cool down

Can someone explain the difference in views between a cooled down scope and one that isn't? Thanks!

A cooled down scope will not have any internal disturbances of the image and a warm scope will cause disturbance of the air that will show up as tube currents or eddy currents on the mirror or up the tube.
This will mimic the disturbances seen when viewing on a poor night or when trying to view over a warm building or parking lot.
The image will not be steady or sharp and will seem to shimmer or have areas of that are unsteady.
On a great night a warm scope will not give the perfect images that it would if it were properly equalized.
As someone else stated, holding a hand in front of the scope while looking at the Moon or a bright object will show the heat rising and this is basically the same thing that a warm scope will do, though it might looks slightly different.

robert wikeen said:

Can someone explain the difference in views between a cooled down scope and one that isn't? Thanks!

Sometimes like atmospheric poor seeing.

In a solid tube newt one can defocus and see waves of turbulence sorta folding their way up the tube. Under other circumstances one can see (again defocussed) air chimneying out the top of the tube.

Optics of significant mass like primary mirrors and corrector plates can radiate heat for hours. This causes refractive air turbulence at the optical surfaces as well as tube currents. And reflectors of any flavor recieve their signal after two or more passed through this potential soup.

My largest scope is a 13" plate glass. Even after hours of cool down, long after the sonotube has shed it's thermal load and tube currents are undetectable, the mirror can still produce boundry layer thermals at the mirror surface. At this point they look a bit like the seeing is softening. Just a tad. Field stars at the limit of view will not sharpen. But when it is cooling issues, turning on the blower cleans up the image. Stars sharpen, tiny fuzzies don't. Planetary details sharpen also.

My first scope was a 4" newt. Not much thermal mass there. When I started using larger scopes I noticed how more seeing was an issue. Thought I was seeing much more of the atmospheric turbulence everybody was talking about. But a lot of it ultimatly turned out to be seeing conditions inside the scopes themselves.

If one looks down a solid tube newt, because of the heat sensitivity of our faces, heat coming up the tube can be easy to feel.

The term "cooled down" is really short hand for something to the effect of "All the parts of the telescope are at the same temperature as the outside air." Since outside air is usually cooler than the telescope that has been riding in a car or sitting in a house, it is usually the scope that needs to "cool down" to match the air.

Before we go further, imagine looking across the desert or a hot pavement on a hot day......Notice the "heat waves" making a distant object shimmer. (They are not actually "heat waves" but different pockets of air at different temperatures--which cause the light passing through them to refract differently, and thus shimmer.) This is "bad seeing." It happens in between the top of the atmosphere and your eyeball, and makes stars twinkle. This is called "atmospheric seeing." It also happens with local sources of heat, like a warm rooftop, or a chimney, or hot car. This is called "local seeing." (It also makes the stars twinkle.)

It can also happen between the mirror and the tube. What is happening is that some part of your scope (usually the mirror since it has the most mass and retains the heat the longest (i.e., cools the slowest) is warmer than the air. It heats the air around it and makes it move, and makes the same "Pockets" that exist in the atmosphere. To see it, defocus a star and watch how the image moves. If it moves back and forth, comes in and out, you have seeing problems.

If there is differential heating of one part of the mirror, you could also get a warped "figure" or shape to the mirror, which would make the focus less than ideal, but this is a relatively rare effect of a heated mirror.

One of the other things the heated mirror does is to heat the air, which then rises up the tube. As it moves up the tube, it disturbs the view.

Any kind off bad seeing makes it so that the object does not stay in one given focus, and also (usually) makes for a little shimmer in the object.

Probably more than you needed to know, but every once in a while it is fun to spout off.

Alex