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Equipment Discussions >> Reflectors

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btom
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Reged: 02/07/06

Loc: Virginia
Re: Thermal Issues and Fans Successes new [Re: btom]
      #5657866 - 02/02/13 10:29 AM

Just noticed you have a 200mm mirror and 220mm tube ID. Yeah your friend is probably correct that you'd need a pretty low flow rate - probably < 10 cfm. But use a variable fan that could for example do 5 - 20 cfm. Crank it to max for cooling and then reduce as needed for observing. Please update the thread after you've had a chance to experiment. Thanks.

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Sarkikos
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5657998 - 02/02/13 11:57 AM

Quote:

Mike I do know about the don't shine a flashlight on the mirror rule.




It's not a "rule" by any means, but IME and that of many others, shining a flashlight on the mirror can easily give a false impression about its condition. But seems like you realize this.

Mike


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Sarkikos
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Reged: 12/18/07

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Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5658012 - 02/02/13 12:03 PM

Quote:

I used foamboard I got at Walmart.




Yes, that's what I use. It's inexpensive, relatively sturdy, light-weight, probably less of a vibration conductor than other materials, comes in different thicknesses and is easy to work with. I usually get mine from local arts and crafts stores, such as Michael's.

I don't have a workshop or basement, don't have proper tools for wood or metal-working, and don't know anyone who could do the work for me. So it's black foam core for me!

Even though the foam core I use is black, I still spray paint it with flat-black paint. And then I dope it with clear, matte-finish outdoor sealant.


Mike


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johnnyha
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5658848 - 02/02/13 08:14 PM

I'll be very interested in your results. I'm starting to wonder if maybe the exhaust fan in the back is better for a square truss dob mirror box, and the fan blowing at the back of the mirror and up the tube is better for a closed tube scope.

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Sarkikos
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Reged: 12/18/07

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Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5658933 - 02/02/13 09:10 PM

Johnny,

That's what I was thinking, too. It's up there in one of my posts somewhere...


Mike


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5659004 - 02/02/13 10:14 PM

One of the reasons I think the negative airflow might work bettr in truss dobs is its got more places to take in the air unlike the closed tube which must pull it all in through the one open aperture. With a truss, the shroud around poles is hardly airtight so Im guessing whatever opening in the shrouds exist are pulling in air before the open aperture gets it. So less intake upfront, less chilling of the diagonal and possibility of exhalation intake, or even thermals off the guiding hand holding the open end. What does make it down the path is again mixed with other air leaking in from the sides. Too, the two or three fans seem to have less draw per diameter of scope compared to the 5" behind my 8" mirror.

Im just trying to rationalize the success others have had with the negative airflow - particularly it would seem with truss dobs. And as much the horrendous results I had.

Pete

Edited by azure1961p (02/02/13 10:17 PM)


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johnnyha
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Reged: 11/12/06

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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5659019 - 02/02/13 10:24 PM

I think when you blow the air up into a square truss dob mirror box from behind, if you close up the back end, you're cooling the mirror but creating turbulence in the box. Whereas an exhaust fan with the box closed up behind, will gently draw the air from around the mirror. It won't necessarily cool the mirror but it should help create a more laminar air flow around it.

The tube dob is a different beast and easier to control the airflow, that's why I think you might be better off blowing the air out from behind which has proven results. But a better fan configuration would be three fans placed well behind the primary, and an extra-wide tube.


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Sarkikos
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5659053 - 02/02/13 10:43 PM

Remember, though, that some - at least one, anyway - posters on this thread have said that pulling air in from below the primary can bring in rising warm, humid air from the ground. Not a good thing. I believe this was in Florida. So local conditions might have some bearing on optimal direction of airflow from the fan below the primary.

Mike


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johnnyha
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Reged: 11/12/06

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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5659096 - 02/02/13 11:09 PM

Good point Mike and actually the "proven results" on the blow fans were from newts that were well off the ground.

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azure1961p
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Reged: 01/17/09

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Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5659119 - 02/02/13 11:30 PM

I will say guys, some of the best seeing in warmer temps on asphalt was after I hosed it down. For thirty minutes at a time - wow - seriously good. Among the best seeing I've ever ever had. But it does need reapplications!

Pete


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ed_turco
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Reged: 08/29/09

Loc: Lincoln, RI
Re: Thermal Issues and Fans Successes new [Re: btom]
      #5662517 - 02/04/13 09:27 PM

Quote:

Just noticed you have a 200mm mirror and 220mm tube ID.




That 220mm tube idea is really a tight squeeze when considering how air currents work inside a tube. I believe the recommended tube id should be 250mm giving a clear 25mm around the mirror. Tube currents tend to congregate closest to the wall of the tube; using the larger diameter lets them kinda do their thing at the walls and away from the light path.

I've seen the difference in my own work.


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Sarkikos
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Reged: 12/18/07

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Re: Thermal Issues and Fans Successes new [Re: ed_turco]
      #5662555 - 02/04/13 09:51 PM

For most of us who have Chinese Dobs with a narrower clearance between primary and OTA, we just have to do the best with what we have. How many of us are willing to replace the OTA with a larger one or the primary with a smaller one or build a Dob from scratch? The side clearance in my 10" is 19mm. In my 8" the distance is 17mm. So I guess I should give up trying to improve thermals in these scopes?

Mike


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Scanning4Comets
Markus
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Reged: 12/26/04

Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5664140 - 02/05/13 06:39 PM

I got out with my 10" solid tube dob reflector last night. I let the scope cool down for just over an hour leaving the fan on it's highest speed. Once I got outside, the mirror was significantly cooled. I observed M-42 with a 30mm 1rpd, (images were super bad as is expected of this EP), my 22mm Vixen LVW and 14mm Denkmeier. M-42 looked really nice using both the 22mm LVW and 14mm Denk. I tried with a 2" Orion Ultrablock and without to look at the stability of some stars as well.

I observed with the fan on all three speeds and in all three instances, images remained fairly close, with the best leaving the fan speed on the medium speed setting. I have my fan in a closed back, so I took it apart today and I am going to try it with the fan suspended on elastics to see the difference. The baffle was just made out of really thick cardboard to see how it would work. I am not sure if a closed system with a baffle is better than an open system with elastics...so I am experimenting. Another reason I like the "open" system with elastics is because I can at least have the mirror exposed to the air if the batteries die and I am out of luck, lol. I'm glad to see I can leave my fan on at ALL TIMES while observing now!

I should have tried some high power on Jupiter, but it was -10C with a windchill feeling like -19. Brrrr . Too cold for my liking so I went in after 30 minutes.

PS: This is GREAT THREAD to learn from others as to what they have done!

Cheers,


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johnnyha
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5664406 - 02/05/13 10:04 PM

Update: Last night using my new exhaust system on my mirror box, I had the best views of Jupiter ever in my lifetime. So it certainly isn't hurting to switch to the exhaust fan. I was able to go over 250X for the first time in months and months, with a perfectly stable, sharp image. I have never seen such detail on the disc, it was simply amazing. I was truly gobsmacked, I did not think my scope was capable of such incredible views from my backyard in Los Angeles. I recently switched to a thinner 14.5" Zambuto mirror and Protostar quartz secondary, and that combined with this new exhaust fan system has delivered the goods - finally - a beautiful, thermally stable image.

This is a simple 3 speed exhaust fan with a closed back, I don't have a di Lio baffle or any boundary fans installed at this point. I also had my 12V box fan about 8 feet from the scope creating a light breeze blowing my body heat away.


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azure1961p
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Reged: 01/17/09

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Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5664465 - 02/05/13 11:05 PM

Johnny Im glad it s working for you. The consensus seems steady in your favor again I'm guessing the truss design huge mirror difference.

Mike S. I think my Parks has an inch or less (slightly) but the only time I noticed BAD tube wall currents was with fan off. Oh my, the out of focus star grows hair!! You can see the encroaching tube wall thermals dissolving the sharper out of focus star edge. Turn the fan on tough and it tidies down nicely. It's never eradicated like its a vacuum
Ofcourse , but the difference is clear.

We gotta go observing together sometime, alas, Maryland is quite a ways out there from CT.

Pere


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Sarkikos
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Reged: 12/18/07

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Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5664718 - 02/06/13 07:03 AM

Looking good. I was thinking about doing something similar. But I'm not sure if I want to cut holes for anything other than the fan itself. If the baffle is attached by Velcro strips at the bottom edge of the OTA, I can collimate before I put on the baffle. There's probably a better chance of the baffle being air tight without all the holes. Besides, then I won't have to cut out all the holes.

That all depends, though, on how close the collimation screws are to the back of the OTA. If they are very close, I suppose it's best to cut out holes for them. I wouldn't want vibration from the fan to be transmitted to the screws and then to the mirror.

Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5664807 - 02/06/13 08:43 AM

I think my reflector must be vibration proof. I even hung the 5" from the mirror holding cell and nary a vibration. The long fiberglass tube might be extra good at vibration deadening.

Pete


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5664882 - 02/06/13 09:46 AM

Nice to read all the updates from everyone. I've been dead in the water due to weather lately but am excited to give the new 120mm exhausting fan a try. I should also be setup to monitor the temp changes by time the weather clears using this neat little kit:



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Sarkikos
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5664935 - 02/06/13 10:28 AM

Quote:

I think my reflector must be vibration proof. I even hung the 5" from the mirror holding cell and nary a vibration. The long fiberglass tube might be extra good at vibration deadening.

Pete




I've never seen any vibration in the image no matter how I set up my fans in the 8" and 10" Dobs. This includes when they had the factory installed fans directly on the back of the mirror cells.

Maybe folks worry too much about this? I suppose vibrations are possible, though, if the fans are powerful enough and/or are vibration-prone to begin with.

Mike


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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5664941 - 02/06/13 10:32 AM

It's a matter of magnification, too. A fan's vibration may not be visible at low power but becomes visible at high. My rear fan can be used at every magnification, but my side fans, larger and more powerful, can only be used up to about 200X, at which point the image starts being noticeably "jittery".
It seems to be more visible on stars and planets than on extended objects.


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