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Seeing rules! No more fan for me?

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#26 Jon Isaacs

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Posted 23 April 2014 - 10:40 PM

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

#27 Jon Isaacs

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Posted 23 April 2014 - 10:51 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-proto...r/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.


:waytogo:

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

#28 GOLGO13

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Posted 23 April 2014 - 10:58 PM

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.

#29 Jon Isaacs

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Posted 23 April 2014 - 11:23 PM

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

#30 sn1987a

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Posted 23 April 2014 - 11:48 PM

Thanks Jon ( and Floyd! ),

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

#31 beatlejuice

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Posted 24 April 2014 - 02:06 AM

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

#32 Mark Harry

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Posted 24 April 2014 - 06:35 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.

#33 bunyon

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Posted 24 April 2014 - 12:45 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.

#34 orion61

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Posted 24 April 2014 - 03:54 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..

#35 Alein 1630

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Posted 25 April 2014 - 10:34 AM

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


How did you do this?

#36 Alein 1630

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Posted 25 April 2014 - 10:41 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. :jump: :rainbow: :grin:

#37 Eric63

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Posted 25 April 2014 - 03:54 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.

#38 Jon Isaacs

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Posted 25 April 2014 - 04:55 PM

;)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. :jump: :rainbow: :grin:


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

#39 herrointment

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Posted 26 April 2014 - 02:24 PM

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

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#40 DocFinance

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Posted 26 April 2014 - 03:05 PM

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.


:lol:

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...

#41 pstarr

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Posted 26 April 2014 - 04:42 PM









:lol:

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...


If the scope and mount are beefy enough you don't have to worry about fan vibration.

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#42 azure1961p

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Posted 26 April 2014 - 06:04 PM

I've had my 8" at well well over 1000x - the atmospheric dispersion was godawful but there was no vibration at all.

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#43 nevy

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Posted 26 April 2014 - 08:50 PM

I don't get any vibration.
Posted Image
Noise blocker with its own rubber suspension for vibration free blowing.

#44 azure1961p

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Posted 26 April 2014 - 08:57 PM

Really nice job on that Nevy.

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#45 beatlejuice

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Posted 26 April 2014 - 10:47 PM

Again, no vibrations:

Posted Image

Eric

#46 Cotts

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Posted 26 April 2014 - 11:19 PM

I've seen a very clever fan idea for larger Newts, probably not possible with scopes smaller than 12 inches... There's a fellow named Joe Wambow (sp.?) who goes to WSP and OkieTex with a 32 inch dob. He has a fan suspended above the center of the mirror with wires (guitar strings?) blowing down onto the face of the mirror. The fan is in the 'shadow' of the secondary mirror so there's no blockage or extra diffraction issues. There don't seem to be vibration problems - the views at above 400x on Jupiter defy description.

The smallest fan of this sort that I've seen is about 2 inches on a side so if your secondary is larger than this you could try it! Difficulty: You'll have to line up the wires with the spider vanes so as not to increase diffraction spikes....

Dave

#47 okieav8r

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Posted 26 April 2014 - 11:28 PM

I've seen a very clever fan idea for larger Newts, probably not possible with scopes smaller than 12 inches... There's a fellow named Joe Wambow (sp.?) who goes to WSP and OkieTex with a 32 inch dob. He has a fan suspended above the center of the mirror with wires (guitar strings?) blowing down onto the face of the mirror. The fan is in the 'shadow' of the secondary mirror so there's no blockage or extra diffraction issues. There don't seem to be vibration problems - the views at above 400x on Jupiter defy description.

The smallest fan of this sort that I've seen is about 2 inches on a side so if your secondary is larger than this you could try it! Difficulty: You'll have to line up the wires with the spider vanes so as not to increase diffraction spikes....

Dave


I saw that fan set-up also Dave, it was on a Webster, I believe. The views through that telescope were indeed fantastic. Clever, the way the fan was mounted over the mirror. Such a setup would probably make me a little nervous though.

#48 Cotts

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Posted 26 April 2014 - 11:34 PM

What about sealing the tube of the newtonian entirely? Put the mirror in a solid tube, seal the back and get an optical quality window at the front end. It seems to me I've read that refractors have much reduced internal tube currents because of their sealed OTA's... Also, it has been said that Dob mirrors being near the ground are overly affected by ground temperature issues. Then there is the observer's body heat going through the truss tubes and porous shrouds... Seal the tube and these issues can go away..

Can't get an optical window for the front? How about stretching some clear mylar over the front of the tube. The coated mylar works for solar filtration without degrading resolution....

Ambitious folks could glue the diagonal to the center of a glass optical window and lose the spikes from the spider...

Wacky ideas are fun....

Dave

#49 schang

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Posted 27 April 2014 - 07:49 AM

The main issue is the mass of the glass, not closed tube. Large refractors, especially those air spaced triplet APOs, also take a long time to cool down.

#50 Jeff Morgan

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Posted 27 April 2014 - 08:51 AM

Can't get an optical window for the front? How about stretching some clear mylar over the front of the tube. The coated mylar works for solar filtration without degrading resolution....


This idea has appeared in Sky & Telescope several times. Specifically July 1978, September 1978, and August 1991. Probably not the best solution for high definition work, but fine for DSO's.

There was another interesting article I was trying to track down. The author used external ducts and fans to recirculate the air from top to bottom within a closed tube. Basically, the idea was that even within the closed tube there were temperature gradients. IIRC, he reported excellent results from the moment the scope was set up (although the scope didn't look particularly pretty).






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