Applications in Optical Instruments
KIDSpec
Astronomical spectroscopy
KIDSpec (O’Brien et al., 2020) is a novel concept for a medium spectral resolution, wide passband, high throughput spectrograph, that can leverage the maximum gains from the large collecting area of modern telescopes. It builds on the original concept by Cropper et al., (2003) using a linear array of MKIDs combined with an Echelle grating to observe the dispersed light across several orders. The light from each order overlaps onto the linear array, but the orders are sorted using the intrinsic energy resolution of the energy sensitive MKID. Thus, instead of needing a large 2-D semiconductor detector, a single 1-D array of MKIDs can be used. Each order from the diffraction grating will overlap on the detector so unless the orders of the grating can be separated or all but a single order blocked, the resulting spectrum will be unusable. In the case of a non-energy-resolving detector, the light from different orders is typically separated using a further dispersive element whose dispersive axis is orthogonal to that of the Echelle grating (and hence is typically referred to as the ‘cross-disperser’). This is how X-Shooter on the ESO VLT is designed but this stage is unnecessary in the case of an energy-resolving detector, as the intrinsic energy resolution can determine the order of the incident light. Simulations of KIDSpec on the VLT show that we expect a gain in signal to noise over X-shooter due to the zero read-noise, as well as gains in cosmic-ray removal.
Dynamical masses of Black-hole X-ray Binaries
The dynamical masses of the compact objects in Black Hole and Neutron Star binaries can be determined using radial velocity measurements of components of the binary. This in turn can be used to constrain the masses. Most X-ray binaries are only optically bright during outbursts, which limits our ability to observe lines characteristic of the donor star. However, Steeghs et al 2002 used the Bowen-blend fluorescence lines to track the velocity of the secondary star. This enables the mass function to be constrained while the system is in an optically bright state. This method enabled us to determine masses for around 10 Neutron Star and Black Hole binaries, including the first dynamical mass for the Black Hole in GX339-4 (Hynes et al. 2003). The high throughput and exquisite time resolution of KIDSpec would enable us to study many more systems, including those at shorter orbital periods, such as the ultra-compact sources, which are thought to be the progenitors of gravitational wave sources.
MKID Microscopes
Ultra sensitive, spectroscopic imaging
Fluorescence microscopy is an incredibly versatile and widely used tool to understand small scale systems such as cells, and MKIDs are of interest as a detector for these instruments because they have several characteristics of interest. A common limitation is often detector noise, which is overcome with a higher signal (i.e. more sample illumination, or longer exposure) which can perturb the sample or cause photo bleaching or photo toxicity. MKIDs overcome this because they have no detector noise, and capture individual photons, meaning much lower light levels can be used. Given their spectral sensitivity, it is also possible to image several fluorophores simultaneously, when a monochrome detector would require multiple exposures using different filters. Because MKIDs have time resolution, it opens up possibilities for temporal studies such as fluorescence lifetime imaging, or simply how a cell reacts to a stimulus. The wide spectral sensitivity means that systems which would previously have required multiple cameras and dichroic, can be much simpler and so more optically efficient. This system could be used to make observations that are otherwise impossible to achieve with existing detector technology.
Adaptive Optics
Ultrafast Wavefront sensor
The next generation of extremely large ground-based telescopes, such as ESO’s 39-m ELT (image below, right) are limited for high resolution imaging by the turbulence of the atmosphere. Disturbances caused by changes in the temperature and density of the atmosphere, cause a distortion of the optical wavefront. This is responsible for the ‘twinkling’ of stars and limits the ability of astronomers to take advantage of the diffraction limited performance. Adaptive optics is a technique that corrects the wavefront distortions by introducing a deformable mirror (DM) that corrects the wavefront before the scientific instrument receive the light from the telescope (below left image). To do this, part of the beam is deviated to a sensor that continuously detects the wavefront deformation and sends it to the DM. This is the so-called adaptive optics loop. There are several types of wavefront sensors. One of them that is becoming increasingly popular is the pyramid wavefront sensor. It consists of a prism that divides the incoming light into 4 different images of the telescope pupil on a detector. We are then able to recover the wavefront distortions from the differences of these 4 images. After software processing, these differences are translated into a geometrical form understood by the DM. By combining the pyramid prism with an MKID array, we can use the exquisite sensitivity of the photon-counting array together with the ability to resolve the wavelength of the light to deliver the optimum image quality. This system could be used to study extra-solar planets to search for earth-like planets.
The Future…
MKIDs promise to open up new areas of research across a broad range of disciplines. We are developing novel instruments in the fields of astrophysics, biomedical imaging and adaptive optics. This multidisciplinary approach has often lead to exciting new discoveries. For more information, please contact one of us and we would be happy to discuss.