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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?)
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  • \$\begingroup\$ The main purpose for giving an optical instrument time to reach temperature equilibrium with the ambient temperature isn't to avoid turbulence. It's to prevent the various optical elements and the structures spacing them from continuing to contract, and at different rates for different materials, as the instrument cools during long exposures. \$\endgroup\$
    – Michael C
    Commented Feb 13, 2023 at 5:36

2 Answers 2

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It is just a tradeoff... the risk of thermals reducing maximum IQ versus unusable images.

It is not BS and it does not only affect Newtonian telescopes. It can also affect camera lenses and refractor telescopes. The primary difference is that for lenses/refractors the issue is with the objective element, which reaches ambient temp much quicker than the Newtonian's protected internal mirror. But lens hoods can amplify the issue by trapping the radiating warm air and slowing cooling.

The point is to keep the lens away from the dew point, not match the air temp. I.e. in a high humidity scenario dew point and air temp can be nearly the same. But in a low humidity scenario the dew point temp will be much lower than the air temp (dew point is never greater than the air temp).

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  • \$\begingroup\$ This makes so much sense! (of course, the Newtonians are the only ones where I've seen the thermal stabilization fan — they're the ones that would take the longest if we leave them to stabilize by themselves!) I also didn't know about the issue with lens hoods! (the irony!! one of the common pieces of advice I've seen in relation to dew is to take advantage of the dew shield — equivalent to a lens hood — to slow down condensation; now it turns out that it can aggravate the issue of thermal distortion) \$\endgroup\$
    – Cal-linux
    Commented Jan 30, 2023 at 23:28
  • \$\begingroup\$ There is also tubus seeing and large telescopes can suffer from dome seeing, caused by warm air trapped in the dome. \$\endgroup\$
    – Grimaldi
    Commented Feb 4, 2023 at 19:32
  • \$\begingroup\$ @Cal-linux Lens hoods can also allow one to avoid having to heat the front of the lens if the dew point is very close to the ambient temperature. \$\endgroup\$
    – Michael C
    Commented Feb 13, 2023 at 5:42
  • \$\begingroup\$ @Steven Kersting Supersaturated air occurs when the temperature falls below the dew point but there are no condensation nuclei in the air on which the moisture can condense. glossary.ametsoc.org/wiki/Supersaturation and encyclopedia.com/earth-and-environment/… \$\endgroup\$
    – Michael C
    Commented Feb 13, 2023 at 5:47
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Warm air distortion works by having different air densities which the light traverses. Those inhomogeneities causes light rays to curve, change direction. The largest effect (in the form of displacing the point where a beam hits the sensor) of such curving will be in the middle between camera and subject. Right at the lens, the effect will be small. So it makes for a good tradeoff with the dew avoidance.

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  • \$\begingroup\$ If you really want to see the effect of thermal distortion, point your telescope to a point in the sky that puts the optical path directly through the air above your next door neighbor's external HVAC/Heat Pump Unit a few hundred feet away. \$\endgroup\$
    – Michael C
    Commented Feb 13, 2023 at 5:39

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