form a six-sided star, with equilateral triangles at the point of each one.

Step 5.

If you have a secondary mirror protruding from the corrector plate, then a hole needs to be cut out to accommodate it. Once again, using the compass and pencil, mark a circle in the centre for this.

Step 6.

Using a drill, put a hole in each piece that is to be cut out, just off the lines to be cut, and big enough to get the jigsaw blade into.

When you insert the jigsaw, make sure it is not running until you have the tool bit all the way into the hole and the mask is firmly supported on a flat, sturdy surface. Remember to cut very slowly and unless you are experienced, cut just inside the line by one or two millimetres. You can use a file later to file down to the line accurately.

Check the mask on your telescope for fit and file to adjust as necessary. Clean the whole thing up with a bit of sandpaper.

Step 7.

Paint or lacquer whatever finish you like. I would suggest a matte finish on the inside and inside edges of the cut hole to avoid any reflected light causing anomalies in your focussing.

Using the Focusing Mask.

Place the mask over your telescope and point it at a bright star. If it is out of focus, you will see three images of anything in the field of view (FOV). Turn your focus knob until the three images become one. This is a fun thing to watch if you do it with three Saturn's or Jupiter's!

You will also note that there are six sharp light spikes coming from the star. try to make these spikes clear and sharp. You are now perfectly focused. Happy observing/imaging.

If you have any questions or problems please do so here.