The Ultimate Location for Amateur Astronomy?
The Canary Islands are well known to the professional community as one of the worlds finest observing locations, but how does the observing fair for the amateur observer at sea level?.
volcanic island of Tenerife, is dominated by the 3718m (12’200ft) snow-covered
peak of El Teide – the highest mountain in Spain..
The Canary Islands archipelago, located off the coast of NW Africa, has long been recognised as a site with excellent qualities for Astronomy. Ever since the first atmospheric seeing monitoring campaigns during the 1970s, it became clear the volcanic Mountain slope sites of El Teide and La Palma, both located at 2400m elevation were superb locations for the situation of major observatories. The many clear nights, and steady seeing allowed, and continue to allow scientific studies and experiments to be conducted using telescopes as large as the new GTC 10 metre reflector, currently under construction at La Palma.
But how does the observing fair back down at sea level on these Islands?. Do the sun drenched towns and villages also make excellent locations for an amateur observatory?. I spent 6 months, from September 02, to March 03 on the Southern coast of the largest Island, Tenerife, using my 28cm reflector, and 1.5 months on the North side of the island at Puerto de la Cruz. During this time I kept constant and detailed records of the prevailing conditions, and made several interesting findings, which I report in this article.
Tenerife - The Island & its weather
Many readers will
be familiar with Tenerife. Not just being the largest of the Canary Islands, it
is also visited by millions of tourists from the UK and Europe each year, to
enjoy the year round sunshine. The island was forged over millions of years
through volcanism, and at the island centre lies the amazing landscape of the
Parque Nacional de Las Canadas – a huge caldera ~20 km across, with the 3718m
peak of El Teide at its centre. It
lies 180 miles off the coast of North Africa, but differs greatly, both
geographically and Climatically to its neighbour.
steep local orography creates many micro-climates across the island, and one can
almost experience all four seasons in a trip from sea level, into the mountains.
The northern windward side of the island is in general cooler at sea level, and
suffers much more from clouds due to the frequent onshore winds, and frequent
The weather of Tenerife (and the other islands) is governed by the North East trade winds, which prevail over the islands most of the year, and especially during the summer months. This trade winds regime is partially responsible for the presence of a near permanent temperature inversion layer over the area, which is actually a commonplace occurrence across the Earth’s subtropical regions. An “Inversion” refers to the opposite state the atmosphere normally takes (with warmer air located nearer the ground, and cooler air higher up, giving rise to convection.) An Inversion occurs when cooler air is located near the ground, and warmer air, above which suppresses local convection. This effect across the Canary Islands is caused by the trade winds blowing at sea level, and warmer dry air, subsiding above.
Fig01a: A diagram showing the two different states of the atmosphere, and the state is takes when a temperature inversion occurs.
vary very little throughout the year across the islands; at least at sea level
locations across the South side of the island. The North is generally cooler and
wetter than the South; especially up on the mountainside. At altitudes of around
1000m and up snowfall can occur along the northern coast during winter, and the
Teide itself often presents a wonderful snow covered appearance during the
280 clear nights per year are experienced on the South side of the Island
(rather less on the windward northern side), and weather often consists of well
broken fair weather clouds, rather than blazing clear blue skies most days.
Relative Humidity is often high (65-90%) throughout the year, and rainfall is
low, with around 25-30mm in the wettest month (November.) The sea temperatures
also vary little throughout the year.
importance of the Temperature inversion is a major factor in the islands being
chosen as a major site for professional astronomy. As already mentioned the
existence of a temperature inversion layer is a characteristic of subtropical
regions, and in the Canary Islands it is registered around 90% of the time. The
efficiency of the inversion layer in separating the marine boundary layer from
the free troposphere is clear on the humidity profile, where typically 60% of
the humidity is situated below the inversion and 20% above it. The low humidity
is highly desirable to professionals, as study into IR wavelengths can be
conducted. The inversion suppresses local convection, which is clearly visible
in the presence of a broken stratocumulus cloud layer whose top is just below
the base of the inversion.
low altitude of the inversion layer (typically around 1700m) means a significant
portion of the land areas of Tenerife (and the neighbouring island of La Palma)
lie well above the inversion layer, making them ideal locations for
Observatories as the clouds and humid air remain trapped below. As studies have
shown conducted at La Palma’s Roque de la Muchachos (ORM) observatory, and
Tenerife’s Observatorio del Teide (OT), the atmospheric transparency and
astronomical seeing is frequently superb. I can say from experience, having
observed from the OT site at 2387m, and also the Las Canadas site at 2200m, the
sky transparency and darkness is astonishing, with stars visible down to 7th
magnitude with the naked eye. Other rarely seen sites from amateur locations
such, as the zodiacal light is commonplace from these locations.
more importantly to the amateur observer, is the observing experience even close
to comparable back down at sea level?.
my observatory, I chose a small,
affordable apartment, with a good sized balcony, located in the small town of
Costa del Silencio, on the very Southern coast of Tenerife (28 02 N, 16 37 W.)
The apartment itself was located a mere ~0.5km from the coast, and the facing in
a SE direction. The balcony proved a sturdy location for the telescope, where it
was left permanently set-up, and polar aligned.
Directly opposite of my apartment, was a Scottish bar, which provided almost nightly observing “entertainment” – especially during the weekly Karaoke night!. Despite this “distraction”, no problems were ever caused during observing, and the light pollution was bearable. The primary task for the telescope during my stay was for high resolution CCD imaging of Jupiter and Saturn during their 2002-03 apparitions for the BAA, ALPO and IOPW observing programs. I also intended to begin observing Mars at the start of the great 2003 perehelic apparition, since its southerly declination wouldn’t prove problematic at the 28 deg N latitude of my site.
The author’s balcony observatory in the town of Costa del Silencio, Tenerife,
from where 110 nights of Jupiter observations were obtained by the author during
his 6 month observing campaign.
Observations of Sky Transparency
of sky transparency from my observatory were actually conducted from the roof of
my apartment building, which enjoyed darker skies than the location of the
telescope. During the 7 months of observations, I would initially report that
the sky transparency is frequently good and occasionally excellent. But a
well-known weather phenomenon of the islands could sometimes reduce the
transparency severely – and does so around two to three times each year.
located close to the huge Saharan desert comes with a price. Several times each
year the islands are engulfed in large dust storms, reducing the transparency to
a level one would more associate with thick fog in the UK!. These huge dust
clouds erupt from the Libya/Algeria regions of the Sahara, and can be
transported across the Atlantic Ocean as far away as Florida. Even the high
altitude of the observatories are not spared, as the dust is “kicked up”
some 6km into the sky. These “calima” as the locals call it, usually last
between 3-5 days, and are also accompanied by a sharp rise in temperatures,
reaching 37 deg C (100 deg F) at times.
above the inversion, such as the ORM indicate from past data collected over the
last 20 years, around 3% of nights each year are lost due to dust, though having
observed from above the inversion during such an event, it does seem more dust
is concentrated below the inversion than above it. During the notable dust
events I witnessed on the Island (April, Oct 02 & Feb 03) it usually took
around 5-7 days for the transparency to return to normal. During the Feb 03 dust
event, I observed from 2200m altitude, and at sea level on the same night at
around the peak of such an event and the different in sky transparency was quite
remarkable, with around 3 magnitudes loss occurring at sea level, but much less
at the high altitude site.
The photograph taken at Puerto de la Cruz by the author on October 5th,
2002 show just how severely transparency could be reduced by Saharan dust
below is the data collected during the October 02’ – March 03’ period:
nights available during the period.
nights of observations (79% of nights were clear to partly clear.)
October 2002 (88% of nights clear.)
– 3 weeks spent at Puerto de la Cruz.
November 2002 (85% of nights
December 2002 (59% of nights clear.) Month of most consistent good
January 2003 (N/A.)
February 2003 (100% of nights clear.)
2003 (100% of nights clear.)
No figure is quoted for January as I was away overseas for 7 days of the month.
Overall Summary reveals that of 140 nights available for observations from the
site, an impressive 110 nights (79%) were clear enough to conduct observations.
This would indicate, that on average one could expect to enjoy around ~280 clear
nights every year from the site.
it should be noted that many nights were of excellent transparency. Stars down
to magnitude 5.5 were frequently noted toward the Zenith, and on most
occasion’s stars as low as 4 degrees altitude (such as Achernar, Gamma Crucis)
could easily be seen with the naked eye. On one occasion, Alpha Centauri was
observed with the naked eye as it transited the meridian rising 1.5 degrees
above the horizon at declination –61 degrees.
figures probably place Tenerife South among the best amateur locations not only
for the number of clear nights, but the percentage of those nights that are of
Observations of Atmospheric Turbulence
a Planetary observer, this area was of great concern to me, and it was rewarding
to be able to identify distinct and predictable patterns in the seeing. Without
doubt, my Tenerife site was the most predictable location I have observed from
with regard to seeing conditions. It should be noted, that the lee side location
of my site from the outset certainly isn’t an ideal choice for consistent
seeing, however the conditions were not at all different from those experienced
on the windward side of the island during my 6 week stay there during Sep-Oct
The following patterns were noted:
“TRADE WIND SEEING”:
major factor in the seeing conditions at sea level on Tenerife is a phenomenon I
term “trade wind seeing”. When the edge of a high pressure system was
present over the islands, bringing the blustery NE trade winds, seeing was
almost always poor to extremely poor. This occurred fairly frequently it must be
said, and it was possible to identify why poor seeing occurred during these
times by comparing my data with that collected at the weather station at the OT
at 2387m. During these times when the trade winds blow at sea level (typically
at 32km/hr NE wind) the temperature inversion is strongest. The air above the
inversion (monitored by the OT weather station) often shows at these times very
low Relative Humidity (~5%) and high wind speeds from the NE or NW (typically
between 30 – 65km/hr and sometimes more.) Due to these winds of different
temperature and speed “rubbing” past each other, this almost always creates
terrible “fast” seeing due to fast moving wind shear turbulence (just like
what observers seeing when the Polar Jet is overhead at more northerly
latitudes.) This was easily noted when Jupiter was defocused toward an extra
focal direction, and it was possible to focus on a layer of wind shear
turbulence running across the image from usually a NE or NW direction. This
typically rendered at least ~5 nights each month totally unusable for any kind
of serious Planetary or Deep Sky observations. I estimate the FWHM seeing was on
the order of 10 to 15 arc seconds during the worst times!. At these times, the
best seeing conditions are experienced above the inversion at the observatories,
due to the strong suppression of convection from below, and laminar flow off the
also worth noting that when the sea level winds continued blustery, but the high
altitude winds were light (below 10km/hr) seeing conditions improved to usually
fair, and sometimes good. This I think indicates the high altitude winds had a
major affect on the seeing experienced at my location, probably due to them
passing over the mountain peaks located 30km to the north, creating very bad
lee-side turbulence above the inversion.
however was also noted during some nights in October 02 from the windward side
of the island at Puerto de la Cruz (even when the winds were onshore above the
inversion.) However I suspect the nights of very bad seeing here to have been
caused by the location of Puerto de la Cruz when the trade winds were present
from the ENE, being downwind of the Anaga mountain range than runs along the
north-eastern part of the Island, and located directly NE of Puerto.
An image obtained by the author during one of the many nights affected by
“trade wind seeing”. Note that the Jovian disk appears very distorted due to
the fast moving turbulent flow from the upper right. During such nights, serious
observations were impossible.
PRE COLD FRONT/LOW PRESSURE SEEING.
times when low pressure systems were present, sometimes with an approaching cold
front the seeing was almost always good to excellent. This was due to the
breakdown of the inversion layer due to the absence of the high pressure
subsidence, and the sea level and high altitude winds clocking to a W or SW
direction, bring air of more uniform temperature directly off the ocean across
my site, without having to cross the mountain peaks 30km to the North. At least
2-3 nights of superb seeing were experienced under these conditions during
December 2002. These conditions are mostly likely to occur during the winter
months due to the frequent breakdown of the trade wind regime. Overall, December
2002 had the most consistently good seeing, and also was the month where the
trade winds prevailed least.
Also during such times, there were often many broken cumulus clouds present. It was also my observation that as these clouds passed over the Planetary image, this often caused it to stabilise for a few seconds. Seeing was notably worse as the cloud boundary passed over the image, and on one occasion, the seeing abruptly deteriorated from good to very poor in a matter of seconds, and did not recover
THE VERY BEST SEEING CONDITIONS.
very best seeing during the period was experienced in February 2003, when at
least 3 nights of near perfect seeing conditions occurred. February 19th,
is the best night I have ever experienced in terms of atmospheric seeing. Extra
focal star images at over 300x were perfectly still, and at 600x, very
occasional extremely slow flicker. The Jovian Moon Ganymede showed distinct
surface markings, and Jupiter itself at 440x revealed a view close to what the
best CCD images reveal.
conditions were again (amazingly!) repeated a few days later on February 22nd,
when again extra focal star images presented a perfectly still pattern. February
20th was also excellent, but not to the degree of the 19th
and 22nd. So what caused such remarkable conditions to prevail during
rare occurrence for the Canary Islands occurred on this date. A large, tranquil
high pressure system was situated directly over the islands, meaning the trade
winds were absent, and also the high altitude winds were still. Also the low
level inversion layer remained present at around 1700m altitude, as was
identified by the strato cumulus clouds observed on these nights along the
mountain side. Seeing conditions were excellent from Sunset, late into the night
of the 19th and 22nd . It would seem, as noted by many
observers in the past, that high pressure centred over your site, does spell
very good news for seeing, be it a volcanic island or city location.
From Tenerife however, either the presence of a low pressure system, or high centred over the islands I consider very favourable as indicators of good seeing, as is the absence of strong winds above the inversion. Also of important note is that though Jupiter and Saturn passed almost through the Zenith from Tenerife, even on those worst nights I have mentioned, the images even at 80 degrees altitude was terrible. Infact, rare was the night where I obtained stable views and images with Jupiter less than 50 degrees altitude.
An image of Jupiter obtained by the author on the night of February 19th,
2003 under almost prefect seeing conditions. A considerable amount of fine
detail could be seen through the eyepiece, approaching that revealed in this
summary of the atmospheric turbulence as observed from my Tenerife site is as
follows. All judgments of the seeing were made on stars located over 50 degrees
altitude, using powers of 300x – 600x on a 28cm reflector.
Atmospheric Seeing results:
nights of observations were possible.
nights of observations were made (79%)
nights (59%) were of fair to excellent seeing (Pickering 5 – 10 ratings.)
nights (41%) were of very poor to poor seeing (Pickering 0 – 4 ratings.)
17 nights (16%) were of excellent seeing (varying between Pickering 7-10
13 nights (%) were of extremely poor seeing (varying between Pickering 0-3
ratings) and rendered unusable for observing or imaging.
10 nights (9%) were of highly variable seeing conditions.
retrospect, I do not consider my choice of site on Tenerife to have been the
best choice (primarily due to finance limitations.) Having observed from the
windward, leeward, and high altitudes areas of the Island, I did not consider
either the North or South of the island especially good locations for
consistently good astronomical seeing conditions (though more data is needed on
the North.) However, as is well documented the areas above the inversion layer
enjoy very consistent steady seeing conditions.
Some fine thoughts...
I consider my observing sites at Tenerife to have been rather disappointing
sites for atmospheric seeing conditions at sea level (based on my experience
having observed from two different sites on the island.), especially when
compared to other similar maritime locations around the globe where data is
available (such as South Florida, Equatorial Africa etc).
I did not obtain a full 6 months data from the Northern side of the island, my
initial 6 weeks there did not show promising results, and I do suspect it also
to be a rather “average” location. However, I only observed from two
different locations. With much more care (and money!) in selecting a site, I
believe perhaps there are some good locations on the island, though I do believe
these to be confined to the windward side of Tenerife. The far north-eastern tip
of the island between Punta de Hidalgo and
El Draguillo are probably the best sea level locations with regarding to
consistent good seeing – at least in Theory.
Fig05: A colour coded map of the island showing the areas most likely to benefit from better atmospheric seeing (Green) and more inconsistent seeing (Red.) Also indicted are the typical wind directions of the sea level trade winds, and higher altitude winds. From this map it's clear the ideal location is where the sea level and high altitude winds prevail directly onshore, without crossing peaks of the island itself. Such area are confined to the Northern side of the island.
problem lies in that the seeing conditions there are unsurprisingly not
consistent on the lee-side of the island, despite the stable weather. The Trade
wind seeing renders at least 4-5 nights each month unusable, and I would predict
this figure to sharply increase (and possibly double) during the Summer months,
when the trade winds and attendant high pressure system to the NW are present
almost 90% of the time. During the winter months however, when the trade winds
are present only 50% of the time, or less, seeing conditions are prone to be
much more favorable in the South of the island. As an example, 10 of the 18
nights of observations during December seeing was good to excellent. It seems
though; the very consistent seeing conditions of the Canary Islands are confined
to the land areas above the inversion layer, which are not subject to the
atmospheric effects experienced at sea level.
attraction to amateur observers (especially those from the UK and Europe) is the
opportunity to observe from dark mountain locations (which truly are among the
darkest and most transparent skies anywhere in the world) that are only a 3 to 4
hr flight from their homelands. Back down at sea level on the islands however,
most are likely to find it rather disappointing, especially with the ever
increasing number of hotel complexes spreading over the islands, gradually
swallowing the night sky, though this is much less true on the Northern side of
the Island where many dark sites still exist.
I think perhaps to say Tenerife overall is a poor location for good seeing
conditions at sea level is perhaps unfair. The data I collected, though without
question reveals the mediocre properties of the South of the Island, I believe
perhaps more research would be worthwhile along the Northern side of the island
to identify the patterns that exist there, as 6 weeks worth of data really just
isn’t sufficient to make any concrete judgments.
I highly enjoyed my seven-month stay on Tenerife. Above anything I suddenly
realized just what an amateur could accomplish in a location blessed with so
many clear nights, even if the seeing wasn’t so great most of the time.
However those handful of nights where the atmosphere stood still and allowed
extraordinary views and images, I will remember for the rest of my days, and I
consider those few precious nights worth the many hours spent at the telescope.
Finally, I hope to return to Tenerife in the not to distant future to resume a campaign on the Northern side of the Island. This I feel would complete the data I have collected, and give a clearer picture of really where the finest sea level locations are on the magnificent island of Tenerife.
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