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who in Newtonians relies on passive cooling and why?

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25 replies to this topic

#1 Daniel Mounsey

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Posted 30 March 2016 - 09:20 PM

Okay let's have it.



#2 Ishtim

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Posted 31 March 2016 - 07:30 AM

By "cooling" are you referring allowing the OTA to reach equilibrium with ambient?

 

My equatorially mounted sonotube Newtonian employs passive cooling (no fans), but it "lives" outside 24/7 in a ROR building where temps usually reach 10 to 15 deg.F above ambient during the heat of the day.  I do however open the roof and door as soon as the sun begins to dip below the horizon to allow max passive cooling time.

 

As for the "why", it's more of "the scope was made that way" and I have never been convinced (via observation/imaging) that I need a powered cooling solution.  



#3 Redbetter

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Posted 31 March 2016 - 08:03 AM

Assuming that Newtonians includes Dobsonian Newts...

 

When my battery for the fan is gone/dead or the leads need to be reworked (all too recent memories.) 

 

I prefer to set things outside and let them acclimate while pointed at the sky, with augmentation from the fan on the back of the mirror.  While I've done extensive heat exchanger design and troubleshooting, I haven't really put a sharp pencil to mirror cooling which is anything but steady state.  I haven't tried instrumenting the system to get hard numbers on what is happening.  I would cut open the box and add more fans to sweep across the face of the mirror if not for the particulates that also come with that sort of convection near ground level...especially in arid climates.  And if I did I would cover the fan openings when not in use to reduce dust accumulation in the mirror box.  I can build a system that will cool rapidly, but there are consequences.  (Consider the potential consequences of circulating ambient air through an SCT OTA as an example of the tradeoffs that may occur.)

 

Rapid cooling is more of a concern when the ambient temperature is falling rapidly and/or if one wants to observe soon after sunset.  It has been less of an issue for me in warmer months when the air column is typically more stable anyway...resulting in much better seeing...but not such great transparency as real Winter.



#4 Jon Isaacs

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Posted 31 March 2016 - 09:56 AM

Daniel:

 

I generally don't bother using the cooling fans on my scopes that live out in the high desert. They live in a well ventilated garage but that doesn't eliminate thermal issues, as the sun sets, the air temperature can drop rather quickly. Most often the seeing is so-so and in any event, a "wimpy" fan has little added value if the wind is blowing even 10 mph.. My challenge is not cooling the scope, it's dealing with the wind. We have an anemometer, I consider nights when the wind is 10mph or under as calm. 

 

I am not out there to view the planets or to split double stars,  I am out there to view the deep sky, I get good views at high magnifications of the various objects that take magnification, globulars, planetary nebulae, small galaxies and the like..  On the rare occasion when the seeing is excellent, I have gotten some amazing views of the planets but that is the exception and not my goal. 

 

In the San Diego itself where the seeing can be excellent and the weather mild, I use my 10 inch F/5 and 13.1 inch F/5.5 for viewing the planets and double stars, I do everything I can to make sure the scopes are fully cooled and the thermal issues are under control.  I just bought a 7 inch 110 volt fan to assist with the initial cool down of the 13.1 inch, it takes a while to cool and I hope the additional air will speed up the process.  

 

Jon



#5 Max Power

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Posted 31 March 2016 - 10:23 AM

10" and smaller cools off within 1/2 hour, set it out,
pull off caps, come back in 30 minutes, ready to go.
No electricity required.
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#6 Jon Isaacs

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Posted 31 March 2016 - 10:30 AM

10" and smaller cools off within 1/2 hour, set it out,
pull off caps, come back in 30 minutes, ready to go.
No electricity required.

 

I guess your 10 inch must cool faster than mine.  When the seeing is well under an arc-second and I want to split doubles close to the Dawes limit, it takes well over an hour with a fan running to cool down to the point where I can see clean Airy disk structures.. And that's here in San Diego with it's totally mild weather.

 

Jon



#7 Abhat

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Posted 31 March 2016 - 11:18 AM

Here in Northeast and Mid Atlantic I can't imagine using a Dob without active cooling. That is what I love about Zhumell scope. They come integrated with 12V DC powered fan. Even with fan on 8" it can take an hour to cool down completely to be able enjoy sharp views..


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#8 Max Power

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Posted 31 March 2016 - 11:22 AM

I usually start with the easy stuff that doesn't require sub-arc
second seeing, you know like the Pleiades or Hyades, double
cluster etc. After tooling around for 1/2 hour then scope is ready
for planets. So yes 10" probably takes an hour to really cool down, but the heavy tube currents are gone after 20 minutes or so.

#9 scarubia

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Posted 31 March 2016 - 11:24 AM

Thickness has far more to do with rate of cooling than diameter does. I had a 12" discovery with a very thick mirror than never cooled to ambient in the few years I had it, no matter what I did. My 18" with 1.5" thick mirror usually cools to ambient within a range of 1-2 hours on average nights - sometimes more, sometimes less depending on weather conditions. I do have a 90mm fan running on the back centre of it all of the time. I use an infra red digital thermometer on the back of the mirror to take readings a few times during the night.


Edited by scarubia, 03 April 2016 - 07:33 AM.


#10 mdowns

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Posted 31 March 2016 - 11:32 AM

This has never been an issue with the 14" Orion dob. Always stored in the garage,the conical mirror stabilizes very fast. I always set the scope up an hour or so before dark to tweak the alignment and finder and level the scope.Works great without any additional efforts.



#11 macdonjh

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Posted 31 March 2016 - 11:40 AM

My 10" f/4 is stored in my vented but un-air-conditioned observatory, so it's typically close to ambient when I set it up.  I haven't installed fans for two reasons: I haven't seen the need and I don't want to cut into the solid aluminum back plate of what may someday be a classic Parks scope.



#12 scarubia

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Posted 31 March 2016 - 11:58 AM

I have tried all sorts of fan configurations over the years - on the 18". Fans on the back, fans on the front, fans on both sides, and no fans at all. 

 

Here are a few of my general impressions:

 

-Some active cooling/ ventilation is better than non at all. 

 

-No matter how many fans you use, or what locations - front or back or both, or "mirror scrubbing" - or how ingenious your cooling/ ventilation system is, the effectiveness of your system and the rate of cooling is always subject to immediate (and changing) ambient weather conditions. 

 

-The thinner your mirror the better (I have had my eye on those really thin quartz mirrors for a while).

 

-Lots of open space around your mirror (bigger mirror box, or bigger tube) makes a difference. More air volume, increased ventilation, faster cooling.

 

After all of the trial and error over the years I ended up with a 90mm fan (about 100 CFM) on the back of the mirror at centre with a variable speed controller. On average, full speed until it gets to within 1 or 2 deg ambient, then half speed or less to keep it there (depends on conditions).


Edited by scarubia, 01 April 2016 - 09:49 AM.

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#13 Mike Lockwood

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Posted 31 March 2016 - 03:53 PM

My 20" f/3.0 Starmaster does not currently have fans because I haven't had time to add them.

 

Luckily the primary is 1.25" thick, and fortunately here in Illinois we usually have a breeze blowing, so that helps cool off the mirror and strip off the boundary layer.  I just roll the scope out and point it into the breeze and come back 20-30 minutes later for observing.

 

The cell is the open Starmaster type, so the wind just blows through.


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#14 Jon Isaacs

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Posted 31 March 2016 - 07:51 PM

My 20" f/3.0 Starmaster does not currently have fans because I haven't had time to add them.

 

Luckily the primary is 1.25" thick, and fortunately here in Illinois we usually have a breeze blowing, so that helps cool off the mirror and strip off the boundary layer.  I just roll the scope out and point it into the breeze and come back 20-30 minutes later for observing.

 

The cell is the open Starmaster type, so the wind just blows through.

 

Mike:

 

Is the Starmaster cell similar to the Obsession cell?

 

Jon



#15 Diego

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Posted 01 April 2016 - 05:16 AM

My homemade 6" dob has an oversized PVC tube at a whopping 10" diameter! The little mirror looks quite punny at the other end. I bought it like that and had thoughts of replacing it with a smaller lighter tube as it currently weighs about 29 lbs! And that's just the OTA.

 

However now I'm having second thoughts about replacing it with a smaller diam tube. I was going to go with something like 8" but since I don't want to use fans maybe I'll just leave it with passive cooling. I doubt that with 2 inches of free space around the mirror tube currents will be an issue. I've never noted any cool down problems so far. The mirror is a thin pyrex at 15mm thickness. We don't have sonotube around here and the cardboard tube MFR don't sell small qtys so I will probably have a thin aluminum sheet rolled to the correct diam and then line it with cork for a simple all passive reflector



#16 Pinbout

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Posted 01 April 2016 - 08:09 AM

D*mn Daniel ...  back at it again with the fans...

 

I am in the process of equipping all my reflectors with fans. I have to buy a lot of fans.


Edited by Pinbout, 01 April 2016 - 08:09 AM.


#17 Mike Spooner

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Posted 01 April 2016 - 09:02 AM

With truss DOBs I like an open, airy design. With tubed Newtonians I point them up before observing but am considering active cooling due to problems I've seen lately with a painted aluminum tube I've been using. I gently blew into the top of the tube and noticed the star images stabilized for a couple of seconds. Now I'm wondering why the effect was so pronounced at the "wrong" end of the tube. Has anyone done a study or has all the testing been at the mirror end?

 

--Mike Spooner 



#18 Mike Lockwood

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Posted 01 April 2016 - 11:44 AM

Is the Starmaster cell similar to the Obsession cell?  Jon

 

In the sense that both are open for airflow, yes.

 

In the implementation of collimation adjustment and edge support, no, they are quite different.



#19 scarubia

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Posted 01 April 2016 - 11:50 AM

 

Is the Starmaster cell similar to the Obsession cell?  Jon

 

In the sense that both are open for airflow, yes.

 

In the implementation of collimation adjustment and edge support, no, they are quite different.

 

 

To add to that: Starmaster cells do not have slings and they built their mirror boxes bigger than Obsessions from what I can tell. On my 18" minimum distance from edge of mirror to inside surface of mirror box is 3". 



#20 Steeveaux

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Posted 01 April 2016 - 03:02 PM

I used to use a fan that I built in to my big dob.  It obviously helped with the tube currents.  When the fan bit the dust I removed it and now just let the six inch hole the fan occupied (near the cell and on the tube itself) create a natural draft (the chimney effect).  

 

If it is one of those nights of superb seeing I will sometimes plug in a box fan and just let it blow through the old fan hole for a while.  This seems to kill things pretty fast.

 

Steve



#21 GeneT

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Posted 01 April 2016 - 08:57 PM

I have an older 12.5 inch Portaball, and the fan died years ago. I use only passive cooling. I put the telescope outdoors two hours before I leave for set up. I arrive about sunset. The telescope's optics have reached ambient temperature. The only problem I have ever had is when there is a sudden and dramatic drop in temperature, which does not happen often in South Texas.



#22 Ed D

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Posted 03 April 2016 - 08:01 AM

My 10" Sky Watcher definitely benefits from the 80mm fan I installed, both as to acclimation and air circulation.

 

My 6" strut Dob is an open design made mostly out of wood.  No fan needed, and if there is no breeze I need the 20" box fan, and the strut Dob benefits from it.

 

Ed D



#23 Pinbout

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Posted 03 April 2016 - 08:12 AM

But for my 8in. f3.45 5/8" thk plate glass primary I'll just use passive cooling for now.

 

2016-04-02 19.49.55.jpg


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#24 precaud

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Posted 03 April 2016 - 10:30 AM

At 7,000 ft with generally low humidity, temps fall fast from roughly sundown to midnight. My three Newts are all tube-types, all have rear 12VDC fans blowing on the back of the mirror, drawing air through a hole in an otherwise closed back, which forces airflow around the mirror and up the tube.. I run the fans at 6 to 8 volts. which not only slows down the air volume, it reduces vibration frequency, which the mass it is attached to can more easily damp. By midnight I can generally turn them all off (if I stay out that late!). I would definitely like to try a small boundary layer fan on the 8", its OTA has the least amount of side clearance for the mirror of the three, and (surprise!) it has the worst boundary layer issues of them.


Edited by precaud, 03 April 2016 - 10:52 AM.


#25 JJK

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Posted 03 April 2016 - 11:49 AM

But for my 8in. f3.45 5/8" thk plate glass primary I'll just use passive cooling for now.

 

attachicon.gif2016-04-02 19.49.55.jpg

Lovely scope!




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