Fast photometry and polarimetry using CMOS Detectors
Prof Iain Steele, Dr Helen Jermak and Dr Chris Copperwheat
In the RINGO2 and 3 instruments the ARI instrumentation group pioneered the use of fast readout, low noise detectors in a polarimeter. Such detectors have two key advantages in this application. Firstly they are able to observe rapidly varying sources. Secondly, and most importantly, the very low readout noise allows data to be taken a high frame rates (~0.1 - 1 Hz). This provides excellent temporal resolution when the source is sufficiently bright or, when the source is faint, allows temporal binning of high frame rate data without a significant read noise penalty to achieve the desired polarimetric accuracy. This gives huge dynamic range (~100000:1) and the resulting ability to observe transients of unknown and rapidly variable brightness. For RINGO2 and 3 we achieved the required low noise, fast readout using then state of the art Electron Multiplying CCD (EMCCD) technology. However due to their mode of operation EMCCDs suffer charge multiplication noise which causes a factor 2 reduction in the instrument sensitivity. We therefore propose to develop new photometric and polarimetric instruments based on the use of newly available scientific CMOS (sCMOS) detectors. sCMOS detectors maintain the rapid readout (>100 fps) and ultra low read noise (<0.9 electrons) advantages of EMCCD but add much greater effective sensitivity (>70% quantum efficiency vs ~40% for EMCCDs). In addition sCMOS detectors are available in much larger formats (~4 Mpix vs ~1 Mpix from EMCCDs) allowing larger fields of view 8 to be sampled at optimal (Nyquist sampled) resolution.
The project will involve both laboratory testing of sCMOS devices and their incorporation into new instruments to be mounted on the Liverpool Telescope in La Palma and the NARIT telescope in Thailand. The project will incorporate a significant element of on site instrument commissioning and observing work. During the second half of the project, as part of carrying out a detailed evaluation of the performance of the new detectors on-sky, the student will apply the new instruments to an astrophysical topic in time domain astrophysics such as high precision exoplanet transit photometry, rapid response GRB follow-up, or polarisation variability of AGN.
CALPOL - a miniture calcite polarimeter prototype.
Lab based prototype of a dual camera polarimeter (mini-MOPTOP)