Astrophotography – Starlight meets the computer

According to the American Astronomical Society, the  charged couple device or CCD   was born on 17 October 1969 at Bell Labs in Murray Hill, New Jersey during a brain storming session between Willard Boyle and George Smith.  While it was originally intended as a solid state storage device, the imaging possibilities quickly became obvious.

(Boyle and Smith were awarded the Nobel Prize in physics in 2009 for their invention of the CCD. )

In the approx. 5 decades of technological improvements where our last article left off , what could be discovered by photographing the night sky had film had improved dramatically. By the 1960s and 70s, it was the choice medium for astronomers to record images of astro objects themselves and their spectra.

Images had also moved from largely black and white to full colour, with research by Dr. David Malin at Siding Springs Observatory in northern NSW being a key driver.

Triffid Nebula from Mt Wilson, USA 1920

Part of the Veil nebula in Cygnus,  (David Malin/AAO)

But film was not without its problems.

Firstly, film has a low quantum sensitivity. This simply means that photographic films overall efficiency is low. It “captures” only a relatively small amount of the light falling on it. Even with large telescopes, the amount of light from distant astro objects is small.  The obvious solution to this problem is leave the  camera’s shutter open for longer and longer periods of time. This would allow more light to fall on the film and details of faint objects revealed.

This is what astronomers did.* By exposing the film for hours at a time, they were able to record faint objects vastly beyond the ability of the eye to see at the same telescope. But there was another problem.

Film suffers from “Reciprocity Failure”. After a certain amount of time,  low levels of  light falling on film starts to fail to produce further detail. There’s a limit to the amount of information that can be captured on film.

These two factors – the inefficacy of film and the limits on how long exposures could be  – were some of the drivers for astronomers to replace film with electronic devices.

In the early 1970s, astronomers started trying to capture images using devices similar to ones used by TV cameras of the time called vidicons. These worked by scanning an image line by line. They’d also been used on spacecraft including Apollo mission and the Voyager probes to the outer Solar System.

In 1976, the first images using a CCD were taken of Saturn using  the Kuiper 61″ Telescope.  CCD cameras quickly became the gold  standard from that time onwards for astronomers. For example, the Comet Halley which was seen in 1986 in our skies was first spotted in 1982 using a CCD camera on the 200″ Hale Telescope on Mt Palomar. They were more sensitive and images could be more easily reproduced.  Early issues like small CCD sensor size compared to film size have gradually been overcome.  There’s some interesting reading on the detailed history of CCD cameras, such as this article here.

The efficiency of film is around 5%. CCD cameras are capable of capturing around 90% of the light they receive.  Exposures became shorter. The increased efficiencies CCD cameras offer mean telescopes could photography fainter object in more detail than they could with film alone.

The costs involved in CCD production meant they were not widely used by amateur astronomers for deep-sky photography. ** They largely kept to film cameras throughout the 1980s and 90s, or used various forms of video cameras to capture objects through telescopes.  That changed when a different type of sensor  – CMOS – became wieldy available.

CCD vs CMOS

CMOS (complementary metal oxide semiconductor) cameras use a different sensor technology compared to CCD cameras.

It’s easier to move the data from the CMOS camera chip compared to a CCD. This means they are better suited to shorter exposures like videos.  They are prone to more background electronic background noise.   CMOS cameras are also less sensitive to light than CCD cameras.

Sony CMOS Camera Chip

CMOS Camera chips also use much less power than CCD chips.

Whie it might seem that CCD chips would be preferable to CMOS chip for astrophotography – and they are in many cases – the global take up of mobile phones, tablets, webcams and other devices has meant that CMOS camera chips have become sophisticated, available in vast numbers and more importantly, very affordable.

CMOS based cameras like those from ZWO and QHY allow amateur astronomers to take images of the deep-sky, planets and even the Sun in greater detail and clarity than many filmed based professional telescopes of only a few decades ago.

In the same way CCD camera revolutionised the capturing of astro images for professional astronomers, CCD cameras have done the same for the amateur.

The Future.

The combination of imaging systems, computer guiding and processing is likely to continue to converge. At BINTEL, we’re already seeing advanced, all-in-one telescope like the  Stellina and the Vespera becoming popular.

We certainly expect to see many more!

* One technique for improving film sensitivity is “hypering” where film is soaked in a various gasses including hydrogen prior to exposure.

** Some early webcam such the Philips Toucam used CCD chips.

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