Build Your Own Hartmann Mask for Perfect Focus

Crafting Your Own Hartman Mask for Pinpoint Focus

The Hartman mask has become an absolutely essential tool for me, especially for achieving sharp focus and perfect collimation on my Schmidt-Cassegrain telescope. It’s really nothing more than a simple piece of cardboard with a few holes, incredibly easy and super cheap to make. So, I’m excited to show you how to craft your very own Hartman mask!

Here’s what you’ll need:

• A piece of cardboard slightly larger than your telescope’s aperture
• A utility knife (or box cutter)
• A ruler and/or measuring tape
• A compass (for drawing circles)
• A protractor
• A pair of scissors
• A pencil

The Procedure:

• Measure the inner diameter of your telescope’s front opening.
• Using your compass, draw a circle of this diameter onto your cardboard.
• Next, measure the diameter of your telescope’s secondary mirror holder.
• Draw another circle of this diameter on the cardboard, centered within the first.
• Now, draw a straight line from the center of these circles out to the edge of the larger circle.
• Grab your protractor and, starting from your first line, draw a second line at a 120º angle, also extending from the center of the circles to the edge of the larger circle.
• Repeat this step, drawing a third line at 120º from your second line. You should now have a pattern that looks like this:

• Along each of your three 120º lines, mark a point exactly 3cm in from the edge of the largest circle.
• Use these three points as the centers for three new circles, each with a 2cm radius.

That’s it! Your Hartman mask design is fully traced. All that’s left is to carefully cut out the large outer circle with your scissors and then use your utility knife to cut out the three smaller inner circles…