Temperature/cooling etc

Could someone explain (or point me to a website which explains) why I keep
reading about temperature equilibration, cooling, and so on? Is this temperature
of the mirror? Or some electrical parts?

Thanks,

Boggis

--
-Take out Ron to reply-
Votre mère était un hamster et votre père a l'odeur des baies de sureau

Until the mirror and the surrounding air are at the same temperature
(thermal equilibration), some of the mirror's figure will be lost to
thermal effects.

--
To reply, remove the "z" if one appears in my address
"Dr. Boggis" wrote in message
...
Could someone explain (or point me to a website which explains) why I
keep
reading about temperature equilibration, cooling, and so on? Is this
temperature
of the mirror? Or some electrical parts?

Thanks,

Boggis

--
-Take out Ron to reply-
Votre mère était un hamster et votre père a l'odeur des baies de sureau

"Dr. Boggis" wrote in message
...
Could someone explain (or point me to a website which explains) why I keep
reading about temperature equilibration, cooling, and so on? Is this
temperature
of the mirror? Or some electrical parts?

Basically, moving a piece of glass from a warm environment into a cool
environment means that thermals will swarm around the surface causing tube
currents. You have to wait for the glass to be near the same temperature as
the ambient air before the thermals stop messing up the images. The larger
the piece of glass the longer this takes.

I once stored my scopes in a shed and didn't have nearly the cooling times
that I have to deal with now that I store them in a garage.... I didn't
realize that the (attached) garage wouldn't be as close to ambient
temperatures as a shed.

There's a roll-off roof observatory in my future.

Stephen Paul

In article ,
"Stephen Paul" wrote:

"Dr. Boggis" wrote in message
...
Could someone explain (or point me to a website which explains) why I keep
reading about temperature equilibration, cooling, and so on? Is this
temperature
of the mirror? Or some electrical parts?

Basically, moving a piece of glass from a warm environment into a cool
environment means that thermals will swarm around the surface causing tube
currents. You have to wait for the glass to be near the same temperature as
the ambient air before the thermals stop messing up the images. The larger
the piece of glass the longer this takes.

OK, that makes sense - it's not any sort of warping/expansion of the mirror
itself, but the air in the tube which needs to equilibrate, is that right?

I'm assuming that storing a telescope in the open air or some sort of unheated
structure (and it not being stolen or crapped on by birds) would mean you
wouldn't have to worry about this?

--
-Take out Ron to reply-
Votre mère était un hamster et votre père a l'odeur des baies de sureau

In article 0bbxb.8636$9O5.8088@fed1read06, "Jan Owen"
wrote:

Until the mirror and the surrounding air are at the same temperature
(thermal equilibration), some of the mirror's figure will be lost to
thermal effects.

Right, gotcha. More questions:
Are they microscopic thermals? Or bigger air currents?

--
-Take out Ron to reply-
Votre mère était un hamster et votre père a l'odeur des baies de sureau

There may be tube currents, but beyond that, the mirror's figure may not
match specifications, preventing it from achieving design performance
levels, regardless of tube currents, until thermal equilibration is
reached.

--
To reply, remove the "z" if one appears in my address
"Dr. Boggis" wrote in message
...
In article 0bbxb.8636$9O5.8088@fed1read06, "Jan Owen"

wrote:

Until the mirror and the surrounding air are at the same temperature
(thermal equilibration), some of the mirror's figure will be lost to
thermal effects.

Right, gotcha. More questions:
Are they microscopic thermals? Or bigger air currents?

--
-Take out Ron to reply-
Votre mère était un hamster et votre père a l'odeur des baies de sureau

"Dr. Boggis" wrote in message
...
In article ,
"Stephen Paul" wrote:

"Dr. Boggis" wrote in message
...
Could someone explain (or point me to a website which explains) why
I keep
reading about temperature equilibration, cooling, and so on? Is this
temperature
of the mirror? Or some electrical parts?

Basically, moving a piece of glass from a warm environment into a cool
environment means that thermals will swarm around the surface causing
tube
currents. You have to wait for the glass to be near the same
temperature as
the ambient air before the thermals stop messing up the images. The
larger
the piece of glass the longer this takes.

OK, that makes sense - it's not any sort of warping/expansion of the
mirror
itself, but the air in the tube which needs to equilibrate, is that
right?

Both.


I'm assuming that storing a telescope in the open air or some sort of
unheated
structure (and it not being stolen or crapped on by birds) would mean
you
wouldn't have to worry about this?

It will help, considerably, perhaps, but unless the temperature is the
SAME as the air where you set up, *some* equilibration will still be
necessary. But it will go MUCH faster this way. Especially if you help
it along with fan(s).


--
-Take out Ron to reply-
Votre mère était un hamster et votre père a l'odeur des baies de sureau

On Wed, 26 Nov 2003 16:10:52 -0800, "Dr. Boggis"
wrote:

OK, that makes sense - it's not any sort of warping/expansion of the mirror
itself, but the air in the tube which needs to equilibrate, is that right?

It's both. If any of the optical elements, reflective or refractive, have
thermal gradients then their figures will be wrong. It is unfortunate that glass
is a poor thermal conductor, as this means that it can take a long time to
stabilize. A solid aluminum mirror would be wonderful if someone could figure
out a good way to figure and polish it in the first place. Even without
gradients in the optics, though, temperature differentials can lead to air
currents which impact the quality of the image.


I'm assuming that storing a telescope in the open air or some sort of unheated
structure (and it not being stolen or crapped on by birds) would mean you
wouldn't have to worry about this?

It does help. But such a structure may still have an inside temperature quite
different from the outside air. Also, in some places the air temperature is
changing rapidly during the early evening when you might start observing. Some
scopes may have difficulty keeping up with this change. That's why really large
scopes have an active temperature control system that maintains the temperature
all the time.

_________________________________________________

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

Or launch it into space!! I keep mine in polar orbit.
(evil grin)


"Jan Owen" wrote in message
news:vubxb.8638$9O5.1914@fed1read06...

"Dr. Boggis" wrote in message
...
In article ,
"Stephen Paul" wrote:

"Dr. Boggis" wrote in message
...
Could someone explain (or point me to a website which explains) why
I keep
reading about temperature equilibration, cooling, and so on? Is this
temperature
of the mirror? Or some electrical parts?

Basically, moving a piece of glass from a warm environment into a cool
environment means that thermals will swarm around the surface causing
tube
currents. You have to wait for the glass to be near the same
temperature as
the ambient air before the thermals stop messing up the images. The
larger
the piece of glass the longer this takes.

OK, that makes sense - it's not any sort of warping/expansion of the
mirror
itself, but the air in the tube which needs to equilibrate, is that
right?

Both.


I'm assuming that storing a telescope in the open air or some sort of
unheated
structure (and it not being stolen or crapped on by birds) would mean
you
wouldn't have to worry about this?

It will help, considerably, perhaps, but unless the temperature is the
SAME as the air where you set up, *some* equilibration will still be
necessary. But it will go MUCH faster this way. Especially if you help
it along with fan(s).


--
-Take out Ron to reply-
Votre mère était un hamster et votre père a l'odeur des baies de sureau

Hi there Boggis. You posted

Could someone explain (or point me to a website which explains) why I keep
reading about temperature equilibration, cooling, and so on? Is this temperature
of the mirror? Or some electrical parts?

It is mainly the temperature of the optical components, as the rest of the
telescope tends to cool more rapidly than the glass making up the lenses and
mirrors. In a Newtonian for example, the primary mirror can hold a lot of
heat, and if taken out into a much cooler environment, can end up generating
currents of slightly warmer air in the area just above the mirror suface.
This can cause "tube currents" which tend to mush-up the view at moderate to
high power. The primary may also be distorted from its normal figure slightly
as it cools down, but I think the tube current problem is more significant.
Thus, a cool-down interval must be observed until the optical components are
very close to the ambient air temperture. Such cooling intervals can be
reduced slightly using a small fan to help blow air past the mirror, as well
as helping to disperse the tube currents. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************