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 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | >> (show all)
Bob S.
Carpal Tunnel
*****

Reged: 07/14/05

Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602755 - 01/03/13 11:11 AM

Quote:

Quote:

Well that's the difference between reading about someone else's experience and seeing for yourself. The boundary layer fan isn't needed if you're "scanning for comets" but dust storms on mars for example is another matter. The idea of running no fan at all even if the aperture is 4" is just terrible. Again if all your looking at are low power views of fuzzy comets u can get away with it.

It's not a debating point. Either you get rid of your thermals and see more detail or you don't and u see less. It's one or the other. If you use primarily low power you don't need it but your stars are still going to flare.

Pete




I have to respectfully disagree here Pete. I am talking from experience here. My last quote was just showing something about the boundary layer fan in an article.

I have been observing since I was 19 and I am 50 now. Back when I got my first 10" F/5.6 plate glass mirror telescope I had incredible views of Jupiter and other planets as well. I had no fans on the back, nothing. It's not a matter of "if you're just scanning for comets" here either. I have had my telescope up to 488x and the views were excellent, which is pretty rare in my neck of the woods. It IS a debating point and you haven't seen what I have seen in my telescope as you were not here when I saw what I did.

I'm not saying boundary layer fans or fans at the back of telescopes is useless at all because I have a large 12cm fan on the back of the mirror now. All I was saying was that I have seen incredibly sharp detail on the planets and had my scope up at relatively high powers and I do NOT have a boundary layer fan.

It appears that going with a boundary layer fan even with my 10" scope and smaller is the way to go from what I just read....so now I will be looking into making that happen on my scope seeing as testimonials outweigh not having a boundary layer fan.

Where would be the best place to put it on my solid tube dob? The bottom? or the sides?

Cheers,




+1 on what Markus said. I too have spent most of the past decade not using fans and in my steady Florida skies, have gotten incredible views from Newts from 6" to 28". However, there appears to be a small body of evidence to suggest that fans properly placed may have some beneficial results that might not be comparable to a non-fan situation?

I personally maintain some level of skepticism about fans and I have recently made a rather sizeable investment in a fan system on a new premium Newtonian telescope. Only time will tell me how effective my intended strategy is?
Bob


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602761 - 01/03/13 11:17 AM

A fellow out here in CA decided to optimize his full-tubed reflector for planetary observing.
He did the usual things:
--lots of clearance between mirror and tube so tube currents would be out of the light path
--tube extended a foot or so above the focuser
--flocking
--large rear fan
etc.

He showed me, actually proved to me, that all you need is a rear fan to make the images great. And that the mirror doesn't have to be at ambient temperature to give good views as long as the heat is blown away smoothly.

We were looking at Jupiter (altitude about 50-60 degrees at the time) and the image was mush. Soft and scintillating. The out-of-focus image was filled with what looked like worms and plumes and feathers of heat.

He turned the fan on and within a couple seconds, while I watched, the image got sharp and contrasty and tons of small details all over the planet became visible.

He then turned off the fan and within a few seconds the image was mush again. When the fan was turned on again, the image resumed its incredible sharpness in only a few seconds.

The back of the tube was not closed off. There were no boundary layer fans.

The fan created a positive pressure at the back that blew air up the tube, blowing the heat out of the tube in a fairly laminar manner (due to the clearance between the mirror and tube wall. It kept any body heat from drifting up in front of the tube. You could stand at the front of the tube and feel a breeze on your face.

The difference between fan and no fan was simply stunning. It convinced me a rear fan CAN pull the boundary layer off the face of the mirror with a simple pressure differential between the edge of the tube and the edge of the mirror.

A couple years later, I saw this same test performed with a 6" f/8 and the effects were the same.

It's not just heat removal that fans provide--it's also the ability to dispose of the boundary layer in front of the mirror. And while side fans blowing on the mirror certainly do all this a lot faster, they weren't necessary on his 12.5" scope.

Commercial scopes are rarely built with as large a clearance between mirror and tube, so the effects of air flow *may* be more visible in the image. Difficult to say. But I see a positive effect from just a rear fan on my truss-tubed 12.5", so I let it run all night. Side fans can cool a mirror quickly, but, other than an occasional blast, after the mirror is cooled are probably not necessary. But that rear fan certainly is.

IME, that is.


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


Reged: 12/10/07

Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5602780 - 01/03/13 11:26 AM

Quote:

Mark,

Quote:

I just talked to a good friend on the phone and I'm going to switch to 8 D cell batteries and run my fan on those.....Should last a lot longer than 2 or 3 nights of observing, LOL!




Yep, you should be able to go about a year or so without having to replace any of the D cells. Don't even bother getting rechargeable D's.

I have a hand-held battery charge checker that I check all my batteries with before I go to my dark site. It's a bother to take the eight D cells out and check them, because I know that they'll seldom show a low charge. I take extra D cells with me just in case, too, but I don't think I've ever had to replace any of the D's in the field.

Mike




How long the D cells will last really depends on how often they are used and at what draw.

Typical alkaline D-cells are good for at least 12,000maH. You can get somewhat better if you spend more. Seems rechargeables are much poorer and provide around 5000mah. (Apparently some rechargeable D-cells are actually AA's in a D-cell package.)

If your fan runs at 200ma, then you can get about 60 hours use from a fresh D-cell battery. You get proportionally longer if you can reduce the current by slowing down the fan. You get proportionally less if you run multiple fans. Around here 60 hours observing could take years. It wouldn't be a month in better sites.

If you can slow your fan for most observing, and run it at 100ma then you should expect at least 120hrs of use from good fresh D-cells. Double that if you can get away with only 50ma draw. So yeah, they can last for years.

dan


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

Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602819 - 01/03/13 11:48 AM

Markus and so I've had tremendous views without too, sub arc second binaries beyond Dawes with unequal magnitude components , detail on Ganymede. It's all there without fans but its even better with. I'm 51 now started in 74 - used my first full time fan a year ago and a boundary later fan a week ago - ill never go back. Ill post some picks of before and after images with different configurations.

Pete

Edited by azure1961p (01/03/13 11:49 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Scanning4Comets
Markus
*****

Reged: 12/26/04

Loc: Ontario, Canada
Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5602863 - 01/03/13 12:04 PM

Detail on Ganymede.

Bionic eyes?


Post Extras: Print Post   Remind Me!   Notify Moderator  
rguasto
scholastic sledgehammer
*****

Reged: 11/18/10

Loc: Long Island, NY
Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5602864 - 01/03/13 12:05 PM

There was (and is ) a huge difference in my 8" F8 reflector with a rear fan. The rear of the tube is fully enclosed, with <1/2" space around the cell. The fan is 80mm with a 60mm step-down adapter which is the size of the central rear "vent". A huge improvement was immediately appreciated during star testing. Tube currents are minimal if any at all times. Cool down is about 20 minutes. The fan runs constantly. I could never get the quality of images I do now without the fan.

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

Reged: 11/07/06

Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5602938 - 01/03/13 12:44 PM

Don, I have that program on my laptop, but the issue with it is its based on one dimension. Thickness. While testing with my Parks 6 in F8 and then with a Discovery10 F6 with a 2 in. thick mirror I couldn't come close to the cool down times that program gave me. I had a fan that I would hang up on the top end of the tube and blow into it for cool down prior to viewing as well as one blowing on the back. The single best thing i've found to do is to move the scope to get close to ambient prior to viewing. For instance I'm going out tonight, so at 5:30AM this morning I put my mirror box outside in my truck, temps here are going to top out at about 32 degrees so by 5:00 PM its gonna be pretty close to 32. Don't get me wrong I run a back and front too and they help but they themselves have never been able to over come high temperature differentials at the eyepiece.If I were to make a rough guestimate I'd say getting within 10 degrees of ambient has worked well for me.

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


Reged: 04/23/12

Loc: Buford, GA
Re: Thermal Issues and Fans Successes new [Re: demiles]
      #5602971 - 01/03/13 01:05 PM

Love this thread! With all the talk about fans blowing air this way and that, I see little, if anything, mentioned about filtration. I don't yet have a fan on my 8" closed tube but I'm concerned about drawing in dirty air - especially during the summer when it doesn't rain often and dirt is easily kicked up by just walking around. If it was an open truss design, I would think the air would adequately dissipate but is there any concern about dust and other particulates accumulating inside a closed tube telescope? I'm a little concerned to install a fan on the back of my telescope without a filter on the intake side and a decent baffle. Perhaps my concerns are unwarranted?

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

Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: dan_h]
      #5603011 - 01/03/13 01:31 PM

Dan,

Quote:

If your fan runs at 200ma, then you can get about 60 hours use from a fresh D-cell battery. You get proportionally longer if you can reduce the current by slowing down the fan. You get proportionally less if you run multiple fans. Around here 60 hours observing could take years. It wouldn't be a month in better sites.

If you can slow your fan for most observing, and run it at 100ma then you should expect at least 120hrs of use from good fresh D-cells. Double that if you can get away with only 50ma draw. So yeah, they can last for years.




At this time the eight D-cells only run one fan beneath the primary. I run the fan at full speed for the entire time I'm at the dark site; I don't have a potentiometer to vary the speed. At a conservative estimate, I go to the dark site an average of once a month and run the fan for about five hours each time. This would be about 60 hours a year for each battery. As I said, the batteries seem to last at least a year, so this is about right.

Mike


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

Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5603019 - 01/03/13 01:35 PM

I'm considering taking that three-fan harness I bought from Orion and adapting it to the 10" Dob. I might rig up one fan as the boundary-layer fan, and attach the other two to a baffle beneath the primary.

As long as there is no vibration transmitted to the optics, I'll run them at full speed all night. Maybe I'll have to replace the D cells every four months or so. Big deal. I'll spend much more than that in gas.


Mike


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

Reged: 08/27/04

Loc: New Hampshire
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5603043 - 01/03/13 01:46 PM

The value in using cooling fans is highly dependent on your local temps and region where you observe.
Up here in NH, I often have a delta-T of > 50deg-F, moving the scope from inside to outside. It takes forever for the mirror to approach outside ambient. Without a fan ... forget it. That's not even taking into account the drop in ambient temp during the night.


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

Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5603059 - 01/03/13 01:55 PM

Quote:

Detail on Ganymede.

Bionic eyes?




The problem isn't so much catching some albedo features on Ganymede ... it's having to bump up the power and follow that little bugger without tracking!


Mike


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

Reged: 12/30/06

Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5603278 - 01/03/13 03:58 PM

Anybody considered coring out the center of the mirror, attaching a fan on the back and sucking the boundary layer off out through the middle?

Jeff


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

Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Jeff B]
      #5603534 - 01/03/13 07:03 PM

Sure. I'll get me one of them hole borers and knock that out during lunch tomorrow.


Mike


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

Reged: 02/07/12

Loc: UK
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5603599 - 01/03/13 07:46 PM

Do you think if I put a 12 inch fan on the bottom of my 12 inch dob it would cool down quick?:-)

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

Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5603606 - 01/03/13 07:51 PM

Too big!

Vibration hassles. Try several 80mm fans instead.

Pete


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

Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Quest]
      #5603627 - 01/03/13 07:59 PM

Quest,

Quote:

Love this thread! With all the talk about fans blowing air this way and that, I see little, if anything, mentioned about filtration. I don't yet have a fan on my 8" closed tube but I'm concerned about drawing in dirty air - especially during the summer when it doesn't rain often and dirt is easily kicked up by just walking around. If it was an open truss design, I would think the air would adequately dissipate but is there any concern about dust and other particulates accumulating inside a closed tube telescope? I'm a little concerned to install a fan on the back of my telescope without a filter on the intake side and a decent baffle. Perhaps my concerns are unwarranted?




A filter might be a good idea. I was thinking of putting one on my fan. But dust is not a big deal here in Maryland. It's so dewy, any dust settles quickly as mud before it can be sucked into the fan!

Seriously, a good way to keep your area clean is to bring an indoor/outdoor carpet or tarp to put your mount on. I think a nice, black carpet is best to retain your dark adaptation. The bad thing about tarps is that they can be slippery.

I just picked up a couple black carpets with rubber backing I can put together to make about a 6'x8' area under my Dob mount. Cheap, too.

Mike


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

Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5603636 - 01/03/13 08:06 PM

Quote:

Quote:

Quote:

However, there appears to be a small body of evidence to suggest that fans properly placed may have some beneficial results that might not be comparable to a non-fan situation?

Bob




A small body of evidence? How big does it need to be?

Pete

Edited by azure1961p (01/03/13 09:01 PM)


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

Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes [Re: azure1961p]
      #5603699 - 01/03/13 09:07 PM

Filtration I think is overkill. You do want a light mesh foam air conditioner type filter to keep from sending Beatles and such from bring shot at the glass but beyond that I don't think it's needed. Chicken wire underneath the ac filter material adds stiffness. You may find a boundary fan actually does a terrific job of blowing dust off that normally would settle without wind interruption. The other thing to consider is children. You don't want little fingers getting wacked. I'm a solo observer but Id rethink the issue of fingers if I were at a public affair.

Pete


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Thermal Issues and Fans Successes [Re: Dick Jacobson]
      #5603706 - 01/03/13 09:19 PM

Quote:

so I think a boundary-layer fan is needed when your mirror is more than a few degrees warmer than the air.




Quote:

And that the mirror doesn't have to be at ambient temperature to give good views as long as the heat is blown away smoothly.

Side fans can cool a mirror quickly, but, other than an occasional blast, after the mirror is cooled are probably not necessary. But that rear fan certainly is.




That seems reasonable. However you get rid of that layer the views should sharpen (side, back, top fan. Whatever.) If the layer is gone and stays gone, no need to blow something that is not there. But it should only take a few degrees change in ambient to create a new boundary layer.

However, being stable thermally seems important, too. Some folks report various amounts of correction error as the mirror is settling. They are thermally stable, and that seems to mean they - pretty much - retain their shape while resisting changes in temperature. Right?

In Daniel Mounsey's review above, this seems consistent with Mike and my own belief you help warm air do what it wants to do: rise. Pulling it out seems a good solution since it adds low pressure (to which air wants to move) and exhausts the air. He did mention, about his smoke test, the entire tube was evacuated in 5 seconds. That /should/ result in better seeing - instantly.

And, after viewing the laminar flow problem recently, I can see how the air blow across the primary departs from the mirrors surface. It leaves a turbulent layer below (on the primary's surface, much like an aircraft wing at stall) That turbulent air can remain trapped near the primary. There is likely some improvement in the view, but it might be less efficient in terms of total evacuation.

Detail on Ganymede (and elongation "effect" of Io) are certainly doable in modest apertures...provided collimated in good seeing and, of course, cooled.

Edited by Asbytec (01/03/13 09:33 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | >> (show all)


Extra information
18 registered and 32 anonymous users are browsing this forum.

Moderator:  ausastronomer, Knuklhdastrnmr, Phillip Creed, JayinUT 

Print Thread

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


Thread views: 20420

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics