Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Reflectors

Pages: 1 | 2 | 3 | 4 | (show all)
Javier1978
scholastic sledgehammer


Reged: 02/12/09

Seeing rules! No more fan for me?
      #6482968 - 04/23/14 10:18 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 22š and outside 15š, so I thought, this is going to suck. Well, that didnīt 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 didnīt seemed to affect the image at all and they didnīt 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 doesnīt seem to affect the image, for me itīs all about seeing. So as the fan has always been a hassle, I think there will be no more fan for me.


Post Extras: Print Post   Remind Me!   Notify Moderator  
star drop
contra contrail
*****

Reged: 02/02/08

Loc: Snow Plop, WNY
Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6482997 - 04/23/14 10:28 AM

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

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6483024 - 04/23/14 10:41 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. 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
sn1987a
member


Reged: 09/26/10

Loc: Western Australia
Re: Seeing rules! No more fan for me? new [Re: Jon Isaacs]
      #6483083 - 04/23/14 11:11 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
bunyon
Post Laureate
*****

Reged: 10/23/10

Loc: Winston-Salem, NC
Re: Seeing rules! No more fan for me? new [Re: sn1987a]
      #6483100 - 04/23/14 11:18 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Seeing rules! No more fan for me? new [Re: bunyon]
      #6483112 - 04/23/14 11:24 AM

Quote:

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, itīs a 80mm quiet fan that runs with a 6v battery and I use rubber bands to hold it.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Seeing rules! No more fan for me? new [Re: sn1987a]
      #6483116 - 04/23/14 11:26 AM

Quote:

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.




Interesting...


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Seeing rules! No more fan for me? new [Re: bunyon]
      #6483122 - 04/23/14 11:28 AM

Quote:

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 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
bunyon
Post Laureate
*****

Reged: 10/23/10

Loc: Winston-Salem, NC
Re: Seeing rules! No more fan for me? new [Re: Jon Isaacs]
      #6483146 - 04/23/14 11:39 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
JMW
Pooh-Bah


Reged: 02/11/07

Loc: Nevada
Re: Seeing rules! No more fan for me? new [Re: Jon Isaacs]
      #6483156 - 04/23/14 11:43 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Seeing rules! No more fan for me? new [Re: bunyon]
      #6483185 - 04/23/14 11:52 AM

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.




Of course, thatīs what I did a lot of times at hig magnifications. I donīt expect to see vibrations when looking at the fan


Post Extras: Print Post   Remind Me!   Notify Moderator  
Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6483200 - 04/23/14 11:58 AM

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

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6483360 - 04/23/14 01:17 PM

Quote:

Of course, thatīs what I did a lot of times at hig magnifications. I donīt 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Seeing rules! No more fan for me? new [Re: Jon Isaacs]
      #6483409 - 04/23/14 01:32 PM

I donīt get it Jon, if I canīt 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6483420 - 04/23/14 01:38 PM

Quote:

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, itīs 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
schang
scholastic sledgehammer


Reged: 04/24/13

Loc: columbia, sc
Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6483654 - 04/23/14 03:26 PM

Quote:

I donīt get it Jon, if I canīt 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
OrdinaryLight
sage
*****

Reged: 11/06/13

Loc: Massachusetts
Re: Seeing rules! No more fan for me? new [Re: schang]
      #6483713 - 04/23/14 04:02 PM

Quote:

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 5š 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).


Post Extras: Print Post   Remind Me!   Notify Moderator  
GOLGO13
Carpal Tunnel


Reged: 11/05/05

Loc: St. Louis area
Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6483721 - 04/23/14 04:08 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
schang
scholastic sledgehammer


Reged: 04/24/13

Loc: columbia, sc
Re: Seeing rules! No more fan for me? new [Re: OrdinaryLight]
      #6483742 - 04/23/14 04:18 PM

Quote:

Quote:

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 5š 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Adam Taylor
insignificant bystander
*****

Reged: 11/11/10

Loc: Arizona
Re: Seeing rules! No more fan for me? new [Re: Javier1978]
      #6483971 - 04/23/14 06:15 PM

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?


Post Extras: Print Post   Remind Me!   Notify Moderator  
nevy
professor emeritus
*****

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
azure1961p
Postmaster
*****

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)


Post Extras: Print Post   Remind Me!   Notify Moderator  
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)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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)


Post Extras: Print Post   Remind Me!   Notify Moderator  
beatlejuice
Pooh-Bah
*****

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
bunyon
Post Laureate
*****

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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..


Post Extras: Print Post   Remind Me!   Notify Moderator  
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?


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
herrointment
Post Laureate
*****

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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
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...


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Seeing rules! No more fan for me? new [Re: DocFinance]
      #6490396 - 04/26/14 05:42 PM Attachment (5 downloads)









Quote:




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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Seeing rules! No more fan for me? new [Re: pstarr]
      #6490524 - 04/26/14 07:04 PM

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

Pete


Post Extras: Print Post   Remind Me!   Notify Moderator  
nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: Seeing rules! No more fan for me? new [Re: azure1961p]
      #6490853 - 04/26/14 09:50 PM

I don't get any vibration.

Noise blocker with its own rubber suspension for vibration free blowing.


Post Extras: Print Post   Remind Me!   Notify Moderator  
azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Seeing rules! No more fan for me? new [Re: nevy]
      #6490864 - 04/26/14 09:57 PM

Really nice job on that Nevy.

Pete


Post Extras: Print Post   Remind Me!   Notify Moderator  
beatlejuice
Pooh-Bah
*****

Reged: 04/05/11

Loc: Hamilton, ON,Canada
Re: Seeing rules! No more fan for me? new [Re: nevy]
      #6491026 - 04/26/14 11:47 PM

Again, no vibrations:



Eric


Post Extras: Print Post   Remind Me!   Notify Moderator  
Cotts
Just Wondering
*****

Reged: 10/10/05

Loc: Toronto, Ontario
Re: Seeing rules! No more fan for me? new [Re: beatlejuice]
      #6491064 - 04/27/14 12:19 AM

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
okieav8rAdministrator
I'd rather be flying!
*****

Reged: 03/01/09

Loc: Oklahoma!
Re: Seeing rules! No more fan for me? new [Re: Cotts]
      #6491073 - 04/27/14 12:28 AM

Quote:

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Cotts
Just Wondering
*****

Reged: 10/10/05

Loc: Toronto, Ontario
Re: Seeing rules! No more fan for me? new [Re: Cotts]
      #6491077 - 04/27/14 12:34 AM

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
schang
scholastic sledgehammer


Reged: 04/24/13

Loc: columbia, sc
Re: Seeing rules! No more fan for me? new [Re: Cotts]
      #6491426 - 04/27/14 08:49 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.

Edited by schang (04/27/14 08:50 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Seeing rules! No more fan for me? new [Re: Cotts]
      #6491490 - 04/27/14 09:51 AM

Quote:

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).


Post Extras: Print Post   Remind Me!   Notify Moderator  
choran
Carpal Tunnel
*****

Reged: 12/28/12

Re: Seeing rules! No more fan for me? new [Re: Jeff Morgan]
      #6491597 - 04/27/14 11:01 AM

I'm sure there are many reasons why this wouldn't work, but I wonder if anyone has considered initially cooling a large mirror with a cold pack placed against the back of the mirror, the sort of cold pack used to treat an athletic injury. Wouldn't have to be frozen, just fairly cold. (I'm ducking as the answers to this come in).

Post Extras: Print Post   Remind Me!   Notify Moderator  
charles genovese
professor emeritus
*****

Reged: 02/04/06

Loc: Madisonville Louisiana
Re: Seeing rules! No more fan for me? new [Re: choran]
      #6491647 - 04/27/14 11:23 AM

Anthony Westley has peltier cooling for his scope

Post Extras: Print Post   Remind Me!   Notify Moderator  
Adam Taylor
insignificant bystander
*****

Reged: 11/11/10

Loc: Arizona
Re: Seeing rules! No more fan for me? new [Re: beatlejuice]
      #6491756 - 04/27/14 12:44 PM

Eric, is the Velcro at 3 points, or all the way round? And how is the fan mounted to the disc?

Post Extras: Print Post   Remind Me!   Notify Moderator  
Eric63
scholastic sledgehammer


Reged: 06/16/12

Loc: Ottawa, Ontario
Re: Seeing rules! No more fan for me? new [Re: choran]
      #6491764 - 04/27/14 12:48 PM

Actually, that's what I do with my 127Mak. I put an ice pack in my case an hour before I go out. But for my 6" Newt I use a computer fan for 30 minutes or so.

Post Extras: Print Post   Remind Me!   Notify Moderator  
beatlejuice
Pooh-Bah
*****

Reged: 04/05/11

Loc: Hamilton, ON,Canada
Re: Seeing rules! No more fan for me? new [Re: Adam Taylor]
      #6491865 - 04/27/14 01:48 PM

Quote:

Eric, is the Velcro at 3 points, or all the way round? And how is the fan mounted to the disc?




Between each bolt - 6 points. 4 small bolts hold the fan frame to the baffle which had a hole cut out of it to match the size of the fan blades.

Eric


Post Extras: Print Post   Remind Me!   Notify Moderator  
choran
Carpal Tunnel
*****

Reged: 12/28/12

Re: Seeing rules! No more fan for me? new [Re: Eric63]
      #6492215 - 04/27/14 05:00 PM

Great! Seems like there wouldn't be any problem with currents and so forth.
Maybe an adjustable cooling coil would be a possibility.


Post Extras: Print Post   Remind Me!   Notify Moderator  
svdwal
Vendor (mBrain Software)


Reged: 03/10/13

Loc: Leiden, The Netherlands
Re: Seeing rules! No more fan for me? new [Re: Cotts]
      #6492233 - 04/27/14 05:08 PM

Texereau already mentions this in his 1957 "How to make a Telescope". He also points out that the tube must be insulated.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Seeing rules! No more fan for me? new [Re: choran]
      #6493051 - 04/28/14 12:17 AM

Quote:

I'm sure there are many reasons why this wouldn't work, but I wonder if anyone has considered initially cooling a large mirror with a cold pack placed against the back of the mirror, the sort of cold pack used to treat an athletic injury. Wouldn't have to be frozen, just fairly cold. (I'm ducking as the answers to this come in).




I do this now. When I know I will be using the telescope, I place a cold pack in the mirror box. There is a lot of thermal inertia in there (so it can not possibly get cold), but I would like to think the cold pack gets things trending down a few degrees. If you live in a humid climate condensation may be an issue though.

There is another possibility. I have often used a 12" household fan placed on the ground behind my mirror box. I suppose one could put a block of ice (or even dry ice) behind the fan to chill the air being blown onto the mirror.

Or better yet, buy an old refrigerator large enough to hold your primary mirror. On the day of a session, place the mirror in the 'fridge and crank it down to the expected evening temperature.


Post Extras: Print Post   Remind Me!   Notify Moderator  
schang
scholastic sledgehammer


Reged: 04/24/13

Loc: columbia, sc
Re: Seeing rules! No more fan for me? new [Re: Jeff Morgan]
      #6493394 - 04/28/14 08:51 AM

Sound like a lot of "work" to me to observe. Had I read this before I buy a dob, I might be turned away from this thing.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Seeing rules! No more fan for me? new [Re: schang]
      #6493438 - 04/28/14 09:19 AM

Quote:

Sound like a lot of "work" to me to observe. Had I read this before I buy a dob, I might be turned away from this thing.




+1


Post Extras: Print Post   Remind Me!   Notify Moderator  
Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Seeing rules! No more fan for me? new [Re: Alein 1630]
      #6493445 - 04/28/14 09:24 AM

Quote:

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?




Sorry about the delay.

Iīm not an expert, but I slightly defocused a medium bright star to see what was going on. The seeing was amazing so I could clearly see a slow motion tube current pattern.


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Seeing rules! No more fan for me? new [Re: Jeff Morgan]
      #6493553 - 04/28/14 10:32 AM

What do you do when the outside air is warmer, heat the mirror in the microwave?.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Doug Culbertson
Post Laureate
*****

Reged: 01/06/05

Re: Seeing rules! No more fan for me? new [Re: pstarr]
      #6493562 - 04/28/14 10:37 AM

Quote:

What do you do when the outside air is warmer, heat the mirror in the microwave?.




Nah, the aluminum in the coating causes sparks in a microwave. A standard electric or gas oven should do the trick.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | (show all)


Extra information
21 registered and 14 anonymous users are browsing this forum.

Moderator:  ausastronomer, Phillip Creed, JayinUT, okieav8r 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 1865

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics