Planteamiento de las hipótesis del modelo de negocio (BMC)

Two types of optical ghosts have been noted in the TCI data. Firstly, a circular or ringed structure is occasionally observed in the Red camera data approximately 10 arcseconds north of the target star, with the exact position varying slightly, which is most apparent in very good seeing conditions. The shape of the ghost is similar to the defocused PSF of the TCI, and hence it is believed that internal reflections within the telescope or TCI optics result in a defocused image of the target field being superimposed on the main image. This ghost is occasionally accompanied by an in- focus image on the opposite (south) side of the target star, slightly closer in. Both ghosts are much more prominent in the Vis camera data, despite the lower signal typically received, presumably due to the transmission or reflection properties of the (unknown) optical components causing these ghost images. These ghosts are regularly detected as companion stars by the automated detection algorithm – occasionally with multiple detections for the ringed structure – and hence must be removed during the manual review process. Both of these features are illustrated in Figure 3.13.

The second, more troublesome type of optical ghost is an extension to, or in some cases a faint duplicate of, the PSF of all stars in the Red camera data. This is typically located to the southeast of the PSF, but the position, brightness, and separation of the duplicate vary, both from target to target and also in subsequent observations of the same target. In many cases the duplicate image is sufficiently close to be strongly blended with the target star, but it is occasionally well separated (sometimes exceeding an arcsecond) and can appear as a convincing companion star to both automated and manual detection methods. The nature of this ghost was not well understood at the time of publication for the 2014 data, and one such ghost was identified as a bona fide companion, located 100 _{to the south of HAT-P-41. The understanding of this feature}

was improved by the detection of a pair of identical, well-separated ghosts during the observation of the WASP-85AB binary, which were seen to have moved in unison around the two stars in a subsequent re-observation a few nights later, indicating that such objects were clearly not of astrophysical origin. These observations of WASP-85

are shown in Figure 3.14, whilst the false positive companion to HAT-P-41 is shown in Figure 3.15

The cause of these companion-mimicking ghosts is a combination of differential atmospheric refraction and the transmission/reflectance properties of the dichroic used to split light between the Red and Vis cameras. As briefly mentioned in Section 3.5.2.4, the effect of atmospheric refraction varies slightly with wavelength, causing the appar- ent position of an object to vary with wavelength. Whilst many telescope instruments are fitted with atmospheric dispersion correctors (ADCs) to remove this effect, the TCI is not equipped with any corrective optics, and therefore stellar images in the TCI appear be slightly elongated perpendicular to the horizon, with the elongation vary- ing as a function of atmospheric conditions and the altitude at time of observation. The existence of the well-separated optical ghost is caused by a leak in the dichroic at approximately 410nm, which transmits a small fraction of blue light towards the Red camera. Adopting the atmospheric conditions from Section 3.5.2.4 and an altitude of 50◦ _{(airmass 1.3), this blue light will appear offset by approximately an arcsecond}

from the redder light received by the red camera, matching the observed separation of the ghost at similar altitudes. The position of the ghost can be predicted from the telescope pointing at time of observation, and an additional option was added to the interactive analysis software to plot this during the manual confirmation stage, which proved very useful in distinguishing true companions from optical ghosts.

Consideration has been given to fitting a long-pass filter to the red camera to eliminate the effects of stray light through the dichroic. The TCI is itself a prototype instrument for a series of similar instruments on the SONG telescope network (Grun- dahl et al. 2007), with the full design including filer wheels and atmospheric dispersion correctors (Grundahl et al. 2009, Skottfelt et al. 2015b); however, these components were excluded from the installation on the Danish 1.54m telescope.

HAT-P

-41B

False

comp

anion

HAT-P

-41A

Figure 3.15: Falsely classified companion to HAT-P-41. This TCI Red camera image appears to show three stellar sources; however, star-like object marked ‘false compan- ion’ is blue light from HAT-P-41A that passed through the dichroic leak at 410nm. The blue light image is offset by approximately 0.800_{, far in excess of the image resolution,}

0

2

4

6

8

10

Separation (arcsec)

0

2

4

6

8

10

12

14

Magnitude difference

Figure 3.16: Detection limits for targets observed in 2014. Shading on the image indicates the fraction of observations for which a given contrast was achieved, with white indicating 100%, and black 0%. The dashed line indicates the median contrast across all targets. The solid line indicates the contrast of the best observation.