$\eta$ Psc

 The second target star, $\eta$ Psc (HD9270, m_v=3.61, JD=50730.507), was also selected from the Hipparcos binary star catalogue. Coronagraphic images of $\eta$ Psc were recorded without spectral filter, the bandpass being limited by the dichroic beam-splitter and the camera ($\Delta\lambda=650-850nm$). These images illustrate the problem of the speckle noise in the search for faint companions. Indeed, for the same reason as above the number of short exposures is poor (9013). Altough the companion emerges easily above the photon noise, the residual speckle noise inhibits its detection on the raw data (Fig. 5.a). However, a nearby reference star ($\eta$ And, m_v=4.42), has been observed immediately afterwards for map subtraction (Fig. 5.b). The efficiency of the subtraction is limited for several reasons :

(i)

The conditions of turbulence, and thus the correction applied by the AO system, are different for both stars (r₀=8cm for $\eta$ And, and r₀=6.2cm for $\eta$ Psc).

(ii)

The coronagraphic images are not invariant to translation ([Malbet 1996]), and in fact highly sensitive to tracking drift. Any centering difference between both sequences causes spurious speckle noise in the subtracted image.

(iii)

A 76Hz vibration of the telescope drive motor, seen by the Shack-Hartmann wave sensor, is left uncorrected by the adative system.

To overcome these problems, both star and reference have been corrected from Flat-Field and smoothed with wavelet transform. Then, to account for the lower speckle contrast of $\eta$ Psc, presumably caused by unequal seeing lifetime during both observations, the reference star was convolved with a gaussian shape of 4 pixels width. Finally, $\eta$ And was shifted to overlap $\eta$ Psc in the field, and scaled in intensity (Fig. 5.c). After subtraction of the two cleaned negative images, the remaining bright and dark speckles belong respectively to the reference star ($\eta$ And) and the target star ($\eta$ Psc).
The measurement of the SNR, on the cleaned map, is made difficult by the crowded field. Although the companion remains undetected on the initial image (Fig. 5.a), in terms of speckle noise, the comparison of both images eliminates all features common to the target (Fig. 5.a) and the reference (Fig. 5.b), so that the only residual speckle indicates the companion position ($\rho=(0.507\pm 0.01)\arcsec$, $PA=(94\pm 1)\degr$). This process leads to an SNR of 137, instead of 250 as predicted by the model. In this case, the SNR is naturally decreased by the companion's smearing, induced by the Wynne correctors.
The long exposure allows to derive a $\Delta m=4.15\pm 0.20$ in the R band. The Hipparcos V-band data gives $\Delta m_v=3.7$, $\rho=0.64\arcsec$ and $PA=47\degr$. A difference of spectral types for both components can explain the apparent discrepancy. We must also notice a discrepancy in the companion position. As the orbital solution is unknown, further observing runs will be needed to confirm the detection of the companion.

 

$$\delta \eta$$ Star name

m_r 2.99 3.61

$$\Delta\lambda(nm)$$ 10 200

$$PA (\degr)$$ 221/202 47/94

$$\Delta m$$ 3.14/3.48 3.64/4.15

$$\rho (\arcsec)$$ 0.33/0.29 0.64/0.51

$$\eta$$ reference no