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Equipment Discussions >> Reflectors

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nevy
professor emeritus
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Reged: 02/07/12

Loc: UK
Re: Seeing rules! No more fan for me? new [Re: Adam Taylor]
      #6483983 - 04/23/14 06:23 PM

Quote:

Quote:

In fact, itīs a 80mm quiet fan that runs with a 6v battery




Javier, I'm curious if your fan running at just 6V is blowing enough CFM of air on your mirror to make a noticable difference compared to no fan at all?



That's a very good point.


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craigLambert55
member


Reged: 04/05/14

Loc: Oregon
Re: Seeing rules! No more fan for me? new [Re: nevy]
      #6484050 - 04/23/14 06:54 PM

Here is an interesting article about using fans on a dob/newtonian. It seems that when it comes to fan speed, there is a point of diminishing return. The article also addresses vibration.

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

In the OP's case, it may be that a fan isn't really necessary. He has an 8" mirror that is 1" thick. The mass of a 10" mirror 1.5" thick is 2.3X the massof the OP's...a 12" 1.5 is 3.4X the mass.


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6484080 - 04/23/14 07:10 PM

Your at the eyepiece, you make the call. Fan, no fan, whatever makes your views better. Glad to hear you had some sharp views.

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azure1961p
Postmaster
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Reged: 01/17/09

Loc: USA
Re: Seeing rules! No more fan for me? new [Re: Adam Taylor]
      #6484098 - 04/23/14 07:19 PM

I have garbage seeing all winter. All winter. Spring and fall are a mixed bag a d summer is often good to excellent.

That said...

Even if its a Pickering 4 I run both the rear fan and the boundary fan. I eliminate the tube components of blurring and this lets the Oickerung value be as promising as it can possibly be.

To not ran fans even in gross seeing is to go from poor to godawful.

Pete

Ps: 6 volts is perfect for an 80mm fan. On my 8" I use a 125mm fan.

Edited by azure1961p (04/23/14 07:23 PM)


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Seeing rules! No more fan for me? new [Re: azure1961p]
      #6484402 - 04/23/14 09:23 PM

Thanks to everyone, itīs always interesting to read your comments.

Regarding the 6v, I did some tests with 12, 9 and 6 v and I agree with Pete that the 80 mm works very well with 6v.

Danny made a very good point, with or without fan, Iīm very happy too with those extremely sharp views that I got yesterday!

Edited by Javier1978 (04/23/14 09:24 PM)


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Seeing rules! No more fan for me? new [Re: GOLGO13]
      #6484645 - 04/23/14 11:40 PM

Quote:

I would always suggest turning off the fan when doing high magnification.




Why would you turn it off? On an excellent night, splitting double stars at the Dawes limit, I am at 821x with my 10 inch F/5, I see no vibration, I see clean diffraction rings.

The fan does not vibrate, I leave it on...

Is your 10 inch F/5 providing clean, sharp stars that rival a high quality apo at similar exit pupils? This can happen if you pay attention to thermal issues.

Jon


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Seeing rules! No more fan for me? new [Re: craigLambert55]
      #6484662 - 04/23/14 11:51 PM

Quote:

Here is an interesting article about using fans on a dob/newtonian. It seems that when it comes to fan speed, there is a point of diminishing return. The article also addresses vibration.

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

In the OP's case, it may be that a fan isn't really necessary. He has an 8" mirror that is 1" thick. The mass of a 10" mirror 1.5" thick is 2.3X the massof the OP's...a 12" 1.5 is 3.4X the mass.






Bryan Greer raised the awareness of the importance of thermal management in Newtonians with those articles.

One thing Bryan points out, cooling rate is a function of the thickness of the mirror rather than to the mass of the mirror. A 12 inch mirror has more mass but it also has more surface area for heat transfer. Conceptually, it is the thickness of the mirror that determines the time it takes to cool. A 12 inch mirror that is one inch thick will take the same time to cool as a 8inch mirror that is 1 inch thick..

Jon


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GOLGO13
Carpal Tunnel


Reged: 11/05/05

Loc: St. Louis area
Re: Seeing rules! No more fan for me? new [Re: Jon Isaacs]
      #6484670 - 04/23/14 11:58 PM

Quote:

Why would you turn it off? On an excellent night, splitting double stars at the Dawes limit, I am at 821x with my 10 inch F/5, I see no vibration, I see clean diffraction rings.

The fan does not vibrate, I leave it on...

Is your 10 inch F/5 providing clean, sharp stars that rival a high quality apo at similar exit pupils? This can happen if you pay attention to thermal issues.

Jon




Maybe yours doesn't vibrate (I know you have a different fan than I do from past postings)...but I can tell mine does. It affects high magnification. So I just turn it off when needed. I've seen other people with similar situations. Some have the 3 fan situation where it can be noticed at high mag.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Seeing rules! No more fan for me? new [Re: GOLGO13]
      #6484692 - 04/24/14 12:23 AM

Quote:


Maybe yours doesn't vibrate (I know you have a different fan than I do from past postings)...but I can tell mine does. It affects high magnification. So I just turn it off when needed. I've seen other people with similar situations. Some have the 3 fan situation where it can be noticed at high mag.





I agree that if your fan does cause vibration problems, it is best to turn it off. Hopefully it can be resolved so that it can be left on.

Running them at lower speeds seems to be a viable solution. My fans were custom made by Floyd Blue. They are simple but effective, I don't know if Floyd had a handle on vibrationless fans or if his design was just that good.

Basically he just used an 80mm fan mounted to a piece of 1/16th ABS plastic and then used three Velcro pads to mount the fan. The fan seals the back of the scope and the Velcro seems to be quite effective at preventing the transmission of vibration. Floyd no longer makes stuff.

Jon


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sn1987a
member


Reged: 09/26/10

Loc: Western Australia
Re: Seeing rules! No more fan for me? new [Re: Jon Isaacs]
      #6484722 - 04/24/14 12:48 AM

Thanks Jon ( and Floyd! ),

I shall steal that Velcro idea and give it a go I can see how it would work

Edited by sn1987a (04/24/14 12:50 AM)


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beatlejuice
Pooh-Bah
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Reged: 04/05/11

Loc: Hamilton, ON,Canada
Re: Seeing rules! No more fan for me? new [Re: Jon Isaacs]
      #6484870 - 04/24/14 03:06 AM

Quote:

Basically he just used an 80mm fan mounted to a piece of 1/16th ABS plastic and then used three Velcro pads to mount the fan. The fan seals the back of the scope and the Velcro seems to be quite effective at preventing the transmission of vibration. Floyd no longer makes stuff.




I was lucky enough to get one of Floyd's fans and as Jon says there is no noticable vibration. The Velcro really does the trick.

Eric


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: Seeing rules! No more fan for me? new [Re: Jon Isaacs]
      #6485041 - 04/24/14 07:35 AM

I've paid attention to a std thickness and 1" mirrors of 8" sizes. It takes roughly twice as long for a std thickness blank to acclimate on a teststand.
***
To get an idea of cooling deltas, it could be kept in mind that surface area varies as the SQUARE of dimensions, and volume varies as a CUBE. Of course allowances have to be made, but I think if you had good examples hanging around some reasonable extrapolation could be made with a scope that has issues...?
***
I never really had any issues with 1" thick 8" mirrors as long as it was kept outside (sheltered!) and out of the sun with a tube around 10" diameter. No fan was necessary.
M.


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bunyon
Post Laureate
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Reged: 10/23/10

Loc: Winston-Salem, NC
Re: Seeing rules! No more fan for me? new [Re: Mark Harry]
      #6485648 - 04/24/14 01:45 PM

Velcro! I'll have to try that. I started with the fan that came with the obsession and have played with it constantly. It's much, much better now than it was but there is still some vibration blur at high magnification so I have to turn it off. Would be nice to be able to leave it on.

And if you don't need it, don't use it. But I would want the fan there in case you do at some point. A fan can be turned off but no fan can't be turned on.


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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Seeing rules! No more fan for me? new [Re: bunyon]
      #6486043 - 04/24/14 04:54 PM

I resolved my vibration issue with my fan by the most simple way, Rubber bands mounting the fan took 100% of vibration away.
I must admit an 8" F6 is a fantastic tube for Planetary.
OWL tested my Cheap Starfinder 8" Mirror to 1/9th wave!!
I bought the whole tube on Cloudy nights for $75.00!
I retired my 8" Meade SN F4 for the rings..
anybody need a cheap Astrograph!
My LXD-75 handles it so well..


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Alein 1630
super member


Reged: 01/18/13

Loc: Dutchess Count,y NY
Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6487621 - 04/25/14 11:34 AM

Quote:

After five minutes or so I did a star test and yes, there were signals of air currents, but they didnīt seemed to affect the image at all and they didnīt last for much long.




How did you do this?


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Alein 1630
super member


Reged: 01/18/13

Loc: Dutchess Count,y NY
Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6487628 - 04/25/14 11:41 AM

Turn the fan on for about 30 minutes before you observe and turn it off while your observing. It may have distorted the view when on, but it'll help cool off the telescope. it is ment to help cool the mirror quickly. thats your problem.

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Eric63
scholastic sledgehammer


Reged: 06/16/12

Loc: Ottawa, Ontario
Re: Seeing rules! No more fan for me? new [Re: Alein 1630]
      #6488338 - 04/25/14 04:54 PM

That's what I do. I use the fan for quick cool down and then turn it off. I turn it back on if I walk away from the scope. For a bit.

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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Seeing rules! No more fan for me? new [Re: Alein 1630]
      #6488461 - 04/25/14 05:55 PM

Quote:

Turn the fan on for about 30 minutes before you observe and turn it off while your observing. It may have distorted the view when on, but it'll help cool off the telescope. it is ment to help cool the mirror quickly. thats your problem.




I have not experienced a distorted view due a fan being on, other than vibration that is. But I do find that 30 minutes is insufficient to fully cool and 8 or 10 scope so that it is rock solid thermally stable.

Jon


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herrointment
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Reged: 03/12/11

Loc: North of Hwy. 64
Re: Seeing rules! No more fan for me? new [Re: Jon Isaacs]
      #6490141 - 04/26/14 03:24 PM Attachment (5 downloads)

Here's a good use for a weak battery pak....and no vibrations in my eq mounted Newtonian.

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DocFinance
sage
*****

Reged: 01/14/14

Loc: Clear Lake, Texas
Re: Seeing rules! No more fan for me? new [Re: bunyon]
      #6490210 - 04/26/14 04:05 PM

Quote:

Indeed. I don't think you can say the fan doesn't cause vibration by looking at it. The best way to check for vibrations would be to look at something connected to the fan at very high magnification and see if there is a distortion.

Like looking at a planet, say.






I can't imagine having ANY fan running on my scope while trying to use it. There is no fan out there that would be smooth enough...


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