Is 77%-80% humidity good for visual?

[asteele3]

I know that when humidity is 90% and higher that there'll be no dew on the telescope lens.

So when it's between 75 - 89% humidity, how long will dew take to form and if it does how much of the lens will it cover?

At 75% would it be 3/4 of the lens?

[JamesF]

I'm not sure I understand your question. Dew forms once the RH reaches 100%, usually as the air temperature drops and its water-carrying capacity reduces. Why would it affect where the dew appears on the lens?

James

[asteele3]

Last time I went out when humidity was 96%. There was no dew. I take it that the higher the humidity, the less the dew?

[JamesF]

No. You have to account for temperature change as well.

At a given temperature, the atmosphere has the ability to carry a certain amount of water. The amount of water it is actually carrying expressed as a percentage of that maximum is the relative humidity. But as temperature drops, the atmosphere's ability to hold moisture also drops, so the relative humidity rises. Once the RH reaches 100% the atmosphere can't hold any more and any excess will condense out as dew. How much dew is formed depends on what the RH is to start with, what the temperature is to start with and how much the temperature falls by. If you go out when the RH is 90% and the temperature drops only a little you may get no dew, but if it falls a good deal you may get an awful lot. But if you go out when it's already very cool and the RH is 90% and the temperature drops by the same large amount you may get only a little dew because there's far less water in the air to start with.

James

[pete_l]

Once the RH reaches 100% the atmosphere can't hold any more and any excess will condense out as dew.

It's also worth noting that we generally measure the temperature and RH of air at a point away from local distortions: some distance off the ground and away from buildings.

With a telescope that's pointing skywards, the objective will be "seeing" the upper atmosphere which is very cold indeed - minus several 10's of °C. The objective lens or mirror will therefore tend to cool down a long way more than the ground, or any nearby buildings. What that means is that the air in close proximity to whatever's looking at the sky will be cooled by the cold telescope optics and will therefore reach saturation (100% RH) even though the RH of the relatively warm air away from the scope isn't moist enough to form dew.

Dew heaters work by counteracting the extra heat-loss from looking at the sky and keep the optics (ideally) at a temperature just above the local dew point, but not so warm that they induce thermal currents which would spoil the seeing conditions.

So to answer the OP's question. Any water vapour in the atmosphere will absorb and scatter the light we want to see, so the lower the RH the better. Water vapour also reflects / scatters terrestrial lights, so higher humidity generally increases the effect of light pollution too.

[LukeSkywatcher]

Water vapour also reflects / scatters terrestrial lights, so higher humidity generally increases the effect of light pollution too.

This is very true.

[JamesF]

Any water vapour in the atmosphere will absorb and scatter the light we want to see, so the lower the RH the better

When temperature is constant, yes. But there will be far less water in the air (about three times less, I think) at 90%RH @ 0C then there is at 75%RH @ 25C. RH Is also pressure-dependent as well, AFAIR, but I don't think it varies as much as with temperature.

James

[umadog]

The water-carrying capacity of the air depends on the temperature. A surface (e.g. the objective lens) radiates heat into the sky and can cool down to below ambient temperature. Warmer air reaches the objective, where it cools and its capacity to carry water decreases. If the air becomes saturated it will deposit dew on the surface. All these things can happen at 75% ambient humidity of at 90%.

[TheNorthernMonkey]

Humidity is strongly linked to temperature. Dew will form when localised rh reaches 100% but you can see from this chart what the maximum change of te,premature to reach 100% is from a given starting point.

http://www.truetex.com/psychrometric_chart.gif