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Equipment Discussions >> Reflectors

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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Thermal Issues and Fans Successes new [Re: Pinbout]
      #5645819 - 01/27/13 10:58 AM

Quote:

its the resistance in the system that reduces the flow rate.



I assume you mean because the air pressure builds up inside the rocker box, reducing the ability of the mirror to pull air out of the tube?


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5645966 - 01/27/13 12:28 PM

also the length of travel, the tube wall texture can be a resistance, direction doesn't matter. when I had the fan blowing I blew some smoke thru the focuser and it came out the top in a similair flow. I know 4' of travel isn't much but it will start effect cfm's.

also to note the fan plate has a gap between the kydex and the tube of about .25in and there is no draw from this gap. guess that sharp 90° turn is more resistance than the mirror/cell obstruction.


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johnnyha
Postmaster
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Thermal Issues and Fans Successes new [Re: Pinbout]
      #5653441 - 01/30/13 11:51 PM

Well I figured out how to cool my dob mirror down now that I have the back end of the mirror box closed up by the exhaust fan shroud. I just put my RV 12V box fan on my Stardust chair and lowered the scope to horizontal and blew right down the top opening of the UTA onto the top of the mirror. This is of course with the shroud attached and the exhaust mirror on. At least for a 15" Classic size dob, this works great for initial cooldown.

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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5653757 - 01/31/13 07:51 AM

Here's another quality of air in the front of the tube, direction doesn't matter, that was apparent.

the movement of air forms a curtain, similar to the air curtains in a commercial refreigerators with the open fronts, like where they have yogurt, cheese stuff like that.

it was very hard for me to place the smoke into the column of air so it would be pulled down easliy. most of the time it was blown away from the curtain.

this why its hard for me to believe someone breathe will be pulled into the light path from being drawn down.


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5656727 - 02/01/13 04:44 PM

Thought that I would post an update to the baffle(s) I've made.

After a lot of promise shown with the results of my temporary baffle, I began construction on two permenant ones, an 80mm and a 120mm.

I went to FastSigns and had them cut two baffles for me from black PVC. Both having 14" OD's and one with a 3-1/8" ID hole for the 80mm fan and another with a 4-11/16" ID hole for the 120mm fan. They did a nice job with smooth, accurate cuts, much better than I could have done with a knife, cutter, or saw.

I drilled the holes to mount the fan, switch and power jack, wired it up, applied the weatherstrip and velcro for mounting on the scope. The first completed one shown here is the 120mm fan. It will suck or blow at over twice the rate of the 80mm fan and do so at 300 less RPM (1400).

Here's some pics.







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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5656751 - 02/01/13 04:52 PM

con't...

The fan just clears the azimuth motor when pointed at the zenith. I'm powering this through a 12500ma LiPo battery that my Dew Controller is connected to and then down to the fan. I'll be able to switch between 12V and 9V to control the speed somewhat.

At full power, I can feel the breeze sucking into the tube. I still need to make the Mauro Da Lio baffle and then I'll be be able to tell which fan will be adequate.







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rockethead26
Carpal Tunnel
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Reged: 10/21/09

Loc: Arizona, USA
Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5656794 - 02/01/13 05:12 PM

Nice job, Mark! Very neat!

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johnnyha
Postmaster
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Thermal Issues and Fans Successes new [Re: rockethead26]
      #5656808 - 02/01/13 05:21 PM Attachment (14 downloads)

I have been getting excellent results the last few nights with the back end of my Obsession truss dob more or less sealed up and using a small exhaust fan on low to medium speed (I posted a photo of this earlier). I also decided to start using my larger 12V box fan to establish a nice breeze blowing my body heat directly away from the telescope. I placed the 12" fan up on a chair about 8 feet away and it works extremely well even at lowest speed. The last few nights with this system I feel like I am finally getting control of the local thermal issues. The fan blowing away my body heat especially was such an easy thing to do and I definitely felt like the image was more stable throughout the night.

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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5657063 - 02/01/13 08:09 PM

Quote:

I went to FastSigns and had them cut two baffles for me from black PVC




that does look very clean.


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btom
super member
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Reged: 02/07/06

Loc: Virginia
Re: Thermal Issues and Fans Successes new [Re: cjc]
      #5657196 - 02/01/13 09:48 PM

This idea has been around a while. Mauro Da Lio has discussed this several times in the past. I exchanged email with him as long ago as 2004 when I was first looking into build a 10" F/5 solid tube. I believe we came up with something like 20 CFM max flow rate to still be laminar for 3/4 inch clearance mirror edge to tube and annulus just shy of 3/4" width and 2" above the mirror face. This was based on fluid dynamic calculations performed by either he or a friend of his. Looking at my sig will give a clue to how quickly I am building that scope

In the meantime, I've bought and sold several scopes and just this week bought a used 11" Teeter STS. It has a similar situation to your scope with only 10-12mm from mirror edge to tube wall. I think for that situation the flow would need to be no more than about 15 cfm to still be laminar. But that's just a guess. I'd use a variable rate fan and perhaps add some baffles to direct flow around the back of the mirror. That way you could increase flow rate for initial cooling with baffles also directing flow on the back of the mirror. When you start observing lower the flow rate as needed to get best image. Anyway some thoughts. I'm interested to see what you find as I'll probably eventually try something similar on the STS. Good luck!


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azure1961p
Postmaster
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Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: btom]
      #5657290 - 02/01/13 10:35 PM

Mark,

Now that's a beautiful thing - look what you did. I can't say my own handy work has been as clean lined. Bravo on your efforts here - mines quite ugly by comparison !!! Alas I'm still promising myself a CF tube homemade so there is hope.

Do consider the 5" fan too if you question what you've got in terms of efficiency - tho you may very well have reached optimum. You flattered your instrument.

Pete


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lbsgville
sage


Reged: 06/15/10

Loc: Florida
Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5657762 - 02/02/13 09:25 AM

I have been following this thread for awhile and decided to reverse my fan and see what happens. I have a solid tube 10" dob. It's an old Meade starfinder but I have done many mods including replacing the particle board mirror cell, well if you can call a mirror glued to a piece of particle board a mirror cell. The new cell came with a 80mm fan and I have always had it blowing onto the mirror. I only had one test so far and it was only about 3 hrs but the mirror seemed to stay more clear. In the past I have noticed the mirror would look like it was getting dew on it if I shined my red headlight down the tube, but still look good if I just looked at the reflection of my hand held in front of the tube. With the fan sucking out the back the mirror looked like it had less dew on it. Again this is my first test and I will do many more, also keep in mind I don't know any of the technical aspect of this subject so my info will be subject to much scrutiny. When I got the new cell I also made a cover for the back of the scope so the air didn't just bounce of the mirror. It works very well. When the fan is blowing on the mirror I can feel air coming out the front of the tube and when I reversed the fan I could fell air being pulled into the front. Like I said I will have to do more testing and looking at the out of focus star to check for tube turbulence and make the best assessments that I can, but the first test seemed to show that for my solid tube blowing air out of the back my be the way to go. Here is a pic of the setup.

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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5657780 - 02/02/13 09:34 AM

Quote:

In the past I have noticed the mirror would look like it was getting dew on it if I shined my red headlight down the tube, but still look good if I just looked at the reflection of my hand held in front of the tube. With the fan sucking out the back the mirror looked like it had less dew on it.




It's probably not a good idea to shine a flashlight on the primary. That can give you a false idea of the condition of the mirror. If there really were dew on the primary, I don't think there would be any question about it. Just do the out-of-focus star or planet test to check the thermals and see how well the mirror is acclimating.

Mike


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lbsgville
sage


Reged: 06/15/10

Loc: Florida
Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5657781 - 02/02/13 09:34 AM Attachment (20 downloads)

Note to all, do not hit the enter button when spell checking. here is the pic without backing.

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lbsgville
sage


Reged: 06/15/10

Loc: Florida
Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5657783 - 02/02/13 09:35 AM Attachment (21 downloads)

And with the back on.

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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5657786 - 02/02/13 09:37 AM

What material did you make the baffle from?

Mike


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lbsgville
sage


Reged: 06/15/10

Loc: Florida
Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5657808 - 02/02/13 09:53 AM

Mike I do know about the don't shine a flashlight on the mirror rule. I am always amazed of how bad a mirror looks when shinning light on it and I do take that into account. I do it to see how the mirror looks as the night progresses. I use my red light to do this and at the beginning of the night the mirror is clear and as the night goes on it starts to look like it is dewed over. I know it's not really the case because when I shine the light on my hand and hold it in front of the scope and look at the reflection in the mirror it's clear. I made that statement to because when I shined the red light on the mirror after reversing the fan the mirror looked less "dewed over" than in the past. Again I know this is not a true test but if the mirror looked better after reversing the fan I was using that reference as one of my not so scientific results.

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lbsgville
sage


Reged: 06/15/10

Loc: Florida
Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5657814 - 02/02/13 09:57 AM

I used foamboard I got at Walmart.

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lbsgville
sage


Reged: 06/15/10

Loc: Florida
Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5657834 - 02/02/13 10:07 AM Attachment (17 downloads)

Mike, the foamboard is by the school supplies. It's made by Elmers. It comes in black now but I don't think they had black when I made mine and hade to paint it, here is how I made it.

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lbsgville
sage


Reged: 06/15/10

Loc: Florida
Re: Thermal Issues and Fans Successes new [Re: lbsgville]
      #5657836 - 02/02/13 10:08 AM Attachment (17 downloads)

Final result

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