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Seeing rules! No more fan for me?

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#1 Javier1978

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Posted 23 April 2014 - 09:18 AM

As I became a very enthusiastic lunar and planetary observer in the last years, I did my homework with my 8” f6 (1” thick mirror) in terms of the two “C” that depends on you. Now, I think I have been overestimating the benefits of using a rear fan running the entire session. This fall I have had quite bad seeing in Buenos Aires and I had no great observations of Mars and Saturn, in fact, the views were quite disappointing at around 140 x most of the time, always with the rear fan running the entire session.

Yesterday the stars looked very calm and I took the scope out. My rear fan had broken a few days ago and the temperature in my house was about 22º and outside 15º, so I thought, this is going to suck. Well, that didn´t happen. Both Saturn and Mars seemed like cut by a knife, very pristine, very quiet. It was the best observation of Mars I ever had. Saturn was absolutely beautiful, with very distinctive colors between A, B y C rings, with grey zones in the B rings. I have never saw the transition between the dark grey of C rings and the black background so clear. Also, I saw the elusive (to my scope) Enceladus. The magnification used was about 210 x (I used a barlow with an extender and a 12,5 mm plossl), that is my limit for a comfortable hand tracking.

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. I observed for about two hours and I have to say this was my very best planetary observation.

So my conclusion is obvious. In my case, the conditions C y by far more important than the cooling C. In fact, the cooling C doesn´t seem to affect the image, for me it´s all about seeing. So as the fan has always been a hassle, I think there will be no more fan for me.

#2 star drop

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Posted 23 April 2014 - 09:28 AM

No fans for me either. The local seeing is my bane.

#3 Jon Isaacs

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Posted 23 April 2014 - 09:41 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. I observed for about two hours and I have to say this was my very best planetary observation.



My experience:

Without good seeing, I will not get the good planetary images. But without a thermally equilibrated telescope, I will not get the best possible planetary images..

It seems to me that you are at stage one, good seeing means good planetary images but not at stage two which requires good seeing and a fully thermally equilibrated scope.

Jon

#4 sn1987a

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Posted 23 April 2014 - 10:11 AM

I too have had a similar experience. I took the fans off my 28" f4.2 to install motorised collimation on the primary. Being lazy I couldn't be bothered putting fans back on straight away. For the last three times I've been out I've had the best views I've ever had in the scope. I've been using the scope without the shroud, the weather has been cool lately, Mars and Saturn after midnight have been spectacular with a 10mm Ethos and Paracorr II. My Kennedy mirror is 2" thick. I always thought I needed more fans apparently I might not need any. I don't have any fans on my 16" and 18" and they cool down quite quickly. :shrug:

#5 bunyon

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Posted 23 April 2014 - 10:18 AM

My experience is that thermal equilibrium is vital. On the other hand, a fan that introduces vibration will negate any gains from thermal equilibrium. If getting rid of the fan helps, it's likely that the fan you had was producing vibrations that could only be detected under good seeing at high power - I have had this problem.

You can either work to eliminate the vibration or turn the fan off when you want to observe at high power, which is what I do. I use the fan to cool the scope and leave it on when viewing at low power. If I'm doing an extended run at high power, I turn it on periodically for 10 minute bursts. I've not yet been able to eliminate vibration entirely.

#6 Javier1978

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Posted 23 April 2014 - 10:24 AM

My experience is that thermal equilibrium is vital. On the other hand, a fan that introduces vibration will negate any gains from thermal equilibrium. If getting rid of the fan helps, it's likely that the fan you had was producing vibrations that could only be detected under good seeing at high power - I have had this problem.

You can either work to eliminate the vibration or turn the fan off when you want to observe at high power, which is what I do. I use the fan to cool the scope and leave it on when viewing at low power. If I'm doing an extended run at high power, I turn it on periodically for 10 minute bursts. I've not yet been able to eliminate vibration entirely.


I agree with you, but I have done a lot of test with my fan in the two years I used it turning it on and off, and there are no vibrations at all. In fact, it´s a 80mm quiet fan that runs with a 6v battery and I use rubber bands to hold it.

#7 Javier1978

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Posted 23 April 2014 - 10:26 AM

I too have had a similar experience. I took the fans off my 28" f4.2 to install motorised collimation on the primary. Being lazy I couldn't be bothered putting fans back on straight away. For the last three times I've been out I've had the best views I've ever had in the scope. I've been using the scope without the shroud, the weather has been cool lately, Mars and Saturn after midnight have been spectacular with a 10mm Ethos and Paracorr II. My Kennedy mirror is 2" thick. I always thought I needed more fans apparently I might not need any. I don't have any fans on my 16" and 18" and they cool down quite quickly. :shrug:


Interesting...

#8 Jon Isaacs

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Posted 23 April 2014 - 10:28 AM

My experience is that thermal equilibrium is vital. On the other hand, a fan that introduces vibration will negate any gains from thermal equilibrium. If getting rid of the fan helps, it's likely that the fan you had was producing vibrations that could only be detected under good seeing at high power - I have had this problem.

You can either work to eliminate the vibration or turn the fan off when you want to observe at high power, which is what I do. I use the fan to cool the scope and leave it on when viewing at low power. If I'm doing an extended run at high power, I turn it on periodically for 10 minute bursts. I've not yet been able to eliminate vibration entirely.


:waytogo:

I was wondering if Javier fan was vibrationless. It only takes the tiniest vibration to seriously affect the image. If the fan is causing a arc-second of motion of the scope, it's a real problem.. An arc-second of rotation about a 48 inch focal length is about about 6 microns, 10 wave lengths of light.

Jon

#9 bunyon

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Posted 23 April 2014 - 10:39 AM

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.

#10 JMW

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Posted 23 April 2014 - 10:43 AM

I live at 4800 feet and dropping night time temps require a fan when using my Webster D14 with f/4.3 14.5" Zambuto mirror. I run the fan slow but when looking at planetary I can see the effects of turning of the fan within a few minutes. Of course the fan will not fix bad seeing so I don't spend much time looking at planets except when conditions are good to excellent. Most of the time conditions are such that I can see just as much looking at planets with my TEC 140.

#11 Javier1978

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Posted 23 April 2014 - 10:52 AM

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.


Of course, that´s what I did a lot of times at hig magnifications. I don´t expect to see vibrations when looking at the fan

#12 Javier1978

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Posted 23 April 2014 - 10:58 AM

I don´t think a made my point very clear. I´m not negating the advantage of having a fan, I´m just saying that it might be a bit overestimated in some cases, and waaay overestimated in my case.

#13 Jon Isaacs

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Posted 23 April 2014 - 12:17 PM

Of course, that´s what I did a lot of times at hig magnifications. I don´t expect to see vibrations when looking at the fan



It's something of a paradox. Just using high magnifactions is not enough, you also need excellent seeing because you need to resolve the image well enough to see the vibration. In an 8 inch, I would be looking at 400x and probably 600x.

Jon

#14 Javier1978

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Posted 23 April 2014 - 12:32 PM

I don´t get it Jon, if I can´t see the difference with the fan on off (in terms of vibration) when carefully observing at around 200 x wich is about all the magnification I want, why would I care what happens at 400 or 600 x? I agree though that a stable conditions are necesary to do this vibration test.

#15 Jeff Morgan

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Posted 23 April 2014 - 12:38 PM

I agree with you, but I have done a lot of test with my fan in the two years I used it turning it on and off, and there are no vibrations at all. In fact, it´s a 80mm quiet fan that runs with a 6v battery and I use rubber bands to hold it.


I have had mixed results with vibrations. My mounting methods have been the same - a sandwich of low density foam, high density foam, and silicon glue. So I am thinking it is the quality of the individual fan units. These have been the Radio Shack 80 mm computer fans. The happened to be available locally so that is why the were selected. Perhaps I could make a better choice.

#16 schang

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Posted 23 April 2014 - 02:26 PM

I don´t get it Jon, if I can´t see the difference with the fan on off (in terms of vibration) when carefully observing at around 200 x wich is about all the magnification I want, why would I care what happens at 400 or 600 x? I agree though that a stable conditions are necesary to do this vibration test.

I think if the mirror temp differs significantly from the ambient temp, a fan will help to cool it down faster. It is nice to have that option. When your mirror temp is close to the ambient temp before you take it out, the benefit of a fan is mute. I have similar experience like you, and I have not use it since a while back. Just one less hassle for me. Coming from engineering background, I have doubts about its benefits, especially when people talking about laminar flow, boundary layer, heater used (for dew and from fans),. and all other mods to improve the mirror equilibrium. For me there is no better way of having stagnant air in the tube with no vibration. If that is not possible, a natural convection of air flowing upward at very slow speed will beat high air flow (and may be turbulent air) and vibration. To each his own, over-rated or not.

#17 OrdinaryLight

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Posted 23 April 2014 - 03:02 PM

When your mirror temp is close to the ambient temp before you take it out, the benefit of a fan is mute.


It's true that the primary purpose of the fan is to get the mirror temp close to ambient temp. Gary Seronik says in his article on Newtonian thermals, "Your scope will mostly be free from thermal problems when the mirror is within 3° C (5.4°F) of ambient. Chances are, this is something you’ll rarely see."

Even if the mirror temp is within 5º of ambient when the scope is taken out it has been shown that in many climates the temperature difference will increase as the ambient temperature drops at a speed which the mirror can not match without some help. If this doesn't happen often in your climate then you are fortunate (and should probably be looking at getting a bigger mirror). :)

#18 GOLGO13

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Posted 23 April 2014 - 03:08 PM

I would always suggest turning off the fan when doing high magnification. Especially when doing planetary observing. Then just turn it back on if the temps are dropping. Most of this assumes a pretty large mirror situation. I feel that with my 10 inch scope I could use the fan to speed up cool down and then it can be off the rest of the session (unless it became obvious it needed to be turned on again). But a large thick mirror that may not be the case. I'd still turn it off for planetary.

#19 schang

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Posted 23 April 2014 - 03:18 PM

When your mirror temp is close to the ambient temp before you take it out, the benefit of a fan is mute.


It's true that the primary purpose of the fan is to get the mirror temp close to ambient temp. Gary Seronik says in his article on Newtonian thermals, "Your scope will mostly be free from thermal problems when the mirror is within 3° C (5.4°F) of ambient. Chances are, this is something you’ll rarely see."

Even if the mirror temp is within 5º of ambient when the scope is taken out it has been shown that in many climates the temperature difference will increase as the ambient temperature drops at a speed which the mirror can not match without some help. If this doesn't happen often in your climate then you are fortunate (and should probably be looking at getting a bigger mirror). :)

I forgot to mention that I have a 10" mirror, in responding to the OP's 8" dob. People with larger, thicker mirrors may have different experience.

#20 Adam Taylor

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Posted 23 April 2014 - 05:15 PM

In fact, it´s 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?

#21 nevy

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Posted 23 April 2014 - 05:23 PM

In fact, it´s 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.

#22 craigLambert55

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Posted 23 April 2014 - 05:54 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-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.

#23 Pinbout

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Posted 23 April 2014 - 06:10 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. ;)

#24 azure1961p

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Posted 23 April 2014 - 06:19 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.

#25 Javier1978

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Posted 23 April 2014 - 08:23 PM

Thanks to everyone, it´s 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, I´m very happy too with those extremely sharp views that I got yesterday!






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