I have this "cognitive dissonance" regarding heat sources at or near a lens (or more specifically, a telescope acting as a lens, as used for astrophotography).
On the one hand, a common practice in astrophotography is to have the equipment out for a while to achieve thermal stability (temperature inside the optical tube equal to air temperature at the location where the telescope is). The idea is that any difference in temperature would cause turbulence which will cause images to become less sharp.
On the other hand, from my previous question on the issue of dew ruining astropictures, I learned the trick of just placing a source of heat close to the lens (ideally close to the outermost glass) to ensure that it won't reach the dew point. (I have not tried that yet)
My main question is: Can someone help me resolve this cognitive dissonance? In particular:
- Is it simply a matter that yes, there will be distortion due to the heat source, but we have no choice but to live with it because otherwise there will be dew and shooting will not be possible?
- Is the issue of having to wait for thermal stabilization something specific to Newtonian reflectors? (is it less relevant in refractor telescopes or camera lenses that are sealed or close-to-sealed?)
- Is this issue of thermal stabilization perhaps B.S./misinformation spread by telescope manufacturers to upsell you (or project an image of a fancy/technically-advanced product) the optical tubes with fans to speed up thermal stabilization?
- Ultimately: is it a matter of the amount of heat / temperature difference? (e.g., when placing a source of heat to avoid dew, do I need to aim to an exact balance between the heat being injected and the heat the lens is radiating, so that the lens ends up as close as possible to the air temperature?)