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DOB cooling fan - when to run it?

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21 replies to this topic

#1 Gschnettler

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Posted 01 October 2020 - 08:06 AM

I just got a new DOB that came with a fan that mounts on the bottom of the tube.  Do you normally just run this while it cools down to get to temperature or do you also run it while you’re using it?



#2 Asbytec

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Posted 01 October 2020 - 08:11 AM

All the time.
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#3 LIVE LONG

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Posted 01 October 2020 - 08:16 AM

 I run mine all the time. It helps reduce thermal currents from forming inside the telescope tube.


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#4 WyattDavis

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Posted 01 October 2020 - 08:16 AM

Run it all the time!


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#5 Starman1

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Posted 01 October 2020 - 12:59 PM

I just got a new DOB that came with a fan that mounts on the bottom of the tube.  Do you normally just run this while it cools down to get to temperature or do you also run it while you’re using it?

You live in a place where the nighttime temperature doesn't drop?

I.e., run it all the time.

The breeze up the tube will help prevent dew on the secondary mirror, too.


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#6 Gschnettler

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Posted 01 October 2020 - 07:05 PM

It’s a 12” truss tube dobsonian.

I just read the cooling fan instructions and it says to only run it when first adjusting to temperature and then turn it off.

My problem is that my Jackery battery unit only has one 12v DC output and I need that to run the telescope goto hand controller. and motors. The only other outputs on the battery are USB.

2 questions:

1) Is it worth it to buy another battery pack just to run the fan?

2) will the fan really keep the dew away given the truss tubes? Or would the fan keep the dew away if I put the shroud over the trusses?

Thanks,
Gary

#7 Napp

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Posted 01 October 2020 - 07:15 PM

If you live in an area prone to dew running the fan(s) helps keep dew off the primary.  I’m in Florida.  I have a 16 inch truss tube DOB.  It has a shroud but without the fans running the primary is prone to condensation forming.



#8 kathyastro

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Posted 01 October 2020 - 07:29 PM

You need a splitter so you can power two (or more) devices at the same time.  Or, if you need another battery, get one.  Running the fan all the time is an excellent way to delay dew formation.  I would also recommend a shroud, which will also delay dew formation.

 

The instructions in the manual are assuming the fan causes vibrations, but most of them do not.


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#9 Bean614

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Posted 01 October 2020 - 08:45 PM

You asked, and the CN members responded rather emphatically---- RUN IT ALWAYS!   Now, go to the Orion Website and look at the Fan & Battery Pack that comes with their 12" Goto Dob.  The Fan is probably identical to the one you have.  The Battery Pack holds 8 D Cell batteries, and I can tell you from the experience of having owned dozens of these (still have some) that this Battery pack will last 40 to 50 hours, at least!  If you view 4 hours per session,  that's over 12 sessions.

    And, for the Princely Sum of $39.95, you get a Spare Fan plus the Battery Pack.

Or, go to Amazon and just get the battery pack.

But, use the fans all the time!


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#10 Jon Isaacs

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Posted 02 October 2020 - 04:59 AM

It’s a 12” truss tube dobsonian.

I just read the cooling fan instructions and it says to only run it when first adjusting to temperature and then turn it off.

 

Here is the thing.  

 

The mirror needs to be within less than 1 degree C of the ambient air temperature for the scope to be fully cooled. That means running it all the time because if the temperature even drops slightly, it will need to cool.

 

I use a 6000 ma-h Lithium battery to run my fans.  I think it was about $35 on Amazon. 

 

Jon


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#11 WyattDavis

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Posted 02 October 2020 - 08:07 AM

Here are some data I gathered several years back:  https://www.cloudyni...primary-mirror/


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#12 Starman1

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Posted 02 October 2020 - 01:18 PM

It’s a 12” truss tube dobsonian.

I just read the cooling fan instructions and it says to only run it when first adjusting to temperature and then turn it off.

My problem is that my Jackery battery unit only has one 12v DC output and I need that to run the telescope goto hand controller. and motors. The only other outputs on the battery are USB.

2 questions:

1) Is it worth it to buy another battery pack just to run the fan?

2) will the fan really keep the dew away given the truss tubes? Or would the fan keep the dew away if I put the shroud over the trusses?

Thanks,
Gary

1.  Yes.

2.  It will help if you use a shroud.  You should use a shroud anyway to reduce scattered light.

An 8-trusspole scope can use a simple lycra cover.  A 6 trusspole scope needs to have rings sewn into the shroud (like the AstroZap) to keep it from sagging into the light path.

 

And the instructions are obviously wrong.  As long as the nighttime temperature is falling, fans need to run. 


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#13 Starman1

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Posted 02 October 2020 - 01:21 PM

Here is a calculator to play with:

http://www.cruxis.co...rrorcooling.htm

Looks a lot like the data collected by Wyatt.


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#14 Gschnettler

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Posted 03 October 2020 - 07:41 AM

Thank you!  I’ll put the shroud on and run the fan all the time!


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#15 ismosi

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Posted 03 October 2020 - 08:27 AM

I run mine all the time. Plus I have a heater for the secondary. Prevention is easier than the cure I think ;)



#16 Gschnettler

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Posted 03 October 2020 - 02:54 PM

Actually that was my next question....if I run the fan all the time do I need a dew heater? If so, just on the secondary?

#17 Napp

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Posted 03 October 2020 - 03:06 PM

Actually that was my next question....if I run the fan all the time do I need a dew heater? If so, just on the secondary?

That will depend on conditions where you observe.  I have a 10 inch solid tube and a 16 inch truss tube with shroud.  I run the fans continuously.  However, here in humid Florida the nighttime air temperature routinely stays very close to the dew point.  That means a lot of dew.  I keep a 12 volt hairdryer handy because the secondaries can and do dew up.  I am going to build heaters for the secondaries.  I have had condensation on the primaries a few times but in each case I had allowed batteries to run down or forgotten to connect or accidentally disconnected the batteries for the fan.


Edited by Napp, 03 October 2020 - 03:08 PM.

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#18 cuzimthedad

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Posted 03 October 2020 - 04:54 PM

When I had my Obsession built, I knew I would need a secondary dew heater so I included it in the build. I sold the scope before moving to ID two years ago and last year, when I ordered my Teeter, I again included a secondary dew heater to be installed...not because we have problems with dew here, but I may take the scope on trips to places that do have issues and want to be prepared for it. So my recommendation is to just go ahead and install one and be done with it. You won't have to worry about dealing with the dew cuz you will be prepared for it.


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#19 Starman.1948

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Posted 06 October 2020 - 10:11 AM

I have a question on the primary mirror fan . . . does it blow onto the mirror or away from it?

Does anyone have a how to source for how to make a secondary mirror anti-dew heater?

 

Don



#20 havasman

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Posted 06 October 2020 - 10:29 AM

I have a question on the primary mirror fan . . . does it blow onto the mirror or away from it?

Does anyone have a how to source for how to make a secondary mirror anti-dew heater?

 

Don

Airflow should be directed AT the mirror.


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#21 Starman.1948

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Posted 06 October 2020 - 11:03 AM

Thanks Dick.

I've also since found a source for the secondary heater build.


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#22 Starman1

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Posted 06 October 2020 - 03:36 PM

Dick is right--the goal is to get as much cool air into contact wit the mirror as possible, and that is accomplished with the fan(s) blowing directly on the mirror.




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