Telescope Focus Mask for Perfect Focus

In this Instructable we are going to make a Hartmann Mask that helps you focus your telescope. For a couple things to start with:

1: Is this the best focus mask for a telescope? No, The Bahtinov Mask is a much better design that allows for more accurate fine focusing and more light to hit the mirror. But is it more complicated to make but still on the easy to do scale.

2: So then this is the easiest focus mask you can make then? Again no, the simplest one you can make would be a mask with 2 small circles at 180 degrees apart from each other.

3: So why this style? This is because it has a mix of both worlds. You can make this typically with items you already have (ruler, compass and a piece of cardboard). And you get nice little diffraction lines that allow you to get a really nice fine focus that is very close to what you can get with a Bahtinov Mask.

So the idea here has to do with the human sight system.

1: We can see motion better than we can see still objects.

2: We are surprisingly good at detecting symmetry.

When we use this mask if the focus is off we will see 3 triangles, We we adjust the focus the triangles will move closer to each other until finally they will meet in the middle and create a star with 6 evenly spaced spikes or diffraction lines pointing off.

This is typically been used for astrophotography to get a really fine perfect focus before taking a really long exposure but I have found that this also helps with visual astronomy as well. If you are slightly out of focus you will see less stars and details because not all of the light the telescope receives is making it into your eyes.

Enough talk lets make

What you will need

A ruler

A compass

A piece of cardboard that is larger than your telescopes tube.

Some way of safely cutting the cardboard. I used an x-acto knife.

First measure your telescopes tube diameter. This is the outer edge diameter of what we call the optical tube assembly or OTA.

Second take that measurement and divide it by 2 to get our radius and set your compass to the radius and draw a circle on the cardboard.

Next keeping the compass at the same setting we need to make 6 arcs on the circle. The arcs should start from the edge and pass through the center of the circle and end at the other edge. Where on the circle you start the first arc is not important but the center of the next arc will be where the first arc crosses the circle edge. This will make a 6 peddle flower like pattern in the circle.

Then we will take 3 of the evenly spaced peddles and draw a line from the center of the to the edge splitting the peddles. This will make 3 lines spaced 120 degrees apart of each other.

Take a measurement of any obstruction you may have in the center of your scope. Again divide by 2 to get your radius and then draw a circle in center of the larger circle.

Now take the diameter of your primary optic or mirror and again divide by 2 for our radius and with your compass put a mark crossing your 3 lines. This will be the apex of the triangles.

You now have a circle with 3 evenly spaced lines with a cross line. Now connect the 3 crossed lines to make a large triangle.

This part is a little less scientific but since we are going to be cutting out 3 small triangles from this we want the triangles to be smaller than the center obstruction (if any), big enough to let the largest amount of light through but not too big that it will make your mask fall apart after a couple uses. For my 114mm scope I choose a top to bottom hight of my triangle of 35mm. From the top of the triangle measure your hight and place a mark. Then connect the bottom sides of the triangle to each other connecting through the center mark you made. You should have 3 triangles (a tri-triangle?) that are evenly spaced and rotated.

Now cut out your large circle and the 3 triangles and we are almost done.

The last thing to do is to make a way to hold it to the telescope. I have seen a lot of the masks with a ring going around the whole circle but for me I find its much easier to put on and take off without shaking or moving the scope if I just put on a small strip that will just hang lightly via gravity. For mine I did a 6 inch long by 1 inch wide strip with 3 lips that I could glue to the mask. Now just cut, glue and wait for it to dry.

Congratulations your focusing mask is now complete and ready for the night skies!

Unfortunately I do not have a camera showing what the eyepiece and mask will show you as you get into focus but here. Special thanks to the iceinspace.com.au for the original idea and plans that I have modified and simplified here.

Using this is very simple:

1: Point your telescope to a bright star.

2: Adjust the focuser on your telescope until the 3 triangles start moving closer to each other.

3: When the triangles converge in the center and the 6 spikes are evenly spaced take off the mask.

4: Enjoy the universe in focus.

5: Repeat as necessary as the telescope changes temperature or the air temperature changes as the night goes on.