Download Steve Holmes - KWFN October 22 2012 speaker

Steve Holmes – The Night Sky and Astrophotography
Recap by Michael Frind.
Kitchener-Waterloo Field Naturalists, October 22, 2012.
On average, we have darkness 12 hours a day. Although sun-lit landscapes are beautiful in their
own right, the night sky also brings much to see, with a truly breathtaking expanse of space and
time that far eclipses anything terrestrial. A cosmic vista, viewed on a clear night well away from
urban light pollution, can turn anyone into a philosopher.
Steve Holmes, a chemist, long-time hobby astronomer, and local president of the Royal
Astronomical Society of Canada, shared his insights gleaned from over a decade of
astrophotography. His presentation included a number of brilliant, razor-sharp images of
planetary nebulae, galaxies, supernovae, and other celestial objects steeped in space and time.
Of all the concepts in astronomy, the most challenging to grasp are those of time and distance.
At the cosmic scale, distance is measured with a unit that embodies both: the light-year, which
takes advantage of the fact that light travels fast (300,000 kilometers per second, or 1 million
km/hr, or about 10 trillion kilometres per year).
For example, the nearest star (Proxima Centauri) is 4.24 light-years away, meaning that it is 42
trillion km away from us. Other stars are thousands of light-years distant, for example Eta
Carinae at 7500. These huge distances help convey the vastness of deep space and the size of the
universe. They also provide a perspective on time: if we were to observe Eta Carinae to explode
tomorrow, in reality the event would have taken place in 5488 BC.
Steve’s long-time passion for astronomy grew into astrophotography: he delighted in seeing the
night sky, but he wanted to be able to record his observations so that they could be compared
with those taken at other times, as well as shared with other observers.
Many celestial objects photograph well, but supernovae are among the most breathtaking. A
supernova is the end-of-life explosion of a large star. This explosion occurs within a matter of
seconds, and appears as a major brightening that fades out over several weeks or months. The
event sends out a high-speed shock wave that is visible as a supernova remnant: a shell-like dust
cloud that expands into a nebula—a very attractive photographic opportunity. Supernovae are
also notable because they form elements heavier than iron, which means that the essential-to-life
trace elements we have here on Earth came from either the Big Bang (which created the entire
universe 13.75 billion years ago), or from supernovae after that.
Because the night sky is orders of magnitude less bright than daylight, deep-sky photography
requires a much longer exposure time than standard photos of our moon or other solar-system
objects. Steve uses a digital SLR camera mounted to his telescope (Skywatcher Equinox 66).
Exposure time is typically 10 minutes, during which time the Earth’s rotation, combined with the
magnification of the telescope, results in a significant motion of the object of interest. For this
reason, an equatorial-type mount is essential.
Steve notes that the motorized mount and associated automated high-precision tracking
equipment add substantially to the total system cost. Excluding the camera, this type of
telescope-and-mount system can cost $2500, with only about $500 of that accounted for by the
telescope itself.
When choosing the actual telescope, the relative aperture (f-stop: ratio of lens focal length to the
entrance-pupil diameter) is a key parameter—just as it is in standard photography. Each step in fstop (e.g. from f/2.8 to f/4) corresponds to a halving of pupil area (aperture), which means half
the light per pixel on the camera’s image sensor and thus a doubling in the needed exposure
time—thereby halving the number of shots that can be taken per hour.
Longer exposure times also make atmospheric turbulence, which causes stars to move slightly
when viewed through a telescope, more problematic. Aside from choosing a viewing location
with little atmospheric turbulence (e.g. a remote mountaintop), the most practical solution is to
avoid having too long a focal length (too much magnification).
As for the camera itself, Steve notes that DSLR cameras for standard photography come with a
very aggressive red filter in front of the sensor, designed to filter out all wavelengths longer than
600 nanometres. This removes the long-red wavelengths (600-750nm), which are still visible to
humans but which would not focus in the same way as the shorter wavelengths that predominate
in everyday photography. But for astrophotography, such a filter removes the deep-red 656nm
hydrogen alpha line, which the dark red of nebulae.
So, the astrophotographer has three options: remove the red filter manually (and replace it with a
dedicated infra-red filter; a secondhand DLSR camera can be bought cheaply, modified, and
used only for astrophotography), purchase a dedicated astrophotography camera (with an
infrared-only filter and with often with specially cooled image sensor; cost is $2000 and up), or
simply live with the constraint imposed by the original filter (i.e. no richly red nebulae, but can
still photograph everything else very well).
The number of photographic opportunities provided by the night sky literally is infinite: no one
knows how far the universe extends, nor how many objects are in it. Although the
aforementioned practical aspects limit how deeply we can probe the cosmos with portable
telescopes, the unfathomable vastness of the universe lends so much perspective and context to
our lives that it makes humanity seem insignificant.
This space-time vastness also gives reason to appreciate how special Planet Earth is. When one
considers the narrow set of conditions necessary for life (temperature, liquid water, nutrients,
protection against ionizing radiation, etc.), then it becomes clear that there is an extremely low
likelihood of finding another planet which we could colonize. Astronomy and photography,
separately or combined, can do much to foster an appreciation for the natural wonders around us.
Profound thanks to Steve Holmes for highlighting the often-overlooked natural beauty of the
deep night sky, and for sharing his astrophotographic insights.