Hartmann Mask

Parts Required (for 8" Newtonian reflector)

10" x 10" sheet 6mm Ply
Sticky Velcro
3 x right angle brackets
3 x 15mm screws
3 x Old mastic nozzles
3 x M4 washers

What is a Hartmann mask and how does it work?

A Hartmann Mask is a simple device consisting of a set of holes in an opaque lens cover.

The out of focus images generated by each hole merge when the telescope is in focus. They operate much in the same manner as an optical rangefinder found on rangefinder cameras.

The device goes by two different names, with Hartmann mask describing a mask with multiple holes and Scheiner disk describing a mask with 2 holes. It was apparently invented by Christoph Scheiner in 1619, but is most commonly called a Hartmann mask today.

Build Process

1. Measure the outer diameter for the font of the scope (mine was 9.25 inches approx.)

2. Mark the centre of the 10" x 10" ply

3. Using a compass mark the circumference for both the mirror diameter and the outer diameter of the scope

4. From the centre with a ruler draw a line out to the mirror diameter then using a protractor and a ruler draw 2 more lines out from the centre at 120 to each other.

Draw the circumference

5. Here I have tested different sizes of holes, 50mm and 60mm

Draw 120 lines

6. Once you have worked out what size you want the holes dill a pilot hole in the centre of where the large holes will be.

Hole test

7. Using a hole cutter cut out the holes (64mm used here - 2.5 inches), then using a jig saw cut round the outer diameter line.

Cut out mask

8. Using the screws washer and brackets attach the barckets to the mask.

Brackets Brackets

9. Here I have drilled a 6mm hole in the centre of the mask so that I can shine a light through and adjust the mask to dead centre.


10. Attach the sticky Velcro to the inside of the brackets, I put an extra soft bit facing out to protect the side of the scope.


11. Snip the end of some mastic nozzles to go over the screws to protect your fingers and the scope.

Finger protection

12. The finished mask.


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