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Cooling the primary mirror

Started by jimg@sonic.net, 08/10/2011 09:11AM
Posted 08/10/2011 09:11AM Opening Post
I've read a lot about the need to cool - or temperature equalize - the primary mirror. What is actually happen optically during this time? If I don't assist this process with a fan, will I just see distortion or "hard to focus" troubles or ...?

Orion XT12i Intelliscope push-to Dob
10mm and 25mm 1-1/4" Plossil eyepieces
2 1-1/4" Barlows
Orion Q70 26mm and 32mm 2" eyepieces
TMB Planetary II 4mm and 5mm eyepieces
Laser and 9X50 finder scope dual mounted
Posted 08/10/2011 09:54AM #1
You'll see a wavering image, same idea as the wavering above the surface of a hot road (but hopefully not that bad).

Of course atmospheric turbulence produces the same effect (or at least a very similar one). Both are made worse by higher power.

You can see a lot without a fan, although they get more important with larger mirrors. What happens is that when we get more experienced we start looking for our best-view-ever of objects. That pushes us to seek out the last few percentage points in a world of diminishing returns. In other words, at some point you'll know that you think you need a fan (maybe four fans).
Posted 08/10/2011 10:12AM | Edited 08/10/2011 10:12AM #2
Cooling is important, but almost more important is scrubbing the boundary layer (hot air on the surface) from the mirror surface. This can actually have a more profound effect on the way the mirror performs than equilibrium of the overall temperature. This is done with fans that blow across the upper surface of the mirror. Then 1 or 2 fans are placed to blow onto the bottom of the mirror, to make sure both sides are cooling down at the same rate. Unequal cooling can result in changes in the figure of the mirror.

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 08/10/2011 06:42PM #3
Jim Goodenough said:

I've read a lot about the need to cool - or temperature equalize - the primary mirror. What is actually happen optically during this time? If I don't assist this process with a fan, will I just see distortion or "hard to focus" troubles or ...?

Jim:

There are various things going on while your scope is cooling down. The mirror does change shape but more important are the tube currents, these are convection currents inside the telescope itself that, as has been mentioned, distort the image so that it is not as sharp as it could be.

One can certainly enjoy the view while the scope is cooling, a thermally stable, "rock solid" scope is mostly important when viewing at high magnifications. Normally it is the stability of the atmosphere that limits the resolution of a larger telescope, you are looking through a column of air at least 60 miles long, if the air is turbulent, you cannot reach the maximum resolution... The tube currents and turbulence inside the scope add to this.

There are some nights when the atmosphere is very stable, depending on where you live, such nights maybe more common or less common but it is the stability of the atmosphere, called the "Seeing" that generally limits the resolution of your scope. Seeing is sometimes gauged in terms of arc-seconds, 1 arc-second seeing means you can resolve a 1 arc-second double star (if the scope is capable.) 1 arc-second requires a scope of at least 5 inches. To get a sense of just how small an arc-second is, it is equal to two lights about 15 inches apart, 50 miles away... 2 arc-second seeing might be considered average, 1 arc-second is rare most places.

I happen to live in San Diego near the coast, here, the seeing is often quite exceptional, 0.5 arc-second seeing is comparatively common. I also happen to enjoy splitting closely separated binary stars, to split a 0.5 arc-second double requires that exceptionally stable seeing, a stable mount and telescope of 10 inches aperture or more that is thermally rock solid.

I have a couple of scopes I use for this, the one I use most frequently is a rather pedestrian 10 inch F/5 GSO Dobsonian. I have made the effort to setup it so that it tracks nicely at extremely high magnifications and that the mirror and the telescope are cooled down by a fan system that is totally free from vibration... If the tube were to vibrate 3 microns, that is 6 wave lengths of light, that is enough to blur the image so that I could not split a 0.5 arc-second double...

Last mouth I was able to split Zeta Bootes at that 0.5 arc-seconds, the best image was at 821x. The image was perfect edge to edge, the two Airy disks separated by a thin dark line with the diffraction patterns surrounding them.

The key was having that scope cooled down with a fan that is free from vibration, there are many fan systems, not many that vibrationless at 800x...

This long story is not about me bragging.., Rather it is meant to emphasize the importance of a top notch fan system. The system I am using was custom built for my 10 inch F/5 Dobsonian by Floyd Blue... Whatever scope it is you buy, I recommend contacting Floyd, his system is clever but simple and inexpensive as well. You can build one cheaper than you can buy one from Floyd but when that first one doesn't quite get it and you have to start over, Floyd would have saved you money as well.

Jon Isaacs