Dob cooling question

"SPQR" wrote in message
om...

I see frequent references to cooling fans on Dobs;

SPQR--

recommend you read the article "thermal management in newtonian reflectors"
by Alan Adler in the jan 2002 s&t. it starts on page 132. bottom line:
your assumption about turning the fan off is not entirely correct. you can
also purchase the article on line at the s&t website.

clear, dark skies--

mark d.

"etok" wrote You know, for the benefit you get from using a cooling fan
on a 10" dob most
of the time, fans aren't worth the trouble.
(I'm assuming you have an Orion newtonian).
If you get a 16" full thickness mirror, and you're moving it between a 70F
house and a frozen lake for observing, then you may benefit from a fan.
Just
my jumbled opinion.

Sounds to me like you've never USED a fan. There is no "trouble" using a
fan, and they work. From what I've read, knowledgeable people claim even a
1/2 to 1 degree difference can screw up the image.

Howard Lester

"Howard Lester" wrote in message
...

"etok" wrote You know, for the benefit you get from using a cooling
fan
on a 10" dob most
of the time, fans aren't worth the trouble.
(I'm assuming you have an Orion newtonian).
If you get a 16" full thickness mirror, and you're moving it between a
70F
house and a frozen lake for observing, then you may benefit from a fan.
Just
my jumbled opinion.

Sounds to me like you've never USED a fan. There is no "trouble" using a
fan, and they work. From what I've read, knowledgeable people claim even a
1/2 to 1 degree difference can screw up the image.

Howard Lester

Yes, people come up with all kinds of reasons their telescopes aren't
working well.

Regards,
Etok



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Just to add to Howard, with a 12 inch or bigger, you almost
need.....make that want....a fan, or even two.
TW




Howard Lester wrote:

"etok" wrote You know, for the benefit you get from using a cooling fan
on a 10" dob most

of the time, fans aren't worth the trouble.
(I'm assuming you have an Orion newtonian).
If you get a 16" full thickness mirror, and you're moving it between a 70F
house and a frozen lake for observing, then you may benefit from a fan.

Just

my jumbled opinion.


Sounds to me like you've never USED a fan. There is no "trouble" using a
fan, and they work. From what I've read, knowledgeable people claim even a
1/2 to 1 degree difference can screw up the image.

Howard Lester

"etok" wrote

Sounds to me like you've never USED a fan. There is no "trouble" using a
fan, and they work. From what I've read, knowledgeable people claim even
a
1/2 to 1 degree difference can screw up the image.

Howard Lester

Yes, people come up with all kinds of reasons their telescopes aren't
working well.

Indeed, so as intelligent amateurs we investigate. For years, my excellent
6" mirror wasn't performing as well as I thought it should. Thanks to the
writings of Alan Adler and Bryan Greer, I added a small fan. Bingo!
Terrific images.

Howard Lester

"bwhiting" wrote in message
...
Just to add to Howard, with a 12 inch or bigger, you almost
need.....make that want....a fan, or even two.
TW




Howard Lester wrote:

"etok" wrote You know, for the benefit you get from using a cooling
fan
on a 10" dob most

of the time, fans aren't worth the trouble.
(I'm assuming you have an Orion newtonian).
If you get a 16" full thickness mirror, and you're moving it between a
70F
house and a frozen lake for observing, then you may benefit from a fan.

Just

my jumbled opinion.


Sounds to me like you've never USED a fan. There is no "trouble" using a
fan, and they work. From what I've read, knowledgeable people claim even
a
1/2 to 1 degree difference can screw up the image.

Howard Lester

I agree that there are benefits to using a fan on large aperture
(relatively) scopes. I don't think it's worth using on an 8-10" scope (like
the Orions) that the original poster alluded to.

Although a fan on a StarBlaster would be *cute*.

Etok



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"Howard Lester" wrote in message
...

"etok" wrote

Sounds to me like you've never USED a fan. There is no "trouble" using
a
fan, and they work. From what I've read, knowledgeable people claim
even
a
1/2 to 1 degree difference can screw up the image.

Howard Lester

Yes, people come up with all kinds of reasons their telescopes aren't
working well.

Indeed, so as intelligent amateurs we investigate. For years, my excellent
6" mirror wasn't performing as well as I thought it should. Thanks to the
writings of Alan Adler and Bryan Greer, I added a small fan. Bingo!
Terrific images.

Howard Lester

I won't knock what works for you, but I must admit skepticism about those
1/2-degree delta T numbers you're throwing out there.

Regards,
Etok



__________________________________________________ ____________________
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The Worlds Uncensored News Source

"etok" wrote

I won't knock what works for you, but I must admit skepticism about those
1/2-degree delta T numbers you're throwing out there.

Skepticism is good. I only cite the 1/2 degree figure because Bryan used it
and apparently had measured it as so. *I* wouldn't know... no. Again, note
that my mirror is "only" 6 inches and it benefits - sometimes a little,
sometimes a lot - from the fan.

Howard

etok wrote:

I agree that there are benefits to using a fan on large aperture
(relatively) scopes. I don't think it's worth using on an 8-10" scope (like
the Orions) that the original poster alluded to.

You seem quite certain of this opinion. Do you have any experience or
someone else's experience that led you to this conclusion? There have
been many, many people studying this and trying different approaches.
Skepticism is good but conclusions based on ignorance are not. One
could just as easily claim that Earth is flat, that we never landed on
the Moon, etc.

Mike Simmons

"etok" wrote in message
...
I agree that there are benefits to using a fan on large aperture
(relatively) scopes. I don't think it's worth using on an 8-10" scope
(like
the Orions) that the original poster alluded to.

Although a fan on a StarBlaster would be *cute*.


Hi Etok,

Please remember that most of the tunnel testing I did a few years ago was
with a 6" Pyrex blank. Even at that size, the peak-to-valley wavefront
errors were non-trivial when the mirror was within a few degrees of ambient.
Take a look at the strength of the boundary layer in these videos:

http://www.fpi-protostar.com/bgreer/

That said, fan cooling is not for everybody. The extra complexity
(batteries, etc.) required may not be worth it for a novice who is just out
trying to nail Messier objects. However, for someone trying to get the best
planetary images there is a lot to be gained by the use of fans.

Sincerely,
Bryan Greer