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Buying an older Dob/Cooling systems

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#1 ricky64

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Posted 13 September 2019 - 10:36 AM

Hi all;

 

I'm looking at picking up a used Dob. It's clear from reading, that observing once temp equilibrium hits is key for best performance.

I live in CO, so I have large temperature swings. Also, my job and family structure make it so that time is my scarcest commodity...

 

Buying an older Dob is very attractive as they seem to depreciate quite a bit, and excellent optics can be had. However, I note on newer designs, such as New Moon, the fan systems are more sophisticated.

Any thoughts on how critical the existing fan systems are, or how hard they are to retrofit?

The dob I am looking at has a Zambuto mirror; to my reading these are a bit thinner which is helpful

 

Thanks



#2 Barlowbill

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Posted 13 September 2019 - 10:51 AM

Not that I am an expert by any means but I would argue that collimation is "key for best performance".  It is also more difficult than setting up outside and waiting an hour or so.  With my 8" Dob I really haven't noticed much problem with temperature issues.  I have no fan.  I just set it up outside and give it 30 minutes to an hour and jump right in.  I would recommend you use it for a while and see what you think before tinkering with it much.  You may find that the existing fan works just fine.  By the way, you did not mention what brand of Dob you are talking about.  The more information you provide, the better answers you will get.  Good luck



#3 macdonjh

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Posted 13 September 2019 - 12:49 PM

After the quality of your observing site, and the steadiness of the atmosphere, mirror quality is next most important.  Odds are pretty good that a Zambuto mirror would have you covered there (which is why premium mirror makers charge a premium).  Not to be alarmist, but can you verify the mirror is from Zambuto?

 

With a big scope, collimation might be easier than dragging it outside... smile.gif

 

If you like DIY projects, fans aren't hard.  You can install them to blow air at the back of the primary mirror, across the face of the primary mirror, or both.  Simple wood working tools and some basic red-to-red and black-to-black DC wiring is all that's required.  I think it's Mr. Lockwood who has been installing fans in front of primary mirrors, inside the shadow of the secondary mirror.  A bit trickier, that, since you have to suspend the fan with wires (or something else thin and able to be tensioned) aligned precisely with the secondary mirror spider or you'll introduce more diffraction into your scope.  Mr. Lockwood likes that better since it cools the reflective side of the mirror directly and serves as both cooling and boundary layer dispersal simultaneously.

 

We don't get big temperature gradients here on the 3rd Coast (it heats up in May and starts cooling in November, so about 1o per day smile.gif ), so fans have not been very important to me.  I have them on two of my four scopes, but hardly use them.



#4 Oberon

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Posted 13 September 2019 - 04:59 PM

Fan systems don’t need to be sophisticated, they just need to blow air over or under the mirror, or both. More is better provided that you don’t introduce vibrations, but that said, you really don’t need much to be effective. The key is to have air moving over the entire surface, to avoid dead spots. You don’t need to worry about laminar flow, turbulent flow is more effective during cool-down anyway.


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#5 Jon Isaacs

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Posted 13 September 2019 - 06:00 PM

Fan systems don’t need to be sophisticated, they just need to blow air over or under the mirror, or both. More is better provided that you don’t introduce vibrations, but that said, you really don’t need much to be effective. The key is to have air moving over the entire surface, to avoid dead spots. You don’t need to worry about laminar flow, turbulent flow is more effective during cool-down anyway.

 

My 13.1 inch has a full thickness mirror and is slow to cool down. I messed around with battery powered computer fans and the like with mixed results. Since it's a back yard scope and 110 volt power is nearby I decided to use a serious fan. 

 

I am currently using a 12 inch drum fan with a rated capacity of 2000 cfm. 

 

I calculate the flow velocity to be about 30 mph, it moves a lot of air. A 120 mm computer case fan is around 60 cfm. This fan moves as much air in 2 seconds as the computer fan moves in a minute.

 

https://www.tractors...qAaArWAEALw_wcB

 

It is effective in cooling the mirror but even on the low setting it causes the scope to vibrate so I switch to a 9 inch fan for observing.

 

Jon



#6 Asbytec

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Posted 13 September 2019 - 07:41 PM

Fan systems don’t need to be sophisticated, they just need to blow air over or under the mirror, or both. More is better provided that you don’t introduce vibrations, but that said, you really don’t need much to be effective. The key is to have air moving over the entire surface, to avoid dead spots. You don’t need to worry about laminar flow, turbulent flow is more effective during cool-down anyway.

Agree. My simple 80mm USB computer fan blows enough air you can barely feel a draft coming out of the upper OTA. It actually vibrates a little when holding it in your hand, so I worried about transmitting vibration the the tube. As it turns out, the scope is so massive relative to the fan that it just does not have the force needed to shake the tube one iota. It's attached with double sided tape to allow the fan to fit snug against a hole fabricated in the mirror cell. Doing so funnels the air directly onto the back of the mirror without leakage. 

 

It does, however, kind of blow fine dust up the OTA. So, some sort of filter can be fabricated. I got one of those germ face masks people like to wear and ripped it apart to make a very thin filter. A rubber band around the fan case holds the filter in place. Two zip ties cut to fit and crossed over the backside of the fan case push the filter away from the spinning blades. The filter does increase resistance to the air flow slightly, but you can still feel the draft. 

 

Then, about as high tech as I get is adding a USB Power Bank to power the fan for eternity. I did not like the idea of buying batteries regularly. The 0.6kg Power Bank also serves as a small counterweight. This set up works pretty well for a thin 8" mirror. It's just a matter of getting into the ambient air. 


Edited by Asbytec, 13 September 2019 - 07:43 PM.


#7 lphilpot

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Posted 13 September 2019 - 09:35 PM

I am currently using a 12 inch drum fan with a rated capacity of 2000 cfm. 

 

I calculate the flow velocity to be about 30 mph, it moves a lot of air. A 120 mm computer case fan is around 60 cfm. This fan moves as much air in 2 seconds as the computer fan moves in a minute.

 

Jon

Get a squirrel cage blower from a commercial HVAC system and you'll be able to blow the scope around the yard!  :D




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