Cerro Paranal versus the sites near La Serena

After the Europeans chose to site the VLT up north near Antofagasta
at Cerro Paranal, away from their "base" at La Serena, I thought it
was
established that conditions up near Antogagasta were superior to
those
at the three existing sites just north of La Serena

But I see that the Giant Magellan telescope plans on siting at Las
Campanas. Is there a story there? Is Paranal less-good than it was
thought? (There is some discussion to that effect at the TMT
website.)

Bob Ayers, just curious

wrote:
But I see that the Giant Magellan telescope plans on siting at Las
Campanas. Is there a story there? Is Paranal less-good than it was
thought? (There is some discussion to that effect at the TMT
website.)

In the following I'm speaking strictly privately, not for the
organization I work for.

The short answer to your last question is yes. There has been a
degradation of the observing quality, most notably the seeing, at
Paranal after 1998 with respect to measurements during the site
selection process.

A lot of details can be found on ESO's astroclimatology pages for
the two observatories,
http://www.eso.org/gen-fac/pubs/astclim/lasilla/ for La Silla
and http://www.eso.org/gen-fac/pubs/astclim/paranal/ for
Paranal.

While the cumulative seeing distribution at
http://www.eso.org/gen-fac/pubs/astclim/paranal/seeing/singcumul.html
suggests that Paranal has better seeing than La Silla more recent
measurements
http://www.eso.org/gen-fac/pubs/astclim/paranal/seeing/singstory.html
see no difference between the two sites or even favor La Silla.

That being said, precibitable water vapor and cloud cover
statistics still clearly favor Paranal.

I don't think anybody can answer the question whether the VLT
would still be built on Paranal if the decision were made with
present day knowledge. Such a decision of course also has to
take into account the enormous logistics and costs that come with
operating two spatially separated observatories.

Concerning your original question about the site for the GMT one
also has to look at available sites, in addition to
infrastructure and astroclimatology. Cerro Paranal is a singular
mountain in its area and one cannot simply go the next hill and
build an observatory there. The situation is quite different in
the La Silla/Las Campanas area where one can go to essentially
any peak and get good observing conditions. In fact, La Silla was
chosen without any site monitoring campaign ever made on La Silla
itself.

Cheers,
Jo:rg

--
Fortune cookie of the day:
Accent on helpful side of your nature. Drain the moat.

wrote:
After the Europeans chose to site the VLT up north near Antofagasta
at Cerro Paranal, away from their "base" at La Serena, I thought it
was
established that conditions up near Antogagasta were superior to
those
at the three existing sites just north of La Serena

But I see that the Giant Magellan telescope plans on siting at Las
Campanas. Is there a story there? Is Paranal less-good than it was
thought? (There is some discussion to that effect at the TMT
website.)

Bob Ayers, just curious

It is not really a joke that observatory sites selected on the basis of
multiyear site surveys turn out to have been having a 2-sigma run of
unusually good weather then - it's almost guaranteed that statistical
fluctuations give you this result. (Hence the desire to look at
rainfall and runoff records over centuris if possible, but for the
Atacama the answer in most years is "none" for rainfall...)

The other factor is that the GMT partners already operate four telescopes,
including the twin Baade and Clay 6.5m Magellan instruments, at La
Campanas, and thuis have all of their own support infrastructure
inplace. The GMT is probably the most bare-bones of the giant telescopes,
with (IIRC) no national agency as a partner, so they are particular
interested in shaving costs.

Bill Keel