Aperçu du fichier PDF ea2803.pdf - page 7/119

Page 1...5 6 789119

Aperçu texte

PRO-AM collaborations in Planetary Astronomy


to its high frame rate and EMCCD technology (Electron Multiplying CCD) allow
separately the compensation of each of these considerations and partially fill the
lack of adaptive optics in amateur astronomy. EMCCD technology, described here,
increases drastically the sensitivity of CCD imaging systems.
EMCCD are based on a classical CCD. Between the pixel matrix and the readout
gate, a special pixel register is added. The pixels of this register are masked to the
incoming light and are polarized with high voltage (typically higher than 50V).
Under such a high voltage, the electrons transferred in the register are multiplied
by a factor that can reach a few hundreds at the exit of the register. The goal is to
multiply each incoming electron to give an output charge constituted of a number
of electrons always higher than the readout noise. By this way, the readout noise
becomes negligible compared to the initial single electron signal. The multiplication
factor can be tuned by software from 1 (the EMCCD appears to be a classical CCD)
to a few hundreds. A lot of physics experiments show that EMCCD technology
is one of the best ways to reach “shot noise” limitations, instead of read noise in
scientific imaging. There are actually two main EMCCD manufacturers (E2V and
Texas instruments).
Already used in the field of amateur speckle interferometry (with a very high
magnification involving a low photon number per unit of time and pixels surface),
the EMCCD cameras allow reaching a so short exposure time that they take images faster than atmospheric distortion speed. This regime allows a kind of imaging
mode called the “lucky imaging” (see details of the technique in Sec. 2.2.3). The
number of good quality images obtained suffers then from a very low spatial distortion. Regardless of a well-known lucky imaging probability law mainly depending
on telescope diameter and Fried parameter knowledge, these images are often of
an outstanding quality. In the field of planetary science, this involves the possibility
to obtain highly resolved planetary surfaces in narrow band filters (a few tenth of
nanometer bandwidth). These filters absorbing most of the incoming light, it becomes possible, with the sensibility improvement, to obtain a quasi-monochromatic
image with a very high spatial resolution (see Fig. 2). Another type of application using this sensibility increase is the reachable magnitude in stellar occultation
experiments. Using a 60 cm aperture Newtonian reflector, it is possible to reach
magnitude 15.7 at a rate of 25 frames per second, which allows recording stellar
occultations by Trans-Neptunian Objects (see Sec. 8.2).
However, EMCCD still have some limitations in amateur applications. The first
one is the well depth of pixels, which can be rapidly saturated if the multiplication
gain is set too high. This implies that an EMCCD camera for amateur astronomy
shall be limited to the use of very short exposure times, and so suitable for only a few
types of amateur experiments regardless of standard CCD technology. The second
one is the loss of linearity at high multiplication gain, which restricts the amplification domain and thus, the photometric measurement accuracy. Another limitation is
due to the speed reachable by the camera (and not only the link speed to the acquisition computer). This makes the EMCCD technology slower than actual sCMOS,
which can acquire frames up to 400 fps. EMCCD technology is intrinsically limited to 30 fps in full frames and hardly reaches 100 fps with selection of the region