telescope stakes
David Le Page
In five years South Africa will be home to the second-largest optical telescope on Earth, thanks in large part to the ingenuity of Cape Town astronomer Darragh O’Donoghue.
O’Donoghue has come up with an improvement to the plans for the South African Large Telescope (Salt) that will allow it to gather 30% to 40% more light than the Hobby-Eberly Telescope (HET) in Texas, on which its design is based – and of which it would have been a virtual clone, had O’Donoghue not got his hands on the plans.
The new telescope has been on the drawing board for about three years, with construction due to begin next year. O’Donoghue’s modifications come in good time to be incorporated into the final design – without extra cost.
The Salt, to be built near the Karoo town of Sutherland, is a reflecting telescope with a large, immobile primary mirror and a secondary corrector. Sutherland is the main observing centre of the South African Astronomical Observatory.
The primary mirror is actually comprised of 91 hexagonal segments, each a metre wide, which together create an 11,2m diameter and a 77,6m2 area.
Many reflecting telescopes, including the Hubble Space Telescope, have a single primary mirror, of parabolic curvature. The world’s largest telescopes, the Keck reflectors on Hawaii, use single parabolic mirrors.
Parabolic mirrors neatly focus light to a single point, but are extremely hard to grind accurately. An undetected flaw 1/25 of the breadth of a hair in the Hubble main mirror nearly saw the whole mission scrapped. That flaw was called spherical aberration, which is the tendency of a spherical mirror to blur light images even as it focuses them.
Because it’s far easier – and cheaper – to grind spherical mirrors, the hexagonal components of the Salt primary mirror are all spherical, and the reflected images of the heavens they collectively produce are all subject to spherical aberration.
Which is why the corrector is the crucial and most elaborate part of the design. It collects light reflected from the primary mirror, moves in order to effectively focus the telescope on different parts of the sky, and corrects for spherical aberration through an intricate instrument combining four separate complex mirrors, each costing $500E000.
The problem with the original American design for the corrector, as O’Donoghue delicately puts it, was that it was “non- optimal”. His optimised design leaves the corrector, more correctly called the spherical aberration corrector (SAC), the same size as that of the HET. But O’Donoghue’s SAC will work a whole lot better.
The improved SAC will not only improve Salt’s image quality threefold or fourfold over HET, but will also enable it to cover four times the area of sky at any one time. What’s more, while the HET design uses only a 9,2m diameter patch of the primary mirror at any one time, Salt will now be using the full 11,2m mirror no matter what the position of the SAC.
Salt will have other strengths over HET, simply because it’s being built later. This means that the electronic and digital instrumentation that drives the telescope will enjoy the advantage of a decade’s worth of further development.
Unfortunately, while it would be easy in theory for the universities that jointly run the HET to upgrade its SAC and take advantage of O’Donoghue’s optimised design, at the moment they just don’t have the money to do it.
While, for once, we do.