A Scheiner Disk is a mask with two holes, whereas a Hartman mask has many holes (typically three).
They require that you target is a point of light, such as a star or distant planet, therefore are no good for day-to-day use.
The mask is like a lens cap with holes in. The holes are positioned in such a way that when out of focus, they cause multiple diffraction images as shown in the first image. As the lens is focused they merge into a single point, and then beyond focus, they separate again.
The masks can use either a circular hole or a triangular hole. The triangular hole creates diffraction spikes which are easier to detect when in focus, but the circular holes will allow more light to enter the lens resulting in a clearer image. Once your telescope is in focus, the mask can be removed.
I haven't been in astronomy very long, but a Hartmann mask is one of the most useful accessories that I have used. It makes focusing so much easier and sharp images can be had in seconds. What makes it even better is that you can make one for free!
How to Make a Hartmann Mask
My mask is constructed from an old Amazon box. I tacked down the printed template with sticky tape and I cut out around the perimeter, leaving an approx 2cm gap around the edge. This is to allow for the rim to be created.
Next I cut out the three triangles with a Stanley knife, and then cut slits into the 2cm border and fold them up. I taped these up with Duck Tape to allow the mask to slot over the telescope aperture.
I spray painted the bask matte black to avoid any reflections caused by the shiny Duck tape which may create interference which may affect focusing.
Finally I used some 3M double sided tape to stick foam pads to the inside edge to keep the mask in place on the telescope.
Refinements to the Hartmann Mask Design
You could have a mixture of circles for light gathering and triangles for diffraction. I imagine this would create an interesting pattern and I will try it some time.
How to Use a Hartmann Mask
To quickly get the best possible focus, follow these steps to use your new Hartmann Mask.
- Set your camera ISO or gain high and turn on live view.
- Aim your telescope at a star. If you cannot see stars in the live view screen and everything is connected properly, it could be that the ISO or gain needs increasing further, the telescope is so out of focus they are totally blurred - try spinning the focusser to see if anything becomes visible. It could also be that the star you are pointing to isn't bright enough. Not much light gets through the mask so be sure to focus using a very bright star such as Vega, Sirius or Arcturus.
- You will see three blurry circles. Adjust focus until these merge into one single point of light.
- Use shutter priority or bulb mode to capture the point of light. Live view is good, but for the most accurate focusing its always best to try some test images.
- Repeat previous two steps until you have perfect focus. Perfect focus is when zooming into the point of light on the test photo, it is one single point, not three close together.
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