Modified Hartmann Mask

This is the modified Hartmann mask that I use for fucusing my 8" SCT telecope.

I borrowed this idead from Ron Wodaski's book The New CCD Astronomy.

As you can see the shape of the openings in the mask are two opposing triangles. The use of triangles produces difraction spikes that facilitate the focusing process.

Image Out of Focus

The scope is pointed to the star Vega in the constellation Lyra. The scope is way out of focus, so the star "splits" in two triangles.

The frames in this secuence were captured at 5fps with a SC 3.2 modified Toucam Pro webcam.

Overlapping Triangles

As focusing starts the triangles come close to each other, the difraction spikes become thinner and longer.

Almost There

The triangles overlap almost completely.This image shows the advantages of the triangle shapes used in the mask. It is difficult to ascertain whether the triangles overlap completely. However the double difraction spikes show that we are not yet there.

Dim stars north of Vega (North is up) appear blurred.

Proper Focus Achieved

Clear long radial spikes are the signature of sharp focus. The dim stars above vega are now visible.

Mask Removed

Once the mask is removed we can see also the stars in the vicinity of Vega.

The Final Proof

A five seconds long exposure shows a difraction pattern consistent with good focus.