We carried out a three year (2013-2015) observational campaign at the Danish 1.54-m Telescope at the ESO observatory at La Silla in Chile in which we obtained ~1000 astronomical images in the field of 11 Galactic globular clusters. The selection of these stellar systems was focused mainly on the visibility of the targets and their relevant physical properties available in the catalogues, among them were considered the density, variable stars known, colour-magnitude diagrams, and luminosity. The telescope was equipped with an electron-multiplying CCD (EMCCD) with the aim of taking very short exposure-time images. The camera was configured to take 10 frames per second. Due to this, the brighter stars observed were not affected by saturation, it helped to give higher signal to noise ratio to the fainter stars and, importantly, it minimised the effects of the atmospheric turbulence such as blending between stars in the crowded fields. To produce normal-exposure-time images (minutes) we implemented the shift-and-add technique that also enabled us to produce images with better angular resolution than previously achieved with conventional CCDs on ground-based telescopes, and even enabled us to produce images with angular resolution close to that obtained with space telescopes. The detection of the stars in each of the globular clusters and the photometry was performed via difference image analysis by using the DanDIA pipeline whose procedures and mathematical techniques have been demonstrated to produce high-precision time-series photometry of very crowded stellar regions. We produced time-series photometry for ~15000 stars in the fields observed which were statistically analysed in order to automatically extract variable stars. Our aim is to complete, or improve, the census of the variable star population in the globular clusters. In NGC 6715, we found light curves for 17 previously known variable stars near the edges of our reference image (16 RR Lyrae and 1 semi-regular) and we discovered 67 new variables (30 RR Lyrae, 21 long-period irregular, 3 semi-regular, 1 W Virginis, 1 eclipsing binary, and 11 unclassified). This cluster was particularly interesting because apart from the results obtained, it shows the benefits of using the EMCCD cameras and the shift-and-add technique. It is a cluster studied several times including data obtained with the OGLE survey and also with the Hubble Space Telescope and our discoveries were still new. Our new RR Lyrae star discoveries help confirm that NGC 6715 is of intermediate Oosterhoff type. In the other 10 globular clusters, we obtained light curves for 31 previously known variable stars (3 L, 2 SR, 20 RR Lyrae, 1 SX Phe, 3 cataclysmic variables, 1 EW and 1 NC) and we discovered 30 new variables (16 L, 7 SR, 4 RR Lyrae, 1 SX Phe and 2 NC). In NGC 6093, we analysed the famous case of the 1860 Nova, for which no observations of the Nova in outburst have been made until the present study. Ephemerides and photometric measurements for the variable stars are available in electronic form through the Strasbourg Astronomical Data Centre.

<p>Optical lucky imaging polarimetry of HL Tau and XZ Tau in the Taurus-Auriga molecular cloud was carried out with the instrument PolCor at the Nordic Optical Telescope (NOT). The results show that in both the V- and R-band HL Tau show centrosymmetric structures of the polarization angle in its northeastern outflow lobe (degree of polarization ~30%). A C-shaped structure is detected which is also present at near-IR wavelengths (Murakawa, 2008), and higher resolution optical images (Stapelfeldt, 1995). The position angle of the outflow is 47.5+-7.5 degrees, which coincides with previous measurements and the core polarization is observed to decrease with wavelength and a few scenarios are reviewed. Measuring the outflow witdh versus distance and wavelength shows that the longer wavelengths scatter deeper within the cavity wall of the outflow. In XZ Tau the binary is partially resolved, it is indicated by an elongated intensity distribution. The polarization of the parental cloud is detected in XZ Tau through the dichroic extinction of starlight. Lucky imaging at the NOT is a great way of increasing the resolution, shifting increases the sharpness by 0.1 asec and selection the sharpest frames can increase the seeing with 0.4 asec, perhaps more during better conditions.</p>

Optical lucky imaging polarimetry of HL Tau and XZ Tau in the Taurus-Auriga molecular cloud was carried out with the instrument PolCor at the Nordic Optical Telescope (NOT). The results show that in both the V- and R-band HL Tau show centrosymmetric structures of the polarization angle in its northeastern outflow lobe (degree of polarization ~30%). A C-shaped structure is detected which is also present at near-IR wavelengths (Murakawa, 2008), and higher resolution optical images (Stapelfeldt, 1995). The position angle of the outflow is 47.5+-7.5 degrees, which coincides with previous measurements and the core polarization is observed to decrease with wavelength and a few scenarios are reviewed. Measuring the outflow witdh versus distance and wavelength shows that the longer wavelengths scatter deeper within the cavity wall of the outflow. In XZ Tau the binary is partially resolved, it is indicated by an elongated intensity distribution. The polarization of the parental cloud is detected in XZ Tau through the dichroic extinction of starlight. Lucky imaging at the NOT is a great way of increasing the resolution, shifting increases the sharpness by 0.1 asec and selection the sharpest frames can increase the seeing with 0.4 asec, perhaps more during better conditions.

One of the highest resolution astronomical images ever taken in the visible were obtained by combining the techniques of adaptive optics and lucky imaging. The Adaptive Optics Lucky Imager (AOLI), being developed at Cambridge as part of a European collaboration, combines these two techniques in a dedicated instrument for the first time. The instrument is designed initially for use on the 4.2m William Herschel Telescope (WHT) on the Canary Island of La Palma. This thesis describes the development of AOLI, in particular the adaptive optics system and a new type of wavefront sensor, the non-linear curvature wavefront sensor (nlCWFS), being used within the instrument. The development of the nlCWFS has been the focus of my work, bringing the technique from a theoretical concept to physical realisation at the WHT in September 2013. The non-linear curvature wavefront sensor is based on the technique employed in the conventional curvature wavefront sensor where two image planes are located equidistant either side of a pupil plane. Two pairs of images are employed in the nlCWFS providing increased sensitivity to both high- and low- order wavefront distortions. This sensitivity is the reason the nlCWFS was selected for use with AOLI as it will provide significant sky-coverage using natural guide stars alone, mitigating the need for laser guide stars. This thesis is structured into three main sections; the first introduces the non-linear curvature wavefront sensor, the relevant background and a discussion of simulations undertaken to investigate intrinsic effects. The iterative reconstruction algorithm required for wavefront reconstruction is also introduced. The second section discusses the practical implementation of the nlCWFS using two demonstration systems as the precursor to the optical design used at the WHT and includes details of subsequent design changes. The final section discusses data from both the WHT and a laboratory setup developed at Cambridge following the observing run. The long-term goal for AOLI is to undertake science observations on the 10.4m Gran Telescopio Canarias, the world's largest optical telescope. The combination of AO and lucky imaging, when used on this telescope, will provide resolutions a factor of two higher than ever before achieved at visible wavelengths. This offers the opportunity to probe the Cosmos in unprecedented detail and has the potential to significantly advance our understanding of the Universe.