Flintstone Stargazing

September 20, 2007

Making a Hartmann Mask

Filed under: astronomy, Equipment — Ed @ 6:37 pm

Back on the 8th of September, I mentioned that I had made a Hartmann Mask to aid in focusing. I did this at the suggestion of Andrew who had pointed me to these instructions. Here’s how I made it:

Using the telescope cover to trace the circle for a Hartmann Mask

First, I got a piece of cardboard and traced a circle around my telescope end cap. I then used a utility knife to cut around this circle. After cutting around this circle, I cut two smaller circles on opposite sides of the disc I had made. Next I created two long straight pieces that together were longer than the circumference and taped them together.

The pieces of the Hartmann Mask

Next, I taped the long piece around the disk and covered it in tape to attach everything firmly and to make it look stylish. This is the result:

A finished Hartmann Mask
A finished Hartmann Mask

The Hartmann Mask on my telescope
The Hartmann Mask on my telescope

Once I’ve got it attached, I will get an object in focus in the telescope and then put the mask on the top of the telescope (this works best with brighter objects). The view will dim noticably (I’m covering most of the aperature) and if it’s not quite in focus, I’ll see two copies of each star or planet. I then adjust the focuser slightly until the two images merge into one.

Hartmann mask focus example
Hartmann Mask effect viewed through the telescope
Out of focus (left), In focus (right)

That’s it! This is a significant boon to focusing when using the 35mm DSLR since it’s very hard to see through the viewer, but also can help with webcams and astrocams (like my Deep Sky Imager).

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