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Equipment Discussions >> Reflectors

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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: StevenYood]
      #5643107 - 01/25/13 06:26 PM

Quote:

I have done the back fan with velcro experiment over the past several months, myself with a 18 inch Starmaster. My findings were that the exhaust fan configuration was vastly preferable for my situation. It definitely yeilded better results for viewing with the fan running at relatively low speeds.
The cooling also was better accomplished with exhaust in my area. In central GA, we can have very extreme dewing problems. When I was the fan blowing directly on the mirror, I sometimes, inadvertantly, overcooled the mirror inducing dew! After a couple of occurances of this, I went to the exhaust option and never looked back.
I am, now, gathering materials to add a front fan in the shadow of the secondary. I can post details of the project and result in a few weeks.




I wonder if simply lowering the RPMs on the blowing fan would have solved it. At anyrate removing the heat before in works its way up into the optical path only worked for me in theory. Practice had issues that conflicted any benefit. It's interesting that it does work for some however. It would seem large mirror truss dobs shroud or not benefit from negative airflow behind the mirror.



Pete


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5643112 - 01/25/13 06:29 PM

Quote:

It would seem large mirror truss dobs shroud or not benefit from negative airflow behind the mirror.




That's what I'm thinking, but maybe not so much for smaller solid tubes.

Mike


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Ed D
Carpal Tunnel
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Reged: 04/30/10

Loc: Sunny South Florida
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5643212 - 01/25/13 07:31 PM

In my case I have a 6" f/8 that is used in a tropical environment where the ground is very humid and hot, even well into the night. Moisture and heat comes out of the ground constantly most of the year. With a fan blowing into the tube it sucks all the heat and moisture and pumps it up the tube, and soon the mirror starts dripping water. My best setup is to draw the air in from the objective, which is high enough from the ground so the heat and humidity is not so concentrated, and exhausting it out the bottom.

I love my solid tube Dob, especially for planetary from home. But, I'm currently building an open truss scope, mostly from wood, for taking to the everglades. Wood seems to be way less prone to dew and moisture problems down here, and I'm thinking an open structure that can breathe easily will do much better in our extreme environment.

Ed D


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: Ed D]
      #5643685 - 01/26/13 01:22 AM

Ed,

The variables that make one the choice of setup over the other is vast indeed . Connecticut always gets a week or two of Florida type heat waves and good seeing during those too, but there's nothing like the month in and month out of tropical climate you folks get. I'm often envious if your seeing down there especially when viewing via cleardarksky.com. All the same the trade off for cooler fall and spring is really appreciated . It's winter I just really don't like.

Interesting but understandable your ground radiates heat and humidity at night. A dripping mirror -.yikes!

Mike,

Maybe in the case of those big mirrors the necessity of drawing the heat off before it can rise is the best way since there's simply so much heating surface to contend with. I know Gustymars with his 16" also has negative airflow behind the mirror and its a long focal ratio too, but alas thats huge piece of glass.

There's a lot of satisfaction had in arriving at what's best for yourself alone despite all the theory. I love the Velcro idea. It's one of those things where the results are too easy to misinterpret and the adjustments as simple as ever.

I'm looking forward to your side by sides with the. 6s.


Pete

Edited by azure1961p (01/26/13 01:26 AM)


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johnnyha
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Thermal Issues and Fans Successes new [Re: StevenYood]
      #5643740 - 01/26/13 02:54 AM Attachment (22 downloads)

Quote:

I have done the back fan with velcro experiment over the past several months, myself with a 18 inch Starmaster. My findings were that the exhaust fan configuration was vastly preferable for my situation. It definitely yeilded better results for viewing with the fan running at relatively low speeds.




Last year I came up with an experimental exhaust fan system for my Obsession, I was inspired by this thread to re-install it tonight - it only took a few minutes. I used Protostar Flockboard to close up the back, held on by friction and a few pieces of tape. The flockboard is perfect, it is flexible enough to bend on the ends and fold inside the mirror box on top and bottom, and the inside is black velvet. The fan is a 3-speed Antec brushless motor so I can get smooth medium and low speeds. This is a small fan but it's not for cooling, just establishing the laminar exhaust flow. Hopefully.

Earlier I posted a photo of the fan I intend to install on the top of the box blowing down onto the mirror, the reversible 3-speed Antec on the flexible stalk.

I put this together last year but did not really have a chance to test it out. My only worry is closing up the box on the bottom and whether that might cause the box to retain heat, but as long as I keep the fan on low speed exhaust I think this should be okay. For cooldown I can use the top fan or even blow my 12V box fan into the top with the scope horizontal like this.

Edited by johnnyha (01/26/13 04:42 AM)


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demiles
professor emeritus
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Reged: 11/07/06

Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5643898 - 01/26/13 07:53 AM

Johnny, I tried this myself but it did work as well as I liked it to. It didn't disrupt the plumes coming off the center of the mirror. One 80mm at 31 cfm wasn't enough, switching to 3 60mm fans pulling 60cfm total caused vibration issues. The 60's were low seed fans.

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MessiToM
scholastic sledgehammer
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Reged: 12/21/09

Loc: Huntingdon PA
Re: Thermal Issues and Fans Successes new [Re: demiles]
      #5644054 - 01/26/13 09:52 AM

I just whipped this up. I also reversed my fan behind the mirror to suck air with the use of a shroud. It's painted black now. I can wait to try it out


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Ed D]
      #5644181 - 01/26/13 11:02 AM

Ed D,

Quote:

In my case I have a 6" f/8 that is used in a tropical environment where the ground is very humid and hot, even well into the night. Moisture and heat comes out of the ground constantly most of the year. With a fan blowing into the tube it sucks all the heat and moisture and pumps it up the tube, and soon the mirror starts dripping water. My best setup is to draw the air in from the objective, which is high enough from the ground so the heat and humidity is not so concentrated, and exhausting it out the bottom.




I've never observed anywhere else except for Maryland, on the Western Shore and Eastern Shore. Even during the summer, we get few nights when it doesn't cool off. Most nights it gets quite a bit cooler at night. I've never experienced any appreciable heat coming up from the ground. I almost always setup on grass.

I've never had water dripping off my primary in the field! So I'm not surprised the best methods for solving thermal problems would differ in tropical and temperate regions.

Have you tried placing an outdoor carpet or tarp underneath the scope? That may prevent heat and moisture from rising up into the OTA. I do that for my telescope at a dark site.

Mike


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nevy
professor emeritus
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Reged: 02/07/12

Loc: UK
Re: Thermal Issues and Fans Successes new [Re: MessiToM]
      #5644398 - 01/26/13 01:22 PM

Quote:

I just whipped this up. I also reversed my fan behind the mirror to suck air with the use of a shroud. It's painted black now. I can wait to try it out




How would you collimate it if you can't see the hotspot?


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MessiToM
scholastic sledgehammer
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Reged: 12/21/09

Loc: Huntingdon PA
Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5644463 - 01/26/13 02:04 PM

the whole unit lifts off the box and easily "bolts" on or off

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johnnyha
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Thermal Issues and Fans Successes new [Re: MessiToM]
      #5644572 - 01/26/13 03:04 PM

Very clever Tom! Ingenious. A combination da Lio Baffle and center boundary fan! What are those, bicycle spokes?

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MessiToM
scholastic sledgehammer
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Reged: 12/21/09

Loc: Huntingdon PA
Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5644583 - 01/26/13 03:13 PM

Oh it isn't my idea. I need to find his thread in this forum. No its real thin wire rope that I also use as the wires to power the fan.

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MessiToM
scholastic sledgehammer
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Reged: 12/21/09

Loc: Huntingdon PA
Re: Thermal Issues and Fans Successes new [Re: MessiToM]
      #5644594 - 01/26/13 03:21 PM

I first saw a fan set.up like this here
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5529511/page...


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Thermal Issues and Fans Successes new [Re: MessiToM]
      #5644878 - 01/26/13 06:35 PM

I just install a 4in fan on my 8inf6 dob. I set it to exhaust the air out the bottom.

I wanted to see how the air moved into the tube so I gently blew cigar smoke [yuk, I can still taste it in my mouth] toward the front of the tube.

one thing I did notice is that the air has to be directly in front of the tube to be drawn down. I can't see how anyones breath would be drawn down the tube while observing at the focuser.

vid: air currents being drawn down the telescope


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johnnyha
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Thermal Issues and Fans Successes new [Re: MessiToM]
      #5644915 - 01/26/13 06:57 PM

Quote:

I first saw a fan set.up like this here
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5529511/page...



Yes but who's idea was combining the M de L baffle with the crosshair primary fan? Please let us know how that performs! I just realized I need to make some kind of baffle for my Obsession...


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: Pinbout]
      #5645267 - 01/26/13 11:13 PM

Hey Danny, thanks for taking the time to do the video and taking a 'hit' for the team.

So your tube is ~10" diameter and 48" long? What is the CFM rating on the fan?

Looked like it had pretty good suck going on there.


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5645297 - 01/26/13 11:49 PM

tube is 9.5 inside dia. 48in long. and I tilted it high so I could see if it handle the static pressure.

don't know the cfm, grabbed it out of a burnt dell computer. I'll have to check. but it wasn't anything that stood out as wow that's a lot of air. its a gentle amount of air for a 4in fan.


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: Pinbout]
      #5645298 - 01/26/13 11:50 PM

Quote:

I just install a 4in fan on my 8inf6 dob. I set it to exhaust the air out the bottom.

. I can't see how anyones breath would be drawn down the tube while observing at the focuser.





Oh it's simple. Consider first that this occurs primarily when the scope is pointed at or near the zenith. With my JMI NGT1 Crayford focuser my nose and mouth are probably a couple inches from the side of the tube. Naturally being warm the breath hits he side of the tube and rises along it. When the rising air meets the intake airflow it goes right down the tube. It's not theory or a hunch or mental modeling, I can see it entering the light path in the image of a defocused star. There's no other airflow competing with the suction of the open aperture and it simply goes down the tube. Too this intake of air also chills my secondary necessitating the use of a dew heater - something the blowing fan bypasses well. Something NEVER needed from this particular site when it was positive airflow.

Keep in mind its a 5" fan on an 8" mirror.



Pete

Edited by azure1961p (01/27/13 12:02 AM)


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: Pinbout]
      #5645313 - 01/27/13 12:05 AM

Quote:

I tilted it high so I could see if it handle the static pressure.




At the risk of sounding stupid, can you please explain what you mean by 'static pressure' and why it would be greater if the scope is aimed high[er]?


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5645627 - 01/27/13 08:46 AM

its the resistance in the system that reduces the flow rate.

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