News

DragonFly Telescope



11 February 2016

Three previously undiscovered very faint dwarf galaxies were found orbiting the pinwheel galaxy M101 using the Dragonfly Telescope (Source: X ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL Caltech)
Three previously undiscovered very faint dwarf galaxies were found orbiting the pinwheel galaxy M101 using the Dragonfly Telescope (Source: X ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL Caltech)

When professional astronomers need new designs of telescope, they're looking at forests of paperwork, tens of millions of dollars at the very least, and a decade or two. But thanks to one astronomer's hobby of nature photography, he got his radical telescope very quickly and cheaply - and also got us one step closer to solving the mystery of how galaxies spring into existence.

One of our main theories of how galaxies form involves Dark Matter. Way way back, over 13 billion years ago, in fact, just a few thousand years after the Big Bang, practically all the mass in the Universe was this mysterious Dark Matter. The Dark Matter began to clump together, thanks to gravity, and began to shape itself into roughly spherical objects - which began to collapse inwards. Various gases (such as hydrogen and helium) collected at the centres of these spheres, turning into the first stars - and the first galaxies. After a few billion years, the small galaxies merged with each other, eventually evolving into giant galaxies, like our own Milky Way.

But there's a problem with this Dark Matter theory. If that pathway of galaxy formation is correct, around each big galaxy we would expect to see vast fields of messy debris left over from the creation process - random ejected stars, partially eaten halos of gas, bulges and streams of matter and lots and lots of very faint dwarf galaxies. Unfortunately, we haven't seen this debris that should be there - if the theory is correct.

One problem is that current telescopes are inherently not very good at seeing this left-over debris. It's related to the fact that the overwhelming majority of professional telescopes catch the incoming light with curved mirrors, not curved lenses.

Curved mirrors have several advantages - you can make them really big, and they are really good at catching the light from a small bright object. Over the last half-century, there have been tremendous advances and improvements in the performance of mirror-type telescopes. In the specific field of gathering light from small bright objects, they've got 100 times better.

But when it comes to gathering faint light from large diffuse objects, mirror-type telescopes have had no improvement at all over the last half-century. This is for various technical reasons. First, the mirrors themselves cause scattering of the light, due to micro-roughness and dust. Also the secondary mirror in mirror-telescopes creates a large obstruction in the light path, and furthermore, the supports that hold this secondary mirror cause diffraction and bending of the incoming light.

So the ideal telescope for looking at large faint objects would have no mirrors and an unobstructed light path - in other words, it would have lenses, not mirrors. But apart from solar telescopes, professional astronomers haven't used lens-type telescopes for a century.

Now back in 2011, two professional astronomers, Roberto Abraham and Pieter van Dokkum, were in a Nepalese restaurant. After curry and rice and lots of beer, they were trying to work out how to find these large faint bits of debris that should have been left over from the creation of galaxies.

Pieter can Dokkum was a keen nature photographer, and suddenly realized that a new camera lens with a wonder coating on the front might just be perfect for their needs. It was a 400 mm f2.8 SuperTelephoto lens, costing around $10-15,000. The lens coating was called Subwavelength Structure Coating - actually, countless tiny cones or pyramids, all pointing outwards. These cones are microscopic - smaller than the wavelengths of visible light. The Physics is complicated, but the end result is that less light is scattered inside the lens - so there's less of what the photographers call "ghosts" or "flares".

By March 2012, they had spent about $15,000, done some testing, and found that their single lens had captured what other astronomers had previously only got hints of. They saw a clear, but very faint, halo of diffuse matter surrounding the galaxy called M51.

Well, if one lens is good, surely three must better. So they swiped the credit card, got another two lenses, and built a special structure so that all three lenses were perfectly lined up. Very shortly afterwards, in September 2012, they got their results - yep, it all worked. By 2013, they were running eight lenses in parallel. In 2014, they published their findings that the galaxy called M101, or the Pinwheel Galaxy, had three previously undiscovered very faint dwarf galaxies orbiting around it.

The astronomers have since upgraded their telescope to 50 lenses. They can now get images in hours, not weeks. Their credit card bill has run up to about half-a-million dollars - but it's still a lot less than tens of millions of dollars, they didn't have to wait for decades to build it, and they didn't have to run through cubic metres of paperwork. They call their array of professional lenses the Dragonfly Telephoto Array for two reasons. First, with 50 commercial telephoto lenses, it looks like the eye of a dragonfly - not a mirror to the soul, but a lens to previously hidden galaxies. And second, Pieter van Dokkum really likes taking photos of dragonflies …