This normally occurs at the plane of the aperture mask.

This works particularly well using aperture masks.

On December 4, 1984 it became the first telescope to make optical closure phase measurements on an astronomical source using an aperture mask.

When a film is properly projected, the frame lines should not be visible to the audience and are typically cropped out in projection with an aperture mask.

The aperture mask removes atmospheric noise from these measurements, allowing the bispectrum to be measured more quickly than for an un-masked aperture.

In these, the film captures additional information that is masked out during projection using an aperture mask in the projector gate, and is known as soft matte.

If a non-redundant aperture mask is used the bispectrum can be calculated without any noise contribution from the atmosphere (but still with a photon-shot noise error contribution).

However, the way the aspect ratio is created with these films is not in-camera, but rather by placing a cropping device, known as an aperture mask, over the film in the projector.

Higher energy X-ray and Gamma-ray telescopes refrain from focusing completely and use coded aperture masks: the patterns of the shadow the mask creates can be reconstructed to form an image.

Due to differences in the camera gate aperture and projection aperture mask sizes for anamorphic films, however, the image dimensions used for anamorphic film vary from flat (spherical) counterparts.

For simplicity the aperture masks are usually either placed in front of the secondary mirror (e.g. Tuthill et al. (2000)) or placed in a re-imaged aperture plane as shown in Figure 1.

These frames are then cropped in the projector by means of aperture masks used in the projector's gate in conjunction with a wider lens than would be used for projecting Academy ratio films.

Although the signal-to-noise of speckle masking observations at high light level can be improved with aperture masks, the faintest limiting magnitude cannot be significantly improved for photon-noise limited detectors (see Buscher & Haniff (1993)).

The pinhole camera is the most basic form of such a modulation imager, but a single aperture mask can contain many holes, in one of several particular patterns, to improve the throughput for hard X-rays and γ-rays, for example.