mirrors

[morgannz]

seeming as im new to all this,just trying to find out as much info as i can about telescopes before jumping in and buying one. so,a question about mirrors.it seems heat effects mirrors,how? what is meant by cooling down mirrors and what happens if you dont? does it just effect dobs or other telescopes also? sorry for the twenty questions....but i suppose its better to be informed than not! and just on a final note,I PROMISE , just wanted to say how helpful people on this site actually are,especially to us ever questioning novices! i salut you!

[Atlas]

Heat effects mirrors and lenses. If you let the telescope sit ouside for about an hour before using it, the temps inside the telescope (mirrors and such) stabilize. If they are not stable with the outside temp, the lenses can fog and the mirrors will distort. This causes blurry images and loss of detail.

When they settle down in temp, the images and view are much better.

Also heat in the atmosphere causes a ripple/wavey effect in your telescope views. The effect get amplified more as you go up in aperture size. Nothing you can do about that. You get the same effect when looking out a window in your house, or pointed over your neighbors roof.

[JamesF]

Heat differentials affect both reflectors and refractors (and catadioptrics). Mirrors tend to be made from large chunks of glass and larger telescopes tend to use mirrors rather than lenses (SCTs and Maks being counterexamples so they may be the examples people come across most.

When a mirror (or lots of things, actually) cools, it doesn't necessarily cool evenly and uneven cooling can temporarily cause the glass to distort very slightly. Because telescope lenses and mirrors are made to such fine tolerances, even small amounts of distortion can cause problems because the surfaces are no longer the shape they're intended to be. You'll see some mirrors advertised as being made from pyrex rather than plate glass specifically because pyrex is more thermally stable than plate glass. That is, it expands and contracts less as temperature changes and therefore distorts the image less.

The other problem with a mirror being warmer than the surroundings is that in a reflector (particularly ones with solid tubes) the mirror warms the air above it which then rises, causing air currents in the tube. These air currents can also distort the image.

Allowing the scope time to cool to the ambient temperature avoids these problems.

Catadioptric scopes are a particular problem because they can be large and almost completely sealed, meaning the air and mirror(s) inside can take a long time to cool down There are all sorts of solutions to help alleviate this problem, even going as far as inserting a cooling tube into the back of the scope that blows cool air inside and allows warm air to escape.

In terms of how inadequately cooled telescopes affect what you see, you may find that you can't get a sharply focussed image even when the seeing is excellent.

James.

[jimmyjamjoejoe]

There is also the boundary layer in reflectors, particularly dobs. Basically it's the same effect as you see just above some hot tarmac on a sunny day, that "mirage" effect.

It's just a layer of heated air which sits just above the mirror, which distorts incoming light, acting like a bubbling, rippling lens.

As stated above though, all of these issues are solved by just sticking your scope outside for 30-60 minutes, then the only thing you need to worry about are atmospheric conditions.