Art and science meet in my adventures in sketching astronomical objects. Here I detail my travels across Australia with my telescopes, and from home, the people I meet and the technical challenges I encounter. From our closest neighbour, the Moon, to the furthest galaxies. From the seemingly trivial to the mind bending complex. All is fair game for my sketch pad. And continuing the tradition of The Mellish Technique.
Selecting a site for Astronomy Purposes
Selecting a site for Astronomy
Finding an optimal site for the purposes of astronomy is not
just about going bush. As astronomers,
we have a set of requirements that go without question: Dark, minimal impact from light pollution,
access to a good horizon and safe. But
there are other factors that often go unconsidered, and go a long way to
providing us with the optimal site conditions.
One of these factors also goes a long way to effectively eliminating the
bane of amateur astronomers – dew.
As with all things in life, many times it just comes down to
compromise between the ideal with what we have available to us. But being aware of the optimal will go a long
way to helping us find the best site with the locations available to us.
A clue to these site conditions can be had by examining the
location of professional observatories. It
is no accident the location that professional observatories select sites with very
specific factors. Identifying these then
forms the basis for selecting sites that best suit us, regardless of where we
find ourselves. These factors are not independent of each other, but more of a
combination of them.
The ideal site criteria are:
· - Remoteness
· - Altitude
· - Topography and Surrounding Land use
· - Horizon
· - Safety
Remotness essentially refers to being as far away from the
influence of light pollution – a dark sky site.
There are many tools available to us to identify where the darkest skies
are, regardless of where we are.
A simple map is a good place to start. Work out where the urban sprawl finishes, and
we have an immediate guide to where the worst of the light pollution finishes. Maps also identify possible locations for
astro, such as National Parks, Airfields, outlooks, and both public and private
Light pollution maps provide a quick guide to zonal areas
surrounding urban districts. If
transportation is a limiting factor, these light pollution maps can help us
select compromise sites which have the least affected by light pollution.
Below is a light pollution map of south east Australia, showing the inner
city being the worst affected, and conditions improving the further out from
Light pollution map of south-east Australia
Altitude is a triple benefit factor. First, fog.
We are all familiar with those fabulous photos of fog filled valleys
with the ridge lines reaching up above the cloud like floating islands. Setting up in a valley is not a great idea
for this reason. Best situation is to
look to set up above the fog line.
A clear and fog filled Megalong Valley in the Blue Mountains, and the clear ridge tops!
Second, dew. Dew will
follow the path of fog, and be drawn down from the ridge tops down into the
valley below. Also as ridge tops are more
exposed, rarely will there be no breeze, which is of great benefit to keeping possible
dew off our equipment. On one occasion
when I had set upon a ridge top, the wind had stopped and frost started to form
my gear. Then a gentle breeze started to
blow, and to my surprise the frost sublimed away!
Third, reduced air density.
The benefit of being high up means that we leave denser air below
us. This means that moisture is also
reduced, and all this helps keep any light pollution more restricted to lower
altitudes. The reduced air density also
means atmospheric transparency is increased when compared to lower altitudes.
Surrounding Land Use
This refers to the local geography and vegetation. Essentially this is a dew control
measure. What to look for is a situation
where the surrounding topography and vegetation throws up a minimum, and if
possible, no moisture.
Many people would think that a lovely open grassy field
would be ideal. Instead, this is the
worst possible situation!!! Turf means
moisture rich soil, and of an evening, this moisture is released into the air
and is the major source of dew. Rolling
green fields, dairy country, pastures, agricultural land, all this while
seemingly ideal, is actually dew-central.
This is often the situation inside valleys too. Lakes, rivers and the sea will also be major
sources of moisture and hence dew.
What to look for is rocky or sandy land. Ridgetops are typically poor in rich soil,
instead often being sandy or rocky.
Trees surrounding a rocky or sandy clearing is favourable as trees do
not expire as much moisture as turf, and help provide wind breaks to control
the strength of prevailing winds.
Topographic map of Sydney (Google Maps image)
This last factor refers to how open the sky is. The more open the clearing being used, the
lower the treetops, and the clearer horizon that is available. While not necessarily critical nor essential
to have a totally clear horizon, some surrounding trees can be beneficial to
help control any sky glow from urban centres, and act as wind breaks. Particularly on hill tops, these wind breaks
can mean the difference between a site that is cold but manageable, or a site
that is windy and just down right unpleasant to be in as the cold permeates
Site safety should a paramount over everything else. Safety includes from unsavoury people, safe
access to and from the site, of any potential dangerous animals, and the site
itself being free of any potential hazards in the dark. If possible, always go with a mate.
Some examples of
sites that I use
The site that I have most frequented has been Katoomba
Airfield, some 120km west of my home in Sydney.
The Airfield is set on top of a sandstone plateau, 1000m above sea
level. The runways are compacted sandy
clay which retains very little moisture.
The valley either side of the tree-lined runways are up to three hundred
metres below the plateau. While
surrounded by trees, the wide expanse of the runways doesn’t totally stop a
breeze from blowing. Dew has only ever
formed once while I was there for astronomy, and it was due to a moisture rich
easterly wind blowing up the mountains, which also meant I was clouded
out. On another occasion frost formed as
the wind had totally stopped. Curiously
enough, a gentle breeze developed, and the frost sublimed way to leave
everything dry again. As it is also
private and enclosed property, it provides a safe environment to use.
3D map of Katooma Airfield (Google Maps Image)
Looking East along main runway
Looking West along main runway
Victoria Falls Road
On the occasions when the Airfield is not available to us
(which happens if there are military exercises or other community groups are
using the Airfield), our standby site is a ridgetop about 10km further
west. This ridgetop also has deep
valleys on either side of it, and the surrounding vegetation is dry native
shrubbery, typical of this area. The
horizon is spectacular, clear for all of 360°.
The main drawback of the site is that it is totally exposed with no
trees to provide a wind good windbreak – the shrubbery provides some relief,
which is better than nothing. This site
however is in a National Park, so while nocturnal visitors are rare, it is
still open public land.
3D map of Victoria Falls obs site (Google Maps Image)
Looking East from obs site
As mentioned earlier, light pollution maps are a good way to
identify areas less affected by light pollution. Now armed with the above set of site
criteria, topographic maps will help identify those areas of higher elevation. Google Maps also has a topography function for
most regions. It is then a process of
identifying potential areas, and then doing the appropriate research of the
actual feasibility of the potential sites, if these sites are on public or
private land, and approaching the necessary people about gaining access for
using the site for astronomical purposes.
Asking local councils about suitable sites is also a good
way of finding sites. Many times council
people will know of not only other potential sites that you may not have been
aware of, be able to suggest other folks to talk to, and sometimes they may
also know of other astronomers in the area.
Sometimes even approaching private properties can render not
only fantastic sites, but great relationships.
This is how I came to gain access to Katoomba Airfield, just by
explaining my intentions and asking the Airfield manager would welcome a few
amateur astronomers, and the result has been not only access to a great site,
but also a very close friendship.
Remember, site selection is usually a process of compromise. Often few of those ideal conditions are
readily available, nor within easy reach, so we need to make use of sites that
give us the best possible situation. Our
passion for astronomy demands unique conditions. By knowing what site criteria provide the
most ideal conditions, we can then make better decisions on site selection.
All this also applies to finding the best locations within light polluted skies.
I hope this gives you some ideas on where to look for the best locations close to home, or as a distant dark sky site.