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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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As part of my 22-inch Dob construction project, I built some cooling fan units that mount just above the mirror box and direct airflow onto the mirror. In practice, the vibration from these was a serious problem. I added a PWM circuit to the electrical supply and found that by dialing down the speed, the vibration could be dramatically reduced, but even without the vibration, I didn't see any improvement in the field of view.
Some participants here suggested using smaller fans and mounting them on rubber bands to lower the vibration. Both good suggestions. But, I haven't really pursued them because the fan units were a lot of work, modifying them would be even more work, and I'm not convinced they will help that much for a large mirror.
I tried positioning a room fan behind the scope to cool the mirror off initially before an observing session. This gets the scope operational in a hurry, BUT, it also leads to dew problems on the primary mirror in humid Missouri observing conditions. So I have discontinued that.
When observing from our backyard, which is what I do most of the time, I just wait until the sun is off our driveway, in the late afternoon, and then set the scope up so its temperature can reach ambient by the time it gets dark. This works well, though it is not an option at dark sky sites.
I notice some users covering their large Dobs with reflective tarps, but I have found that leaving the scope uncovered so any breeze can flow across the mirror box results in the mirror being much cooler by sundown compared with scopes that have been covered.
A few nights ago, observing from my backyard, the thought occurred to me that I could set a Vornado room fan on a small table near the mirror box, raise the back end of the light shroud to expose the mirror, and then direct airflow from the fan on the face of the mirror. Doing this enables a very large air flow with no vibration problems. I did some comparison views with and without the fan, and setting the fan at low, medium, and high air flow rates, but I couldn't see any improvement in the view with the fan on. It would probably make a difference if the mirror were not already near air temperature, but it doesn't seem to help much at all if the mirror has been given time to cool down normally. Not surprising. I guess what this might indicate is that small variations in temperature between the mirror and ambient air conditions do not cause enough of a convection problem in an open mirrorbox design for forced air cooling to provide much benefit. The mirror box for my scope is shallow compared with many American designs (only 9-inches high), the back is very open due to additional air flow cutouts in the corners, and the mirror is only about 2 inches below the top of the mirror box.
I intend to experiment with this a little more, but so far I haven't seen anything to indicate cooling a large Dob mirror with fans helps improve the view in the eyepiece. I don't doubt the benefit of fans for smaller, closed tube Dobs, but that is a rather different application.
Putting a fan behind the mirror would be easy enough, and it might help at dark sky sites. I guess this is a common application. But I keep hearing Albert Highe commenting that trying to cool a mirror by blowing on its backside is like trying to cool a cup of coffee by blowing on the bottom of the cup...
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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spaceghost
super member
Reged: 01/04/07
Posts: 139
Loc: Olathe, KS
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Hi Friske. Sky and Telescope Jan. 2008 has a good newtonian cooling article. I use a fan on the back of mine, blowing up the tube. It makes a huge difference at high magnification on the planets and moon. Without the fan on, no matter how long I leave the scope outside, I never get a good view like I do when I have the fan on for a while.
One of the best ideas I've seen for a big dobsonian like yours was a small fan strung with wires directly in front of the mirror, in the center, blowing on the mirror. The fan was behind the secondary, and smaller than the secondary, so it wasn't blocking any of the view.
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Justin
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Justin:
Nice to hear from a neighbor! (I live in midtown KC.)
Positioning a fan over the primary in the secondary mirror silhoette (sp?) is a novel idea. I haven't heard of that before. I have to say I would be nervous about having a fan suspended over my primary. If I thought it would help the image, it might be worth working on. The thing is, that Vornado was delivering way more air flow than any muffin fan can provide and I couldn't see any difference even with that.
I'll have to take a look at the January S&T. Thanks for the reference.
From your description, I'm guessing you have a mirror in the 6-12 inch range with a solid tube. I have read lots of content concerning the effectiveness of this arrangement, consistent with what you are saying.
BTW, are you an ASKC member?
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2394
Loc: Doylestown, PA
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Fiske,
I'm guessing from this post that you are assuming your mirror is not reaching ambient temperature and thus you could have better views at the EP. Is this so? I know your mirror is 22" diameter, what is its thickness? Cooling rate is a function of mirror thickness.
Have you determined that your mirror still has a distorting boundary layer or is it that you believe the views should be better?
With my 18" (2" thick mirror) I went through a similar thought process. I do set-up the scope early with a house fan behind the mirror. The small fan I leave run all night.
To determine if the mirror is near ambient I use high power (280-330x) and defocus a brightish star. If I see slow, wiggly blobs and/or streamers then my mirror is still warm. If I see small, fast moving in one direction blobs then it is the atmospheric seeing and the mirror is close enough to ambient. This is the case on almost every night after a few hours. It is very rare to have seeing good enough to push the magnification of an 18" scope.
Does any of this help?
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Walt:
This is a lot of help!
I actually believe the mirror was at or near ambient. For one thing, these were warm nights. The temperature didn't drop much. The mirror is 2-inches thick, like yours. I'm just curious about cooling and wanting to play around with it a bit and understand it better.
As I mentioned above, I have learned NOT to cool my scope with a room fan at dark sky sites in Missouri if there is even a whiff of humidity in their air -- a sure fire way to dew up my primary in the early morning. (Something I really hate.)
Checking the distortions on a defocused star at higher powers is an excellent (and elegantly simple) test. I can see doing it when the mirror has not had time to cool down just to see exactly what boundary layer problems look like and then comparing that to a properly cooled off mirror to compare patterns caused by atmospheric seeing.
My experience has been similar to yours -- although I have plenty of aperture to support a lot of magnification, the seeing conditions rarely permit me to do so. It's a good night when I can effectively use a 5mm eyepiece (447x).
When I first started using the large Dob, I worried a lot that it wasn't collimated well enough. However, I'm using a Glatter laser collimator and a 2-inch Cat's Eye autocollimator and have no trouble getting both tools to agree and keeping the autocollimator reflections stacked up. I finally realized the issue is seeing, not scope performance.
I think one source of confusion about this is that users don't always account for observation reports from areas where sub-arc second seeing isn't that unusual. Most of us aren't so lucky.
Oh, another thought that has occurred to me is the argument that cooling is more important when the temps are dropping rapidly may be underminded by the fact that if temps ARE dropping rapidly, the seeing is not likely to be good anyway. I'm not sure how this would actually prove out, but I suspect that since steady seeing conditions almost always occur when the temps are stable and there is little or no breeze, mirror cooling isn't likely to be much of an issue. In my experience, these conditions are more likely to occur later in the evening, I mean really steady seeing, when, of course, the mirror has had plenty of time to reach near ambient temps.
Thanks for sharing your thoughts!
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2394
Loc: Doylestown, PA
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Good thought about steady temperatures and seeing. I'll try to note this during observing to see if there is a correlation.
I have found that a period of good (better) seeing can be short. Besides the fleeting moments I've have periods of 2 to 20 minutes when the seeing stabilized and planet viewing was incredible. Several times it was when a light cloud cover moved in and the faint fuzzies disappeared. I could still see Mars or Jupiter so moved the scope to them. After some time the seeing deteriorated and then noticed the clouds cleared off.
For planets I've also found that using a higher magnification than the seeing seems to support is better than a lower magnification. This way when the seeing improves, even for a moment, you'll see the details.
So I guess the moral is observe often and don't give up due to what seems like less than ideal conditions.
Oh, Fiske, if I haven't said so yet, that is a nice scope you built. Enjoy.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Walt:
Thanks so much for your kind comment about my scope. Your suggestion about using higher power makes good sense.
I'm still getting used to observing with such a big scope, figuring out how to use it to best advantage. For me it takes some time to learn all the ins and outs of a particular instrument. Each one seems to have its own personality in a way. Its own quirks. When you really get to know a scope, it is like an old friend. You know its preferences, strengths and weaknesses, and enjoy spending time with it.
I don't observe exclusively with the 22-inch either. Last night I was out with my XT-8. I was amused to notice that stellar diffraction patterns were obvious in the XT-8 when I was using a Nagler 5mm T6 eyepiece. The same eyepiece provides 447x in the 22-inch and doesn't come close to displaying a diffraction pattern. I realize this is because the larger aperture provides much more resolution. It is just interesting to see the difference in practice.
Come to think of it, I have never seen a diffraction pattern using the 22-inch. I guess I just haven't pushed the magnification high enough. I'm sure some formula or other is available to determine what magnification would be required to see a diffraction pattern with a mirror this size. I think, if I understand correctly, the requirement is so high seeing conditions rarely permit them from being observed with larger aperture instruments.
Steve Kennedy, who made the mirror, actually moved to Yucca Valley, CA because he performs star tests as part of his figuring process, and needed a location with extremely steady seeing.
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2394
Loc: Doylestown, PA
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I think you need about 50 to 60X per inch of aperture to see the diffraction rings. That's about 1200x in your big scope.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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LateViewer
professor emeritus
Reged: 11/17/05
Posts: 608
Loc: Westchester NY
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Hey Fiske,
While I do not have a pic, I do have my fan isolated by the things women use to hold their pony tails tight. These hold up over time in a way that rubber bands do not.
Al
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12.5" Discovery String Truss Dob
DSV-1 Alt-Az Mount
WO 66mm Petzval
Orion 127mm Mak with WO 2" Dielectric Diagonal
Celestron CG-5 GoTo
8.8, 14, 18, and 24mm Meade UWA Series 5000 EP
32, 40mm Orion Optiluxe EP
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Al:
Well of course they do! We all know how important fashion and style are...
Thanks for the tip. It's a good one.
Walt:
Thanks for the guidance on magnification required to show diffraction rings. To accomplish that, I would need to barlow a 4mm eyepiece, which I don't own at present. But, I've always thought any well-rounded amateur astronomer should own a decent 4mm eyepiece. Hmmmm. I wonder if Tele Vue offers a 4mm T6...
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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kfrederick
sage
Reged: 02/01/08
Posts: 214
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hi nice telescope big mirrors are hard to get cooled down/ my 20inch i use two 12 volt fans to pull the air down and out the bottom if i wire them in series and use a 18 volt cordless drill battery they are runing on 9 volts they do not shake the image much and help alot/ to cool the telescope down when i take it out i run them on 18volts/ i like your motors you have on the bottom where did you get them ? thanks very nice telescope /kevin
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square_peg
Postmaster
Reged: 03/26/04
Posts: 23812
Loc: Maple Valley, WA
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Quote:
I actually believe the mirror was at or near ambient.
That's probably the case. We have so few nights where the temperature is stable that I can't imagine not running the fan. I've done on/off tests on high power images and the difference is obvious. A clear high power planetary image will turn to mush in 3-5 seconds after turning off my fan. It snaps back clear about 1 second after turning it back on. My fan blows on the back of my mirror. The back of the tube is closed off around the fan so that the air has to travel up out the tube. Of course tube currents probably aren't as much of a problem on your trusser.
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Tom (Pegster)
DSH-8 (GSO Dob)
15x70 Oberwerks
SVP 100 f/6 achro
WO 66 Petzval
Sears Discoverer EQ 60/900
8x42 Regals
History is Philosophy teaching by examples.
Thucydides
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Crab
super member
Reged: 02/23/05
Posts: 111
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If you have problems with dew and get the best seeing when the air is still then my guess is that you are observing on flat ground or in a valley. If you can find a ridge where there is a steady breeze coming up the ridge from a valley then the seeing at a point just below the top of the ridge will be better almost any night than any place on the flat or valley. Breezes on flat ground churn the different temperature air in swirls. The ridge causes laminar flow and gives the impression that the altitude is higher.
There is considerable information in the air currents visible with curved mirrors. You can make a schlieren device that records an image of an aircraft in flight where the variations in pressure about the aircraft are visible. From such a photographic image you can determine the speed of that aircraft.
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dan chaffee
sage
Reged: 03/22/06
Posts: 280
Loc: Kansas City
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Hi Fiske,
If seeing is good enough to see diffraction patterns,
they should be visible at between 20x and 25x/inch aperture, irrespective of the actual aperture. But, you
are right in that the seeing would have to be so extraodinarily good to see diffraction patterns in an aperture that size and at any magnication that
the probability of that happening is very low; at least here in Mo. and Ks. What this really means is that probably 99% of the time viewing you'll be left with a strehl MUCH worse than .8--or anybody's definition of "diffraction limited" --after the atmosphere gets through with an optical path that wide, and that includes some pretty good nights.
We can only do so much to ensure that things we can control
aren't the weakest link in the chain and cooling a big
primary (by amateur standards) is way up there. My best result for my 10" chunk is to aim a desk fan across the
primary so it scrubs the face and isn't connected to the
OTA. I suppose one thing to try if you go with fans
mounted behind the cell that might mitigate vibrations is
to look for fans of slightly different sizes (or vary the sizes of the fan blade diameters) such that the
differing vibration frequencies cancel each other out.
Dan
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spaceghost
super member
Reged: 01/04/07
Posts: 139
Loc: Olathe, KS
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Hi Fiske,
To answer your questions, no, I'm not yet a member of ASKC, but hope to be someday. I have a 5" f/8 dobsonian, and a just-finished 12.5" f/6 dobsonian, both tube-style. The 12.5" after a couple of hours of cooldown gave a great view of Jupiter at 274x. I'm using it a few times without a fan to see how it does, and then I'm putting three 10 cm fans on the back blowing up the tube. I'm anxious to see how the fans work with the 12.5". One fan works wonders on my 5".
I agree with Walt, viewing the out-of-focus swimming of a star or planet is a great indication of problems with temperature differences at the mirror. I find that the fan on my 5" greatly improves the steadiness of the out-of-focus image.
For the big thick mirrors like yours, I bet the best thing to do would be to keep it in a cooled chamber matching the nighttime temperature, so when you take it out it is ready to go. Imagine not having to wait for the cooldown. I wonder if anyone does this.
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Justin
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Thanks everyone for your responses and suggestions.
Quote:
If you have problems with dew and get the best seeing when the air is still then my guess is that you are observing on flat ground or in a valley. If you can find a ridge where there is a steady breeze coming up the ridge from a valley then the seeing at a point just below the top of the ridge will be better almost any night than any place on the flat or valley. Breezes on flat ground churn the different temperature air in swirls. The ridge causes laminar flow and gives the impression that the altitude is higher.
This is interesting, but, ridges like you describe are not too common in our area. I'll keep this in mind, though, in case I come across an observing location with these characteristics. It makes sense, I will say.
Dan:
Nice to hear from you.
Quote:
What this really means is that probably 99% of the time viewing you'll be left with a strehl MUCH worse than .8--or anybody's definition of "diffraction limited" --after the atmosphere gets through with an optical path that wide, and that includes some pretty good nights.
This agrees with what I have experienced so far. It doesn't mean using a large aperture scope is not worth the effort, just that the scope isn't going to perform anywhere near its theoretical resolution most of the time. It does consistently outperform smaller aperture scopes.
My experience with fans so far has not been encouraging. I'm still interested in experimenting with them, but do not plan to invest a lot of resources or time in the effort. Pointing a Vornado (or any sort of compact room fan) at the primary is easily done and doesn't require purchasing anything. The next time I mess around with this, I'll try it using much higher magnification. I may try mounting a fan behind the mirror on elastic bands, simply because it is easy to do, I already have the scope wired up for the necessary power, and I already have the fans. I'm skeptical it will make much difference, but we'll see.
Justin:
The ASKC is a fun club, and they have a fabu dark sky site in Butler, Missouri with electrical RV hookups, etc.
Sometime in the next year I plan to build a telescope shed near my observing location in our backyard so I can easily roll the 22-inch out instead of setting it up and taking it apart each time I want to observe. I intend to install a thermostatically controlled exhaust fan to keep the temp in the shed at ambient during the day. Cool down time has never been a problem for me when observing from home simply because I set the scope up in plenty of time for it to cool off and our driveway (where I observe) is shaded from the sun all afternoon by trees on the west side of our yard.
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10441
Loc: Amargosa Valley, NV, USA
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Fiske, wait until Fall weather rolls around, and see if things change with regard to cooling. Humidity in the air tends to retain the day's heat and slow the cooldown to the point that the mirror may have plenty of time to track the temperature changes. Once you start getting frontal systems blowing through, with dryer air behind them, temperature changes will happen much more rapidly. At that point you might find that the fans are useful after all.
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Dave:
Yeah. The thought had crossed my mind.
Thanks for weighing in. I will post back to this thread if I notice this in the fall.
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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dan chaffee
sage
Reged: 03/22/06
Posts: 280
Loc: Kansas City
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Quote:
This agrees with what I have experienced so far. It doesn't mean using a large aperture scope is not worth the effort, just that the scope isn't going to perform anywhere near its theoretical resolution most of the time. It does consistently outperform smaller aperture scopes.
I don't doubt that for a second. A 22 inch scope could have
a strehl of under .7 at the eyepiece and still blow your mind with planetary detail and resolving globulars!
Dan
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Yeah. I have yet to see a large Dob (18-inches or over) with any sort of cooling fan mounted behind or in front of the mirror. I realize some scopes have these, maybe Obsessions come with a rear fan installed(?), and other ATMers add them. But I have seen dozens of big scopes, none of which do. Of course, these are mostly used at lower magnifications on DSOs... 
They do provide stunning views, without any cooling whatever. I just wonder how much difference fans might make. So far I haven't seen any difference, but I will keep playing with them.
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
Edited by Fiske (08/12/08 07:28 AM)
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