Thermal analysis of a 25" dob (preliminary)

clive milne · #1 26-11-2011, 07:44 PM

I have to shoot off to work in the next hour or so, but before I do I thought I would put up some thermal infrared images taken last night showing the heat flow over a few hours of Andrews 25" obsession. I'll have a nibble at giving my interpretation of what is going on, but with consideration of the time I have, this is most likely to be incomplete.

Also... there are a few caveats implicit in FLIR images, so caution in advised in rushing to conclusions.

Anyhoo... here are the pics with time stamps and ambient temperatures added for context:

enjoy,
~c

http://i371.photobucket.com/albums/o...ne/IR_3702.jpg

http://i371.photobucket.com/albums/o...ne/IR_3704.jpg

http://i371.photobucket.com/albums/o...ne/IR_3705.jpg

http://i371.photobucket.com/albums/o...ne/IR_3707.jpg

http://i371.photobucket.com/albums/o...ne/IR_3708.jpg

http://i371.photobucket.com/albums/o...ne/IR_3710.jpg

http://i371.photobucket.com/albums/o...ne/IR_3714.jpg

http://i371.photobucket.com/albums/o...ne/IR_3718.jpg

clive milne · #2 26-11-2011, 07:49 PM

Last but not least is the top cage... perhaps most significant point in this image is that the the kydex baffle is 1.5C below ambient... the spider vanes have also radiated enough heat in to space to drop below ambient as well.

http://i371.photobucket.com/albums/o...ne/IR_3716.jpg

clive milne · #3 26-11-2011, 07:55 PM

Ahh... looks like my text editing (time and ambient for each frame) didn't upload... I'll have to get on to it when I have time..

btw) how do you embed pictures (rather than just having the link visible)?

regards,
~c

clive milne · #4 26-11-2011, 08:00 PM

One quick comment... notice how much heat is trapped in the soil under the telescope... it clearly has more thermal inertia than the primary mirror itself. I suppose the obvious heat flow path is through the ground board (and in to the telescope structure)

alocky (Andrew lockwood) · #5 26-11-2011, 08:11 PM

Thanks Clive;
I should point out that just about every bit of optics on the top cage has a dew heating strap, which I had switched on last night. You can even see the extra one I keep for eyepieces tucked alongside one of the upper cage tubes.
Given that we were still in shorts and sandals at midnight, I probably could have saved a bit of battery power.
Pity about the transparency, I assume we were seeing all the aerosols from the fires down south.
cheers,
Andrew.

alocky (Andrew lockwood) · #6 26-11-2011, 08:15 PM

Purely radiative transfer mechanism - I doubt there's any 'flow' per se.
The main culpirit will be soil moisture, there was some rain in the last day up there. There's not a lot else in the regolith that has any significant latent heat capacity.

Quote:

Originally Posted by clive milne

One quick comment... notice how much heat is trapped in the soil under the telescope... it clearly has more thermal inertia than the primary mirror itself. I suppose the obvious heat flow path is through the ground board (and in to the telescope structure)

clive milne · #7 29-11-2011, 02:31 PM

Is anyone actually interested in this, or am I just wasting my time elaborating further?

alocky (Andrew lockwood) · #8 29-11-2011, 04:27 PM

I'm interested of course, but it mightbe worth putting it on the Obsession owners group - with your permission.
Cheers,
Andrew.

bmitchell82 (Brendan) · #9 29-11-2011, 04:34 PM

its very interesting indeed Clive, as it gives people a good idea on what generally happens with scope cool down times. I guess depending on your mirror substrate the 1-2 deg difference of mirror to ambient wouldn't pose much of a issue when looking at pyrex.

What are your thoughts on that?

clive milne · #10 29-11-2011, 04:37 PM

Cheers guys.. I'm in the process of putting all my thoughts down in a separate web age... 1/4 of the way through at the moment. I'll post it when finished.
regards,
~c

clive milne · #11 29-11-2011, 05:57 PM

https://sites.google.com/site/binocularnewtonian

clive milne · #12 29-11-2011, 06:14 PM

Quote:

Originally Posted by bmitchell82

its very interesting indeed Clive, as it gives people a good idea on what generally happens with scope cool down times. I guess depending on your mirror substrate the 1-2 deg difference of mirror to ambient wouldn't pose much of a issue when looking at pyrex.

What are your thoughts on that?

The issue isn't so much a question of heat induced distortion of the substrate as it is the convective plume at the boundary layer distorting the image through variation of the refractive index of the air.

There is a qualifier to that... if the mirror substrate hasn't been correctly annealed then you will see the mirror figure do bat**** crazy things as it equilibrates thermally. (even if it is pyrex) Basically you will see astigmatism at the focal plane come and go as the optic cools.

bmitchell82 (Brendan) · #13 29-11-2011, 06:19 PM

so if i had the volume of the tube being drawin though and exhusted out the rear of the scope essentially sucking in the cool air this would negate these thermals wouldn't it?

Paul Haese · #14 29-11-2011, 06:23 PM

There are several of us on the site that have been onto active cooling with TEC's for the last 6 years. I had an 18" SDM that I peltier cooled via a cover that went over the rocker box and had a bottom sash to keep the cool air in the box. I found that the 18" would take around 2 hours to cool to ambient (0.5 degrees C). Natural cooling with a fan is ok but you would still be exuding a lot of energy with that scope even after many hours of using the fan. The IR images seem to indicate and confirm my thoughts. My suggestion is to look at cooling techniques and if you like I can find the images of the system I made to cool my then 18".

clive milne · #15 29-11-2011, 06:52 PM

Quote:

Originally Posted by alocky

I'm interested of course, but it mightbe worth putting it on the Obsession owners group - with your permission.
Cheers,
Andrew.

Yes of course... by all means.

alocky (Andrew lockwood) · #16 29-11-2011, 08:38 PM

Quote:

Originally Posted by Paul Haese

There are several of us on the site that have been onto active cooling with TEC's for the last 6 years. I had an 18" SDM that I peltier cooled via a cover that went over the rocker box and had a bottom sash to keep the cool air in the box. I found that the 18" would take around 2 hours to cool to ambient (0.5 degrees C). Natural cooling with a fan is ok but you would still be exuding a lot of energy with that scope even after many hours of using the fan. The IR images seem to indicate and confirm my thoughts. My suggestion is to look at cooling techniques and if you like I can find the images of the system I made to cool my then 18".

How did you avoid the peltier device bringing the mirror below ambient? There's plenty of anecdotes from humid areas about the whole primary dewing suddenly with just normal fan cooling. I've sen plenty of space devoted to just attacking the boundary layer, even putting the fan in front of the primary. Others advocate blowing air across the primary. In the Perth part of WA, it's my opinion that the atmosphere is rarely steady enough for the boundary layer on the mirror to be the main problem with your image - but on those -2 degree winter nights when the jetstream stops I want to be able to exploit it!
cheers,
Andrew

Paul Haese · #17 29-11-2011, 08:58 PM

The thermal mass of that mirror will allow small windows of the mirror being at ambient before the core temperature begins to release heat and create the boundary layer again. You would need several periods of cooling for that size mirror in a falling ambient temperature.

I only once saw dewing with my 18" and that is because I cooled it to 6 degrees below ambient and near morning it started to dew up. A portable hair dryer puts an end to this though and the little heat will do nothing to the mirror and quickly acclimatise back to stable mirror views. Without this you cannot really tell what the seeing actually is like. The jet stream view of fast seeing can also be imitated by core heat from a mirror.

For further information and data on mirror cooling see Anthony Wesleys site. He has comprehensive data on mirror cooling.

alocky (Andrew lockwood) · #18 29-11-2011, 10:47 PM

Thanks Paul, I guess that's why we're experimenting! On the topic of WA seeing however, it's not an opinion I formed in 5 minutes. The seeing in summer here is usually bad enough for the scintillation to be visible at zenith to the naked eye, and it definitely correlates with poorer views through the eyepiece.
The nights when the big girl really shines are when the temperature is plummeting towards zero, there's ice on the cars, and presumably no chance of the mirror being at equilibrium. The stars are steady, and I can crank the thing up past 800 power, no problem. If there's more to be gained by sorting out boundary layers on these nights, I'm all for it.

cheers,
Andrew.

bmitchell82 (Brendan) · #19 29-11-2011, 11:10 PM

Winter with clear skies over here ='s brilliant seeing. go up north it is phenomenal!

Paul Haese · #20 29-11-2011, 11:26 PM

Andrew I recommend you talk to Anthony also about cooling. It is quite pain free overall, you will benefit from the removal of the boundary layer issues even on nights when the seeing is less than perfect. If you want I can find my images of the mods I made for the 18" and I can send them via email. Let me know via PM.