By inserting masks into the pupil plane (P1) (cf. Fig. 1), the pupil function (and thus the transfer function) can be modified as desired. For instance the spider diffraction pattern can be removed by placing a Lyot mask or a four-hole mask (see §3.3), and telescope arrays can be simulated by placing a mask with appropriately located holes.
Pupil masks allow to select a sub-sample of spatial frequencies and more accurately measure the corresponding complex visibilities since they will be less attenuated (as the overlap of the fringes is lower); hence, a better use of the maximum number of photons set by the detector. The price to pay is to perform an interpolation in the Fourier space (aperture synthesis, see methods in §5.1) and complementary observations to make the process more robust.
The main drawbacks are a lower limiting magnitude in the case of small holes and a more dilute uv-coverage.
Some pupil masks have been made by drilling holes into a 5-cm metallic disk, according to the complementary networks from Golay (1971). They are displayed in Fig. 6 and the corresponding (u,v) coverage in Fig. 7. A successful image of HR8652 was restored using these masks and the aperture synthesis method from Lannes (1989, 1991) and Anterrieu (1992) (Cf. paper in preparation).
Figure 6: Set of Golay pupil masks available in PISCO.
Figure: Corresponding (u,v) coverage with
the 3 masks of Fig. 6.