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Telescope Optical Prescription nearing final design

Extensive work between team members within Instituto de Astrofísica de Canarias (IAC), La Universidad de Oviedo and Liverpool John Moores University, is culminating in the final Optical Prescription of the New Robotic Telescope (NRT). This breakthrough is an important step, which will enable work to start on the precision manufacture of the which make up the Primary (M1) and Secondary (M2) Mirrors of the telescope. The optical prescription of the telescope defines the key dimensions, positions and curvatures of the different mirrors in the 'optical train'. Dr Éamonn Harvey of Liverpool John Moores University has used extensive simulations involving different mirror curvatures to analyse image performance. This cutting edge optical simulation using Zemax® has enabled rapid investigations into various optical prescriptions and the optimisation of a many different parameters.

However, there is a delicate balance to be struck. Whilst the optical image quality needs to meet the Scientific Goals of the NRT, there are many other constraints to the optical design. This is an issue that Dr Harvey is all too aware of, "There's always a trade-off. You can design a telescope for the perfect image, but we need also consider what is achievable in manufacture, operation, stiffness of telescope structure, enclosure size and how these considerations impact short and long-term costs."

Optical Sumlations need to look at the Point Spread Function (PSF) at the focal point of the telescope and ensure image quality is maintained across the field of view
Simulating image quality at the Cassegrain focal point

In order to ensure that the Optical Prescription quite literally 'fits' with other elements of the project, there has been multi-disciplinary iterative work across teams within the different project partners. Take, for example, the distance between the M1 and M2 mirrors. This dictates the tube length of the telescope structure, affecting its balance point, oscillation behaviour and stiffness. There is also an impact on how large the clamshell enclosure will have to be to house the telescope safely from the adverse weather conditions on the Observatory site, 2400m above sea level on La Palma. César Rodriguez of La Universidad de Oviedo has been looking into the impacts of different mirror parameters on the telescope structure. Using ANSYS software and a Python interface, he has been able to create a telescope parametric model, looking at the deformations and oscillation modes for different optical prescriptions. This has enabled rapid and iterative analysis to quickly determine which elements of the optical prescription have the greatest effect on the telescope structure.

Ansys® Simulations of the primary Oscillation mode of the telescope

The simulations have enabled all of these factors to be evaluated, ensuring that the required stiffness of the tube structure is achieved. As the telescope will be using more sophisticated design than the common Surrier Truss, which is used on most 4m class telescopes. There is a much higher level of stiffness, essential when slewing quickly to a new transient event and taking data as soon as possible.

As Miguel Torres, one of the team's System Engineers, at the IAC comments, "Most projects try to scale up telescope designs, however with NRT we are looking at the latest technology employed by the large 8-10m class telescopes and we are scaling this down."


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