importance of altitude

I am leaving at 420m above sea level (1300ft) and have the possibility
to drive within 20mns to a 1100m (3300ft) location

q1: how is the highest point likely the improve observation through an
SCT scope of the 8-10" range

q2: how does it affect the choice between a SCT 8" and a 10" ? when
price and weight are not criteria for the choice. I mean for example at
the highest point both will be very good, therfore no need to go for
the 10", etc...

Thanks.

pascal wrote:
I am leaving at 420m above sea level (1300ft) and have the possibility
to drive within 20mns to a 1100m (3300ft) location

q1: how is the highest point likely the improve observation through an
SCT scope of the 8-10" range

All other things being equal (like that ever happens) you're under about
7% less at atmosphere. You *might* notice a difference. You're better
off scoutting out the darkest spot within 20 minutes rather than
emphasizing the highest. Could be the same spot, but dark is what's
important.

q2: how does it affect the choice between a SCT 8" and a 10" ? when
price and weight are not criteria for the choice. I mean for example at
the highest point both will be very good, therfore no need to go for
the 10", etc...

There Is No Substitute for Aperture
I live at 7200 feet and want to trade my 11" in for a 12.5" (Also SCT to
Dob Newt, so there's more to it than just inches). The Keck telescopes
are at 14,000 feet. Big and high.
Altitude doesn't mean dark, or good seeing. Visit Denver :-PPP

Shawn

Pascal,
What is your lat , long ?

Dan


pascal wrote:
I am leaving at 420m above sea level (1300ft) and have the possibility
to drive within 20mns to a 1100m (3300ft) location

q1: how is the highest point likely the improve observation through an
SCT scope of the 8-10" range

q2: how does it affect the choice between a SCT 8" and a 10" ? when
price and weight are not criteria for the choice. I mean for example at
the highest point both will be very good, therfore no need to go for
the 10", etc...

Thanks.

"pascal" wrote in message
ups.com...
I am leaving at 420m above sea level (1300ft) and have the possibility
to drive within 20mns to a 1100m (3300ft) location

q1: how is the highest point likely the improve observation through an
SCT scope of the 8-10" range

q2: how does it affect the choice between a SCT 8" and a 10" ? when
price and weight are not criteria for the choice. I mean for example at
the highest point both will be very good, therfore no need to go for
the 10", etc...

Thanks.


I can't answer the altitude question but, as for the difference between the
8-inch and 10-inch: All other things being equal, aperture rules. My
8-inch Dob was lost in Hurricane Katrina -- I replaced it with a 12-inch Dob
and, although it is a sonofabitch to haul around, once I set it down and
start observing, it's worth the trouble.

pascal wrote:

I am living at 420m above sea level (1300ft) and have the possibility
to drive within 20mns to a 1100m (3300ft) location

q1: how is the highest point likely the improve observation...

It depends on your local climate. If the extra elevation gets you to
above
a climate pattern, it can make a huge difference. On the California
coast,
for instance, it's the difference betwen having fog on most summer
nights
and being in gloriously clean, dry air.

In the Northeast of North America, however, the extra 3000 feet of
altitude would just mean a higher probability of cloudy weather.

Aside from local weather, 3000 feet of elevation will have a barely
perceptible effect on observing. After all, you'd still have 90% of
the atmosphere above you.

- Tony Flanders

It depends on your local climate. If the extra elevation gets you to
above
a climate pattern, it can make a huge difference. On the California
coast,
for instance, it's the difference betwen having fog on most summer
nights
and being in gloriously clean, dry air.

In the Northeast of North America, however, the extra 3000 feet of
altitude would just mean a higher probability of cloudy weather.

Aside from local weather, 3000 feet of elevation will have a barely
perceptible effect on observing. After all, you'd still have 90% of
the atmosphere above you.

Yes, it is not about altitude but rather seeing. Aren't there even
micro-climate issues that can override any generalizations about local
seeing and altitude? I would do what the big boys do before building an
observatory, i.e. test the specific location for seeing by spending some
time observing.

Dennis

Dennis Woos wrote:
It depends on your local climate. If the extra elevation gets you to
above
a climate pattern, it can make a huge difference. On the California
coast,
for instance, it's the difference betwen having fog on most summer
nights
and being in gloriously clean, dry air.

In the Northeast of North America, however, the extra 3000 feet of
altitude would just mean a higher probability of cloudy weather.

Aside from local weather, 3000 feet of elevation will have a barely
perceptible effect on observing. After all, you'd still have 90% of
the atmosphere above you.


Yes, it is not about altitude but rather seeing. Aren't there even
micro-climate issues that can override any generalizations about local
seeing and altitude?

Most of the seeing is in the ground/telescope layer which is controlled
by cooling to the sky and wind. On Mauna Kea the volcanic cinder is very
effective in creating a low level thermal inversion due the the cinders
high emissivity, low thermal mass, and high surface area. This layer can
produce 1 arc second of seeing at times in the first 30 meters above the
summit.

Not only does the mountain create a wake in the atmosphere like a boat
wake,but also is the source of a cold air that feeds density driven
flows much like water flowing down the slopes in streams.(bad seeing)

The night time thermal inversion creates a stable layer above the
inversion usually accompanied by a low level jet just above the
inversion and produces atmospheric gravity waves (AGWs). These waves
interact with the inversion causing a periodic variation in the seeing
as well as propagate vertically. These waves can grow in amplitude until
they break much like a beach wave causing an increase in the optically
significant turbulence (seeing).

So yes, the micro climate can produce seeing in excess of the "free air"
seeing and complex terrain produces a complex atmosphere.

One needs to create a data set that relates the seeing to wind direction
and velocity. For example one site I work at has the best seeing with
wind from the South at 10 mph. less wind, I see more cooling from the
ground and more wind will start making waves. (my best guess)

Observation during thin clouds can reduce the seeing because it reduces
the cooling as long as the clouds are not due to a upper level
turbulence layer. Because of this it is also handy to look at a local
radiosonde plot. (from weather balloon, National Weather Service, twice
a day 0 and 12 Hrs UT)


I would do what the big boys do before building an
observatory, i.e. test the specific location for seeing by spending some
time observing.

To do this takes at least two setups as you need to compare simultaneous
measurements and not estimate what the other site is doing.

The first thing I do when considering a site these days is to look at
Google earths view of the site to see how it relates to up wind
obstructions. I set down on the site, tip it on its side to get a 3d
view and take a spin.

Twice a day about 1 hour before sunset and 1 hour after sunrise the
transition between the daytime unstable layer and the night time
inversion is a low level thermal neutral event that depending on the
location can almost eliminate the ground layer seeing for several
minutes. If I look at features in a long horizontal path with a
telescope I can tell when the low level seeing is improving and even
though it's bright, see improvements in the view objects like the
sun,moon, and bright planets.

Thus timing of seeing measurements is also an issue.

Have a look at the book:

Boundary layer Climates 2nd Edition (1987)
by T.R. OKE
for a good intro to micro climates.


Dan

Dennis

Dan,

You seem to have studied thouroughly the subject. I leave near Geneva ,
switzerland and Geneva is in a hole surrounded by 2 chains of mountains
about 1200-1600m high, one of them is the "Jura" from which originates
the word "jurassic". During high pressure phases in winter (like now)
it is totally under a stratus which culminates at about 900m and even
when the sky is clear I always suspected that it would be clearer at
1200m since an astronomy club meets up there. I guess I will join them
and check out.

Thanks.

Pascal.

Pascal,

Please send me the latitude and longitude of your present and proposed
site.

Dan

pascal wrote:

Dan,

You seem to have studied thouroughly the subject. I leave near Geneva ,
switzerland and Geneva is in a hole surrounded by 2 chains of mountains
about 1200-1600m high, one of them is the "Jura" from which originates
the word "jurassic". During high pressure phases in winter (like now)
it is totally under a stratus which culminates at about 900m and even
when the sky is clear I always suspected that it would be clearer at
1200m since an astronomy club meets up there. I guess I will join them
and check out.

Thanks.

Pascal.

Pascal,

I live 40 miles from you in Lausanne. From here, I'll say that the
difference in altitude will have a huge effect: it will get you out of
the winter fog we are experciencing (for californians, an equivalent of
your marine layer), which will trap the light pollution (in winter at
least). The winter skies will be extremely clear, whether you climb the
alps or the jura mountains. 1200 m high will get you most of the time
out of the fog. Seing is not usually very good though.

Generally, in switzerland, getting up will get you tremendously better
observing conditions, because you'll get ou of the fog and smog and
generally away from city lights. Heck, I bought a chalet in the Jura
mountains for that very reason!!!

Cheers! good skies are not so far from you,

Pierre, in Lausanne