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Cooling for the Vintage Orion Optics 8" Newt


philjay
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The past few nights trying to image Saturn have shown up a limitation on this scope, due to the closed mirror cell that was in favour at OO in the 80s and 90s the mirror takes an age to cool and there are loads of tube current problems. These aren't too bad when purely visual but with a smaller chipped planetary camera attached they become very obvious.  


 


So a bit of planning was required as the cell back is just a dish of 10 gauge ali plate and if I go drilling whacking great holes in it I will weaken it and cause problems when collimating with the push pull screws.


 


So instead of 1 big fan I have chosen three smaller 50mm fans which will be linked to my homemade dew heater controller which will control the speed (its just a Maplins Pulse Width Modulating 12v drill speed controller so is perfect for the job)


 


The main problem is retaining or increasing the rigidity of the cell back. This I have decided to do using 3 40 x 3mm spreader plates set at 120 degrees. These will spread the load of the collimation screws plus the round feet/protectors I am adding to the end of each plate.


 


Because the cell is flush with the rear of the tube and not inset like modern scopes the collimation screws (and now fans) are a little exposed and vulnerable to knocks therefore I am fitting the 3 bobbins to act as feet and protectors.


 


 


Yep it would have been easier to fork out 170 quid for a modern OO cell with fan but I still want to retain some of the vintage identity of the scope and also the modern cell is a flush type and my tube is not long enough for one.


 


Yesterday I planned it out and drew up a quick sketch to get the concept right in my brain cell then today I started making the bits.


 


The fans were from Bob Potts in Derby and the dual temperature gauge was from Maplins, (the sensor will be stuck to the rear of the cell so I will be able to see what temperature gradient there is between the outside and the inside of the scope at a glance. 

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And here are the relevant bits made up and ready for fitting to the cell when I can get time and pluck enough courage up to take a drill to the mirror cell


 


The black round thing in the middle is the terminal box for the cables, I had a bit of delrin in my bit box so I made this up on the lathe. The 3 feet are 40mm white nylon left over from a previous project


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I wouldnt like to peltier cool a mirror Leigh, you are only after maintaining an equilibrium with the miror and ambient temperature plus reducing tube currents which a fan does efficiently. A peltier would have to be the size of the mirror to avoid thermal deformation and it would have to be controlled to ambient as well as stuck directly on to the mirror rear. I shall stick with fans I think :-)

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Ah, no I wasn't thinking of directly attaching it, that probably wouldn't end well at all. I was just wondering if you were looking to speed up cooling. Scratch that idea then!

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Had a few hours playing again and plucked up courage to drill the holes in the cell and get it altogether. Here's the cell fully assembled and just waiting for wiring and mirror fitting. Oh and I got rid of the nut and bolt collimation adjusters and made my own Phils Knobs :D

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Here's the cell in place fully assembled wired and whirring away. The fans were running for half hour and brought the temp to within a degree or so before I had to close the roof of the obs cos it started raining in.


 


Just got to put some cable clips on it to tidy things up


 


The other pic is of the temperature readout.


 


All I have to do now on this scope is make some decent finger adjusters for the secondary collimation, its screws and nuts at the moment then that's it a fully rebuilt and modernised planetary imaging scope.


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How about some large bore heat shrink, to put over the whole of the wiring from the box down to the fans? 


 


Another tidy job there Phil.


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You read my mind Baz, I thought I had some but I only had thinner stuff and it was a beggar to slide over the cables, so had to do it in small lengths. I should have gone out and bought some from Maplins or Potts but I couldn't be doing with going out again, wanted to get it up and running. The clips'll tidy it up some though when I get them 


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Very nice Phil, I like the addition of the thermometer so you can measure the benefit!


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Thanks Dave, the sensor for the thermometer is stuck to the rear of the mirror with silicon adhesive, same as youd use on the diagonal. This is not really a necessity just curiosity on my part to determine whats actually happening to the image over the cool down time.

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Thanks Dave, the sensor for the thermometer is stuck to the rear of the mirror with silicon adhesive, same as youd use on the diagonal. This is not really a necessity just curiosity on my part to determine whats actually happening to the image over the cool down time.

Yes that will be interesting to see, have had chance to give it a go yet Phil?

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Yep Dave but not done a cool down graph yet. The thermometer shows that the fans help alot and also when I swith the fans off if the mirror hasnt fully cooled the temp will rise to show the real mirror temperature instead of just the cool air blowing over it.

The image is pretty stable even with a couple of degrees differential but best with 1 or less difference.

I switch the fans off when imaging to avoid any vibration then switch them on between exposures, this regime works well.

Other investigations by other folk suggests a fan at the front of the mirror to take away the thermal sheer layer but I dont think I will go that far.

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Yep Dave but not done a cool down graph yet. The thermometer shows that the fans help alot and also when I swith the fans off if the mirror hasnt fully cooled the temp will rise to show the real mirror temperature instead of just the cool air blowing over it.

The image is pretty stable even with a couple of degrees differential but best with 1 or less difference.

I switch the fans off when imaging to avoid any vibration then switch them on between exposures, this regime works well.

Other investigations by other folk suggests a fan at the front of the mirror to take away the thermal sheer layer but I dont think I will go that far.

Interesting stuff! Saw a Dob at the IAS show that had a couple of fans just above the mirror.

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I think i've got the same 'scope as you.


Expect some 'brain picking'


 


My first upgrade I think will be the scope rings. It's still got the thin Orion types, which don't seem too clever - any advice ?


 


cheers.


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Hi Pete, no problem fire away with the questions.

Rings, the oo ones are pretty grim arent they, I got some rings for the Sw 200 and packed them out with cork, using b&Q cork tilesglued on the inside, then I felt lined them using the wilko velour stuff and they fitted perfectly. These are far superior to the oo ones now and one can safely loosen themto turn the tube now without fear of the tube dropping out.

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Those are orion usa not orion optics uk and they are the same as the sw rings. My tube was smaller than 235mm, a pair of sw ones came up (thanks Sheila) and I used those. Check your tube diameter before buying as the older scope tubes are different from the modern ones. If you fancy a trip out your welcome to come round and have a butchers at mine.

Has yours got the enclosed cell as well then?

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Phil

As far as I know I have the exact same bit of kit as you. Before your mods :)

Piccies are here

http://www.eastmidlandsstargazers.org.uk/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=2161

I'm off on a weeks hols shortly by it would be interesting to take a butchers at your setup. There are a few obvious mods needed. Rings and focuser first I think

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