Offered coronagraphic modes

Different masks are offered for coronographic observations:

• classical dark mask (Lyot stop): 2 opaque disks of diameter 0.7 & 1.4".
  These masks are held by metallic wires which cross in the middle and create another small mask that is offered under the name C_0.7_1.4.
• one semi-transparent mask (C_0.7_sep_10): on a transparent substrate over a diameter of 0.7" the transmissivity is reduced to 0.4% (0.3%) for H-band (Ks-band).
• two new four quadrant phase masks, one optimized for the H (4QPM_H) and one for the K band (4QPM_K) are offered as of P80 and replace the old 4QPM optimized to work in K band. These masks reduce the intensity of a source by adding a phase shift of pi to the wavefront. Both masks also include a small Lyot mask 0.15" diameter located in the center, which mitigates the effect of residual jitter. Peak attenuation for the masks is ~100, a gain of ~2 with respect to the old 4QPM. More details below and in the User's manunal.

All filters and cameras can be used. See the user manual for restrictions with the 4QPMs.

4QPM coronagraphy can be used in combination with pupil tracking.

4QPM Night tests

AB Dor was observed between 0h39UT and 0h51UT, followed by a reference star (HD 41371) at the same parallactic angle and observed for the same duration. The Right Ascension of the reference is therefore chosen with a difference equal to the observing time (here about 30 arcmin) and at the same Declination (differing by about 1°). In these case the orientation of the telescope pupil with respect to the instrument is more or less identical and as a result, static phase aberrations or amplitude effects (like spiders) keep the same orientation in the FOV.
Seeing ranges between 0.66 arcsec and 1.11 arcsec with a median value of 0.90 arcsec. AO correction was good and stable with Ec=45 ± 5%.
The two stars are very similar: same spectral type K1III and K0III respectively, same IR magnitudes about mK=4.7 and mH=4.9 according to the 2MASS catalogue, and visible magnitudes are close mV=6.93 and 7.1 respectively. We combined 6 images of DIT=1s, NDIT=120.
The reference is used to calibrate the residual pattern in the image of the star and hence the intensity ratio between the 2 images has to be determined. The optimal intensity ratio is found while trying to minimize the residual intensity in the image. Re-centering of the 2 images was not necessary. Raw and subtracted images are presented in Figure 1. The companion AB Dor C is clearly visible even on the direct coronagraphic image. Its distance to AB Dor A is measured to be 0.185 ± 0.005 arcsec.

Fig. 1: Direct (left) and coronagraphic images, raw (middle) and subtracted (right) of AB Dor. The companion AB Dor C is clearly visible even on the direct image when contrast is numerically enhanced although it is as bright as some other PSF structures. The field of view is 2.5 arcsec. North is up, East is left.

Figure 2 shows the radial contrast on the coronagraphic image and the detection level once the halo is calibrated with a reference star. The 5 sigma detectability of a point source reaches ΔKs=6.7mag at 0.2 arcsec and ΔKs=9mag at 0.5arcsec

Fig. 2: Radial profile of PSF, raw and subtracted coronagraphic images of AB Dor as observed in the K band with 4QPM_K. The blue line corresponds to the averaged intensity in the subtracted image while the red line stands for the 5 sigma detection level.