Larger telescopes will need to be fitted with an aperture mask
with an opening not exceeding 75 mm; too large an aperture will lead to an excessive buildup of heat, especially at the eyepiece.
David Brady and his colleagues avoided that obstacle by combining a "coded" entrance aperture mask
with a high-resolution CCD detector to produce a high-throughput, high-resolution spectrometer with no moving parts.
A hexagonal coded aperture mask
is located 1.7 m above the detection plane in order to image a 16[degrees] section of the sky with 2[degrees] angular resolution.
The left photo was taken with the lens set to f/5.6 using the internal diaphragm, while the other was taken with the diaphragm fully open but with the lens stopped down to f/6 using the test aperture mask
. It worked just as well as I'd hoped--the second photo appears to have been shot with a small refractor, rather than with a camera lens.
A thin steel aperture mask
on the front includes precision-cut pinholes measuring 50, 100, 150, 200, and 250 microns that will guarantee you'll have the perfect "star" to adjust your optics with.
The breakthrough came when some unknown person did something counterintuitive: instead of using the entire objective lens, he put an aperture mask
on it that blocked the outer parts of the glass so that only the small central portion let light through.
If you own a 10-inch or larger Newtonian reflector, you can simulate this scope with a circular aperture mask
placed to avoid the secondary mirror and its supports.
I was particularly impressed with a low-tech focusing aid that consists of two triangles cut into an aperture mask
, which gives the effect of diffraction spikes when the image is focused (see page 112).
With special optics they essentially cover the 10-meter primary mirror with an aperture mask
containing 36 small holes.
Most often I would wind up using full aperture because, even on nights of substandard seeing, there would be occasional moments of steadiness, sometimes lasting only a fraction of a second, in which the scope would show detail that simply could not be seen with the aperture mask
These include the Tranquillity's Gallery, a design inspired by the X-Ray coded aperture masks
used on the British Skylark space rocket.
See his article on aperture masks
on page 66 of this issue.