$\delta$ Per

 

 

Figure 4: b. The same image enhanced with an inverse square law, and smoothed to the speckle scale with a wavelet transform filter. The faint companion ($\Delta m=3.5$) emerges above the speckle noise, although no reference star was available in this case to subtract the fixed speckles.

The first target star observed is $\delta$ Per (HD22928, m_v=2.99, JD=50725.646), a binary system previously observed by Hipparcos ([ESA 1997]). We used the $0.45\arcsec$ occulting mask and 400 $\mu$m pupil stop. The turbulence conditions were characterized by r₀ = 8cm during observation. 9503 short exposures of $\delta$ Per have been acquired, corresponding to an observing time of 190s. We used an $H\alpha$ filter (653.6nm) of bandwidth 10nm. The faint companion appears at $PA=(202\pm 1)\degr$ and $\rho=(0.293\pm 0.027)\arcsec$. The sampling on the camera is 144 pixels per arcsecond. The image (Fig. 4.a and 4.b) presents some fixed patterns, mainly produced by the spider arm diffraction which is visible as four symmetrical bright speckles. An AO gain of 6, azimuthally averaged, is obtained at $0.3\arcsec$ from the star. An SNR of 53 is measured on the speckle size ($15\times 15$ pixels), while the model predicted an SNR of 133 ([Boccaletti et al. 1998]). The discrepancy with theory can be an indirect effect of the computer failure mentionned above; speckle noise becomes dominant owing to the small number of exposures. The intensity of the companion was derived from the CP20 long exposure and corresponds to a $\Delta m$ of $3.48\pm 0.20$.

 

Figure: a. Negative cleaned map of the binary star $\eta$ Psc, observed with the $0.45\arcsec$ mask and the Wynne corrector's spectral band ($\lambda_0 = 635 nm$, $\Delta\lambda=650-850nm$). Due to the strong turbulence (r₀=6.2 cm) and the small number of exposures (9013), the faint companion (arrow) remains buried among the speckles. A flat field correction has been applied to the map.

 

Figure: c. Difference of $\eta$ Psc and $\eta$ And cleaned maps. The companion now appears as a dark feature (arrow). The image has been smoothed with a wavelet transform filter.

 

Figure: b. Reference star ($\eta$ And) observed in the same way just before $\eta$ Psc with r₀ = 8cm, 3645 short exposures. The suspected "companion" peak of Fig. 5.a is here absent, although the static residual speckles are similar. The scattered halo level is significantly lower than on Fig. 4.a.

 

Figure: d. Positive cleaned image obtained with an inverse square law, and a threshold at 3$\sigma$ level.

The Hipparcos data gives $\Delta m_v=3.14$, $\rho=0.33\arcsec$ and $PA=221\degr$, which is in good agreement with our results.