Led by project manager Tony Fox-Leonard, the team at Glyndw r University St Asaph has polished the 1.5 metre optic down to just 7.5 nanometres - which equates to around the size of a haemoglobin molecule.
It is the best measurement ever managed in the UK and was produced for ESO's (European Southern Observatory) European-Extremely Large Telescope.
This is the first time such a feat has ever been achieved using computer-operated machinery.
Telescope mirrors have to be polished down to an amazing level of smoothness to receive clear image from the cosmos.
Once completed, the giant eye on the sky will gather light from distant stars and galaxies, be 39 metres in diameter, made of 798 segments and located on Mount Cerro Armazones in Chile, where it is set to gather 15 times more light than the largest telescopes around today. Work began this week on blasting away part of the 3,000m mountain's peak to make way for the new observatory, due to be completed by 2022.
Mr Fox-Leonard said: "Reaching 7.5nm is a significant achievement and marks another major milestone for the UK's optical manufacturing industry."