Why do you cool a telescope?

Yes, I agree....it seems that image stability (and
resolution) in my scope tends to improve as the night
goes on, assuming that the 'seeing' remains relatively constant...
I figure about 4-5 hours, with 2 fans running continuously,
to stabilize a 30 inch (2" thick) chuck of 110 lb. pyrex.

(That's why I get out to our observing site, and get set up,
some 2 hours before sunset...allowing lots of time for
mirror cool-down)!
Clear Skies,
Tom W.





Chris L Peterson wrote:
On Fri, 11 Jul 2003 12:49:54 -0400, bwhiting wrote:


Ok, guys, all well and good on deserts...but the original
question was...the title above...and I think we all agree
that a scope....any scope including refractors, must be
brought to, or very close to, the ambient air temperature,
to work at maximum efficiency.....just a reminder as
to the original question of 'cooling' a scope. :-)


Absolutely. However, there are some other interesting related issues. First of
all, what is the actual performance difference between a system where the mirror
is warmer than ambient and one where it is cooler? I suspect the optical effects
are worse in the first case, where convection effects will be more severe. The
main equilibrium effect is more likely to do with figure. Because glass is a
poor thermal conductor, it will be at different temperatures throughout its
volume, and the resulting stress will produce deformation (there are very few
truly zero coefficient of expansion mirrors out there.) Too bad aluminum doesn't
take a good polish- its thermal properties would make it a good material for
mirrors.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Paul Schlyter" wrote

OK, I've never been there so I you can tell that better than me. But
why is the city (town? village?) called Palm Springs?

For the same 'reason' we have rivers in Tucson. ;^) (they're all dry
riverbeds)

Howard Lester
(Don't give me this 'dry heat' crap)

Hey, you guys are giving Paul a bum-rap....
I've never heard him bad-mouth the USA...
You are confusing him with that
Kayak-khan-al\mohameed-Maji-Hameed guy...
that A-rab who was the terriorist from
the mid-east who wished the USA ill-will on 7/4.
Clear Skies,
Tom W.

Mike Simmons wrote:

Dan,

I know your intentions are good but do you realize that your informed
opinions, real data and solid references are out of place in s.a.a.?
Nothing kills a good slugfest between ignoramuses like us like a bit of
solid evidence.

Mike Simmons


Ha ! fooled you mike, typical of a California observatory, I made it all
up. lies lies lies
The only credibility of Mt wilson these days is the outstanding picture of
planet X
from the solar tower.
Dan


P.S. I hope you know I'm just kidding. I read every one of your posts
-- even in threads I'm not interested in!

why is the city (town? village?) called Palm Springs?

Because there are palms (in the nearby mountain canyons) and springs =
(both in the nearby mtns and hot springs in the open desert).

-Florian

I figure that if you're given enough rope, Mr. Schlyter, you will eventually
hang yourself and choke to death on your anti-Americanism...and you're
almost there. It has already been established that you don't know ****
about history, politics, the military and we can now add deserts to the
list. Don't look now, Mr. Schlyter, but your stupidity is showing.

Al


"Paul Schlyter" wrote in message
...
In article ,
John Steinberg wrote:

Paul Schlyter wrote:

Apparently, the North American "deserts" are quite different from
the deserts in Sahara and on the Arabian peninsula.... so I'd like
to ask you if YOU have been in a desert? And then I mean a _real_
desert, not a "desert" with palms and springs and casinos....

Good grief! Now he's even bashing our deserts! Is there no end to
your anti-Americanism?

WHAT?????

Are you having a severe superiority complex, or something? Do you
really require your country to be the BIGGEST and BEST on ABSOLUTELY
EVERYTHING ??? Sorry, but you'll just have to face the fact that the
largest deserts of the world resides outside the US -- and saying so
is merely stating a fact. Claiming it's "anti-Americanism" is just
ridiculous !!!!!!!!!!!!!!

BTW I'll readily admit that the deserts in Sweden are even smaller
than the deserts in the US .... as a matter of fact they're
non-existent, since we don't have any deserts here. So by your logic
I've now been bashing my own country Sweden even more, and thus I
have devoted myself to endless anti-Swedishism.... .... by your
logic, that is, not by my logic...... evil grin


Tell you what, we'll start a fund drive to have you fly over and spend
two weeks in a tent in Death Valley. Then will we'll see what you
think of our deserts.

Probably easier to endure than the middle of the Sahara desert in,
say, southernmost Algeria..... I would of course want to try them
both before passing any definite judgement.


P.S. Saab is owned by? Volvo is owned by? Neener, neener, neener.

I know these car companies nowadays are owned by Ford and General
Motors. And I couldn't care less .... to me, a car is a means of
transportation, not a means to try to boast my ego....

BTW the US cars by which you drive into Death Valley would most
likely fail if you tried to use them to drive across the Sahara
desert... sure, they could probably stand the heat, but the sand
which creeps in the machinery everywhere would make them fail perhaps
after a week or two....

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/

(Paul Schlyter) wrote in message ...
After I had returned home, it took almost a week before I had washed
away that orange-ish very fine sand from my hair.....

At least you got rid of it. I have to *observe* through some of it,
as it blows clear across the Atlantic Ocean.

In northern colder climates you'll get a lot of snow on the roads
in winter, so you'll have to drive cars which plow away the snow
from the roads. In Nouakchott they have a similar problem, but
all year round: they'll have to plow away SAND from the roads
regularly..... there are sand storms there quite often, which moves
around large amounts of sand in an uncontrollable way.....

And for all anyone knows, the American southwest could become that way
any day now. It *has* been like that, perhaps as recently as 400 or
500 years ago. The 1930s saw years of massive dust storms in the
central and western USA--yet, the disturbing geological evidence is
that this "Dust Bowl" was pretty trifling in the big picture.

Do you have similar problems in Palm Springs or Las Vegas? g

Nouakchott is in the middle of a sand box the size of the entire USA.
Is this a contest?

Anyway, the dust storms aren't as bad nowadays; they merely cause
50-car pileups on the highways. If there are any sand plows, i
haven't heard of them, but then i don't live there.

Apparently, the North American "deserts" are quite different from
the deserts in Sahara and on the Arabian peninsula.... so I'd like
to ask you if YOU have been in a desert? And then I mean a _real_
desert, not a "desert" with palms and springs and casinos....

Two words: Colorado River. The American lifestyle is deceptive in
appearance--surely you're aware of that, given your air conditioner
comment. I once read that the Colorado supplies seven states with
water, and is a political hot potato in four; without it, the American
southwest might well be uninhabitable in large numbers. By the time
the Americans are done with it, the Wadi Colorado doesn't have enough
water left to dribble into Mexico.

I've heard that some southwesterners have taken to *spraypainting*
their front yards to give the appearance of the obligatory American
lawn--anything to make it look like they're not in the desert.

I don't know if there really is a spring in Palm Springs, but i'm sure
that the Sahara and Arabian deserts have their share of oases.

Then of course, Palm Springs and Las Vegas aren't too far from a
quaint little oasis we call "Death Valley."

I think two points are being missed he
1) (off-topic) Not all deserts behave the same way.
2) (on-topic) It stays hot overnight in some places. Whether we
choose to call such a place a "desert" is irrelevant.


Clear skies!

--
------------------- Richard Callwood III --------------------
~ U.S. Virgin Islands ~ USDA zone 11 ~ 18.3N, 64.9W ~
~ eastern Massachusetts ~ USDA zone 6 (1992-95) ~
--------------- http://cac.uvi.edu/staff/rc3/ ---------------

On 11 Jul 2003 14:09:58 -0700, (Cousin Ricky) wrote:

I don't know if there really is a spring in Palm Springs, but i'm sure
that the Sahara and Arabian deserts have their share of oases.

On the north end of town, you can hike up into Palm Canyon, with a perennial
stream under the shade of thousands of date palms. One of the richest aquifers
in California lies directly under Palm Springs. The town is right at the base of
Mt San Jacinto, with a height over 10,000 feet. Even though it is on the dry
side, the mountains collect plenty of snow and a lot of that moisture works its
way down to the east side.

Deserts are created primarily by surface conditions. There are many where the
water table is surprisingly high, which means oases in depressions, artesian
wells, and shallow dug wells for property owners.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Paul Schlyter wrote:

BTW the US cars by which you drive into Death Valley would most
likely fail if you tried to use them to drive across the Sahara
desert... sure, they could probably stand the heat, but the sand
which creeps in the machinery everywhere would make them fail perhaps
after a week or two....

Having owned and maintained US and various European vehicles, I can't
say that the better European vehicles had any advantage over US
vehicles. The design and construction is largely the same. As far as
dealing with deserts, liquid cooling is essential, easy brake
maintenance is desirable, frequent filter and lubricant changes (daily
at times) is helpful and frequent inspection of seals a must. In the US
deserts, occasionally it is necessary to drive in water higher than the
wheel hubs for prolonged periods. This is especially hard on the running
gear wherever the car came from. I once wore out the brakes on a
Wagoneer in just five miles of driving in sandy water above the door
sills.

Chuck
--
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons

Cousin Ricky wrote:

Anyway, the dust storms aren't as bad nowadays; they merely cause
50-car pileups on the highways. If there are any sand plows, i
haven't heard of them, but then i don't live there.

The sand doesn't pile up on the roads. The Sonoran Desert has monsoons
and the biggest sand storms are in monsoon season. It is really shocking
to be driving at night and have one hit. No warning. The road ahead
vanishes from view instantly. You pick your way to a "safe " place to
stop knowing it will rain mud soon and allow driving again.

Apparently, the North American "deserts" are quite different from
the deserts in Sahara and on the Arabian peninsula.... so I'd like
to ask you if YOU have been in a desert? And then I mean a _real_
desert, not a "desert" with palms and springs and casinos....

Two words: Colorado River. The American lifestyle is deceptive in
appearance--surely you're aware of that, given your air conditioner
comment. I once read that the Colorado supplies seven states with
water, and is a political hot potato in four; without it, the American
southwest might well be uninhabitable in large numbers. By the time
the Americans are done with it, the Wadi Colorado doesn't have enough
water left to dribble into Mexico.

I've heard that some southwesterners have taken to *spraypainting*
their front yards to give the appearance of the obligatory American
lawn--anything to make it look like they're not in the desert.

When I lived in Tucson back in the 70s, I bought a place with "desert
landscaping." I did not paint the lawn. This had pluses and minuses. My
water bill was quite low. In wet years, the weeds were a problem. The
tumbleweed in the back yard was especially annoying. The worst was a
pair of century plants in the front yard that decided to bloom at the
same time. I filled my F-1 truck at least 4 times carting off that
disaster.

I don't know if there really is a spring in Palm Springs, but i'm sure
that the Sahara and Arabian deserts have their share of oases.

Then of course, Palm Springs and Las Vegas aren't too far from a
quaint little oasis we call "Death Valley."

I think two points are being missed he
1) (off-topic) Not all deserts behave the same way.
2) (on-topic) It stays hot overnight in some places. Whether we
choose to call such a place a "desert" is irrelevant.

Desert areas can have pretty good seeing. I recall about 1 arcsecond as
pretty common in Tucson. It is better on some of the mountains that
stick up out of the desert floor. I don't have any data from other
deserts. The Sonoran Desert, however, is pretty good even at lower
altitudes. This has led to my utter disappointment with Colorado,
however, I occasionally have access to a light bucket 150 miles from
here. About 2 arcseconds is normal.

Chuck
--
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons