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?.

By Damian Peach

Fig01: The volcanic island of Tenerife, is dominated by the 3718m (12’200ft) snow-covered peak of El Teide – the highest mountain in Spain

Introduction

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.天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪( z5 U. u7 ?! z! V4 z
It lies 180 miles off the coast of North Africa, but differs greatly, both geographically and Climatically to its neighbour.

Tenerife’s 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 inversion layer.

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.

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Temperatures 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 winter months.

Around 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.

The 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.

The 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.

But more importantly to the amateur observer, is the observing experience even close to comparable back down at sea level?.

My Observatory

As my observatory,天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪! u  K2 n& l5 [! l; i% g; U3 f5 S
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!.
4 \! k+ J1 J" S; `4 y国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材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.

  

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Fig02: 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
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Observations of Sky Transparency

Observations 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.

Being 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.

Locations 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. Despite this however the islands closer to the Sahara (Lanzarote, Fuerteventura) are affected even more so by these events, with visibility being reduced to as low as 50m at times!.  

Fig02a: 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 events.

Presented below is the data collected during the October 02’ – March 03’ period:

140 nights available during the period.

110 nights of observations (79% of nights were clear to partly clear.)

  

• ·天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪" |4 m; @3 ]0 u7 _/ u
  October 2002 (88% of nights clear.) – 3 weeks spent at Puerto de la Cruz.

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  November 2002 (85% of nights clear.)

• ·国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材% U$ h0 L1 o9 l9 F. H
  December 2002 (59% of nights clear.) Month of most consistent good seeing.

• ·
  ' W( E$ s, s9 h, A: n天文家园欢迎您！January 2003 (N/A.)

• ·天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪+ A- A5 y) P" [' Z; C. N
  February 2003 (100% of nights clear.)

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  March 2003 (100% of nights clear.)

Note: No figure is quoted for January as I was away overseas for 7 days of the month.

An 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.

Also, 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.

These 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 excellent transparency

Observations of Atmospheric Turbulence

As 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 02.

The following patterns were noted:

1 “TRADE WIND SEEING”:
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A 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 ocean

It’s 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.

This 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.

Fig03: 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

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2. PRE COLD FRONT/LOW PRESSURE SEEING.

During 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

3. THE VERY BEST SEEING CONDITIONS.

The 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.

These 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 this period?.

A 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.

Fig04: 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 image.

My 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.

Overall Atmospheric Seeing results:

140 nights of observations were possible.

110 nights of observations were made (79%)

65 nights (59%) were of fair to excellent seeing (Pickering 5 – 10 ratings.)

45 nights (41%) were of very poor to poor seeing (Pickering 0 – 4 ratings.)

  

BEST: 17 nights (16%) were of excellent seeing (varying between Pickering 7-10 ratings.)

WORST: 13 nights (%) were of extremely poor seeing (varying between Pickering 0-3 ratings) and rendered unusable for observing or imaging.

VARIABILITY: 10 nights (9%) were of highly variable seeing conditions.

In 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...

Overall, 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).

Though 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国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材3 A7 h( R' L( s: b  V. \' u; m; ^- b. E
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.

The 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.

The 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.

However 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.

Overall, 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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