APO cooling down

On Sat, 05 Feb 2005 00:51:05 -0500, Stephen Paul
wrote:

RichA wrote:
On 4 Feb 2005 09:03:14 -0800, wrote:


"But what kind of temperature change are you taking the scope into?"

In the summer, the house is 80, the garage is 105dF-110dF and the
outsides are 100dF at setup time
In the winter, the house is 65, garage is 55dF and outside is 50dF at
setup time
In the spring and fall, the house is 80dF, the garage is 90dF and the
outsides are 85dF-ish
Its central Texas.


You poor baby! Try going from a 70 deg temp to -22 with a 12" SCT.
In other words, during a 3 hour observing session, it never reached
bottom and never achieved full resolution.
-Rich

What you need Rich, is a good 102mm ED F9 Doublet to tide you through
the cold months. Cool down is "comparatively" non-existent. ;-)

-Stephen

I know. I do miss my FS-102. My five inch is too heavy.
It was a "cloudy night" project using a tube from a Russian Newtonian
that has a 1/8" wall thickness!
-Rich

I posted this message yesterday, but it didn't appear on SAA, so here
is my second attempt.

Thank you very much for all your replies. So a fast scope will need
longer cooling down than a slow scope, if aperture is equal. A doublet
APO cools much faster than a triplet, because the later has more
glass. Even an 3" or 4" APO needs some time to cool down, though it
can be used immediately with low power viewing.

I searched the Internet and found the following interesting post of
Valery Deryuzhin:

" Our former 6" oil spaced triplets settles within 50min
from +20C to - 12 C while 6" air spaced doublet achromat
settles under same conditions within 35 min.
I think that this give you some info. In the case with air
spaced triplets cool down time will be around 2x vs air-spaced
doublets. But for smaller instruments this ratio can be
not so large."

For the thread please see the following link:
http://tinyurl.com/5qvym

Has anyone experience with the above statement? My next question is
how long will it take approximately for a 5" or a 6" APO to cool down
from room temperature say 70 dF to 32 dF for a doublet and triplet?

Well after this post I had some feedback on other newsgroup about cool
down of the 5" doublet and triplet apo's. It seems that a 5" apo needs
about roughly 40 minutes to cool down from 70 dF to 32 dF.

Thanks.

Lukman

"Enyo" wrote in message
...
A lot depends on your definition of cool down and how you cool down. If
you mean no detectable thermal impact with the star test, it is a lot
longer than having good looking star images or decent looking planets. If
you remove the lens cap, dew shield, diagonal and point the scope down so
the warm air vents out the top you get a lot quicker cooling than if you
leave it in the case. Going from 72 to 20F for my AP130, decent images in
10 minutes. The first thing I do is take the OTA out of the case and let
start cooling while I set up so it gives good images as soon as I am setup.
For critical viewing of planets I need about 40minutes. For an TMB152
under similar conditions the times are in the range of 15 and 90 minutes.



As a data point, my 11" f/16 needs about 45 minutes to settle down assuming
a 20 degree difference in temps between the lens and outside air. The
images are quite degraded at first, but usable after fifteen or twenty
minutes. The tube assembly is at ambient because of the way I transport it,
so this is just the objective equalizing. With the long aluminum tube, I
still notice further temp changes by having to refocus occasionally due to
tube contraction.

Jeff

On Sun, 13 Feb 2005 04:55:40 GMT, "Jeff R. Schroeder"
wrote:


"Enyo" wrote in message
...
A lot depends on your definition of cool down and how you cool down. If
you mean no detectable thermal impact with the star test, it is a lot
longer than having good looking star images or decent looking planets. If
you remove the lens cap, dew shield, diagonal and point the scope down so
the warm air vents out the top you get a lot quicker cooling than if you
leave it in the case. Going from 72 to 20F for my AP130, decent images in
10 minutes. The first thing I do is take the OTA out of the case and let
start cooling while I set up so it gives good images as soon as I am setup.
For critical viewing of planets I need about 40minutes. For an TMB152
under similar conditions the times are in the range of 15 and 90 minutes.



As a data point, my 11" f/16 needs about 45 minutes to settle down assuming
a 20 degree difference in temps between the lens and outside air. The
images are quite degraded at first, but usable after fifteen or twenty
minutes. The tube assembly is at ambient because of the way I transport it,
so this is just the objective equalizing. With the long aluminum tube, I
still notice further temp changes by having to refocus occasionally due to
tube contraction.

Jeff


Most SCT owners are pretty familiar with that, especially since the
contraction of the SCT aluminum tube results in a much greater focus
shift (owing to the primary mirror focusing) than such a long
refractor.
-Rich

For the life of me I cannot understand how someone can afford to heat
their home to 80F and can still afford a high-end large aperture apo.
But can't afford the simple luxury of a secure environment for their
telescope at close to outdoor temperatures?


You'd laugh at people demanding to fly their kites in dead calm
conditions. Or those who wanted to sunbathe in an overcast. Or fish in
the desert. Or cycle up Everest. Or keep their scopes indoors?

Are you guys just plain obtuse? :-)

Chris.B

On 13 Feb 2005 14:54:55 -0800, wrote:

For the life of me I cannot understand how someone can afford to heat
their home to 80F and can still afford a high-end large aperture apo.
But can't afford the simple luxury of a secure environment for their
telescope at close to outdoor temperatures?


You'd laugh at people demanding to fly their kites in dead calm
conditions. Or those who wanted to sunbathe in an overcast. Or fish in
the desert. Or cycle up Everest. Or keep their scopes indoors?

Are you guys just plain obtuse? :-)

Chris.B

Most people wouldn't want to store an expensive scope in a shed, or
other possibly easily compromised structure. However, some people do
store scopes in their garages.
-Rich