Figure 1.
An individual
speckle frame
(top left), the
integrated image
of 1,000 speckle
frames (top right),
the Fourier power
spectrum (bottom
left), and the
resulting recon-
structed diffrac-
tion-limited image
(bottom right).
Adapted from
Scott and Howell
(2018).
Figure 2.
The design of Zorro. A
pickoff mirror deflects
the light coming from
the tertiary mirror, redi-
recting it into Zorro.
Inside Zorro, the light is
split by a dichroic into
red and blue channels to
their respective cameras
equipped with electron-
multiplying CCDs.
Yet another solution, far less expensive than
the latter and easily implemented at optical
wavelengths, is speckle interferometry. First
proposed by French astronomer Antoine
Labeyrie in 1970, speckle interferometry is
based on the idea that atmospheric turbu-
lence can be “frozen” when obtaining very
short exposures. In these short exposures,
stars look like a collection of little spots, or
speckles (Figure 1), where each of these
speckles has the size of the telescope’s dif-
fraction limit. When taking many exposures,
and using a clever mathematical approach,
these speckles can be reconstructed to form
the true image of the source, removing the
effect of atmospheric turbulence.
One instrument capable of doing speckle
interferometry is the Differential Speckle
Survey Instrument (DSSI, Horch et al., 2009),
which visited Gemini North and South on
multiple occasions since 2012. Visiting in-
struments expand the capabilities of what
the facility instruments can offer, but come
with a significant burden in logistics: permis-
sions must be obtained, agreements signed,
the equipment shipped, a dedicated crew of
people must travel, some facility instrument
must be removed, and finally the visiting in-
strument must go through testing and com-
missioning. Is there another viable solution?
In other words, is it possible to make the visi-
tor feel truly at home?
Enter Zorro!
Zorro (and its sibling ‘Alopeke at Gemini
North) is a new dual-channel, dual-plate-
scale (field of view) speckle interferometer
permanently mounted on Gemini South.
In simpler words, Zorro can obtain two
diffraction-limited images with different
filters simultaneously. Besides the speckle
mode (which gives a field of view of only a
few arcseconds), Zorro also has a wide-field
mode with a field of around 1 arcminute. The
speckle mode reaches the diffraction limit of
Gemini (15 miliarcseconds at 500 nanome-
ters), while the wide-field delivers an image
quality between the diffraction limit and the
natural seeing. Limited testing has shown
images with an image quality of around 0.15
arcsecond.
Zorro (the Spanish word for Fox) is indeed
small and clever, like its furry namesake.
Mounted between the instrument support
structure and the calibration unit at Gemini
South, it solves the perennial problem of
which facility instrument must be displaced
by not displacing any. Since it doesn’t re-
quire a port of its own, Zorro is free to take
up residence as a “permanent visitor.”
46
GeminiFocus
January 2020 / 2019 Year in Review