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
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 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.
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
For measurements of samples in Reflection, aperture masks
of 30, 8 and 3mm are available.
2%, and BMC, producers of aperture masks
for the television industry, with 7.