(This was a recent project done between plumbing and building an awning.)
A Hartmann Mask is a simple focusing aid used on telescopes and telephoto lenses to aid focusing.
Its main use has been in astrophotography, but it can be used anywhere. It is particularly useful with
cameras without live view, but also to properly focus celestial objects for optical viewing - when no other
means are handy. There are many references to Hartmann Masks, and they can be made in many different
patterns here is one:
The one shown here was recently made to help focus the borrowed MTO 1000/10, a Russian (actual)
f=1100mm mirror lens operating at f/10.
The materials were simple - a washed out Bushells coffee tin, 500 gram size (made of tin ends and lid and
cardboard cylinder). The tools used were almost as simple - a box-cutter knife; a small rigid wooden box
(about 15 x 8 x 6 cm); a computer printer to make a precise template of the mask; a 30mm diameter metal
hole-cutter saw and battery-powered drill; matt black and gloss black spray paint.
The small wooden box was the rigid platform to hold the box-cutter level and in the same place, while the
coffee tin was placed on a table and rotated by hand past the protruding blade. This made a progressively
deepening cut around and through the cardboard cylinder. Two such cuts were made, one to get the
container to the right height and the other to later fashion an inner sleeve. The problem was to use materials
that nearly matched the diameter of the lens and of its extended sliding lens hood. The outside diameters of
the lens hood and coffee tin were similar, as were the inside diameters of the lens hood and coffee tin. The
inner sleeve - used to join the two items - was slit lengthwise to remove a small, measured width of
cardboard. This was then slid firmly into the extended lens hood. The remaining coffee tin then slid firmly
over this resulting sleeve.
The coffee lid - made of tin - was used to form the actual mask, by cutting two circular apertures into it. To
find the optimal design, an on-line formula was used. It can be found here:
To maximise the amount of light entering the system, the two apertures of the mask required the maximum
possible diameters. This came out to 30 mm per aperture. The lid was stamped with the Bushells logo and
its surface was also stepped, so the apertures had to fit inside these features. Ideally - for maximum
resolution - the outer edge of the apertures would have to match the outer edge of the objective mirror of the
lens. In the end, the two measurements were pretty close.
In the construction, the tricky part was using the hole cutter saw and drill, as the pitch of the teeth was
rather coarse for the thin metal of the lid. Ideally, the lid should be firmly sandwiched between to pieces of
plywood, but becuase of the size and different levels of the lid, only a firm baseboard - and much care -
was used. The result was quite successful just the same, but care should be taken with the power tools in
particular. Very low speeds and a very low clutch setting were used.
In between rain periods the resulting mask pieces were painted - matt black on the inside surfaces, and
gloss black on the outside. The finished result can be seen in the images below.
Fig 1. Interior of Hartmann Mask.
Fig 2. Mask mounted on MTO1000 lens
During overcast weather the mask was assembled and attached to the mirror lens and tested on some distant
trees. The results were successful. The out-of-focus (OOF), double image is typical of what is seen before
focus is reached.
Fig 3. Distant tree through mask: Triptych of OOF image, focused image, and image with mask removed.
Today the setup was tested on the sun - USING A PROPER AND SAFE SOLAR FILTER. Again, the
results from the mask were successful. The Baader film ND 5 (100,000X) solar filter is shown attached to
the mirror lens in the following figure.
Fig 4. Baader film solar filter attached to MTO1000 and camera
Three phases of the focusing procedure through the Baader filter are shown in Fig 5, along with
AN UNEXPECTED PROBLEM that occurred when the mask was removed for a normal, full aperture shot of
the sun through the solar filter. A BRIGHT STRAY PATTERN OF LIGHT marred the image.
This was later found to be due to a PINHOLE size break in the filter material, rendering the filter useless in
that state. The filter will be made useful again by blocking the pinhole with a small amount of opaque
material, as this would have negligible effect on the image. It does, however, serve to show the tremendous
filtration required for safe viewing and photography of the sun. The following figure shows the small hole
that caused this effect.
Fig 6. Pinhole break in Baader film filter.
For comparison, an earlier image of the sun using the same equipment is shown below.
Fig 7. May 2013 Partial Solar Eclipse seen from Sydney
An image of the sun through the repaired filter will be posted as soon as possible.