Milky Way and the Dew Point Temperatures

[CatherineA]

I want to try photographing the Milky Way and have been reading up in preparation. There is an App that is available in the USA, but not Canada, that includes the predicted humidity percentages and dew point temperatures for whatever dates and location you plug in. It seems a quick and easy way to decide whether to take along a dew heater.

If you don't have this App, how else do you get this information?

[DanK]

I may be wrong, but I think you may be confusing two things.

Astronomical events can be computed accurately, and there are apps that will tell you, for example, the elevation of the moon over the horizon from any location at any time at any date. The two I know of are The Photographer's Ephemeris and PhotoPills.

Humidity can't be predicted that way. It's a weather phenomenon, not an astronomical one, and it's predicted short-term and quite inaccurately. AFAIK, the only way to get this is to look up a local weather forecast. In the US, Dark Skies (which Apple bought and may disappear soon) provides that information. I don't know whether it does in Canada.

I would expect that you can find the historical average dew point for some locations on the web. Dark skies does provide a prediction of humidity for any date in the future, but I think that's nothing but historical data. That would be of some use, but I think limited, as there can be wide variations from the historical average day to day, depending on the emergence of weather systems.

Dark Skies can show you this, as its "time machine" allows you to see detailed data from the past. On the night of Sept 10, it was very humid here; the dewpoint was 71 Fahrenheit, 22 degrees Celsius, at midnight. Two days later, a front had gone through, leaving behind much drier air, and the dewpoint dropped to 54 degrees Fahrenheit, 12 degrees Celsius, at midnight. That sort of swing can't be predicted far in advance for every point in the world.

[xpatUSA]

I want to try photographing the Milky Way and have been reading up in preparation. There is an App that is available in the USA, but not Canada, that includes the predicted [relative] humidity percentages and dew point temperatures for whatever dates and location you plug in. It seems a quick and easy way to decide whether to take along a dew heater.

If you don't have this App, how else do you get this information?

Can't answer the question, Catherine, but what would the dew heater be for? Living near Houston, I should know, but I don't.

Is it to do with taking a colder-than-ambient camera out of a car and it misting up inside and out?

Paraphrasing Dickens:

"Relative Humidity 99 percent, result - happiness. Relative Humidity 100 percent, result - misery.”

[DanK]

Ted,

I'm assuming that it's to deal with the temperature dropping during night photography. I've done some in the Adirondacks when the temperature dropped below the dew point, and condensation formed on the camera. My camera and lenses are weather sealed, but condensation on the front of the lens or on a filter ruins the shot.

Then again, I had never heard of this device. I googled it (well, to be precise, duckduckgoed it): https://astrobackyard.com/dew-heaters-astrophotography/

Dan

[xpatUSA]

Ted,

I'm assuming that it's to deal with the temperature dropping during night photography. I've done some in the Adirondacks when the temperature dropped below the dew point, and condensation formed on the camera. My camera and lenses are weather sealed, but condensation on the front of the lens or on a filter ruins the shot.

Thanks for the clarification and the interesting link, Dan.

https://astrobackyard.com/dew-heaters-astrophotography/

Did you pick up on this?

"When the surface of your telescope objective or camera lens drops below the ambient temperature of the air outside, moisture begins to condense."

As you know, it's wrong - pardon my pedantry.

[CatherineA]

thank you so much for replying Dan, I really appreciate it.

I hadn't thought that that might be the way the Dark Skies Weather app provided its data, but it makes sense. That is the App that the instructor (Eric Kuna) recommended on the video that I watched. That App is not available in Canada although Dark Skies Finder, which gives light pollution maps, is available.

I am not sure how many days in advance Dark Skies Weather provides forecasts for but it looked so useful because it gives hourly break-downs for variables including cloud cover, temperature, precipitation, humidity and dew point. If it is all based on GPS coordinates then that would be even more useful because for night skies a photographer might be a long way from a city and data for the city might not be as accurate.

It looked such a quick and easy way to prepare for the possibility of condensation: if the App said that the dew point is going to cross the outside temperature, then pack a dew heater. However it is not much harder to look up a local weather forecast the day of the planned shoot to see what is predicted for that night and prepare that way. I've never actually taken dew point into consideration before when shooting.

[CatherineA]

hi Ted! They are just like the hand-warmers and foot-warmers that people who walk their nutty cold-loving dogs in Ottawa winters know all about. The instructor said that he just wraps one around his lens with an elastic band on nights when the dew point crosses the outside temperature.

He (Eric Kuna) said that he often uses them in his home state of Florida but he didn't need one the night of the shoot in the video which was in the Mojave Desert.

Maybe it is just a term that the instructor coined. He used it a number of times so I thought that it must be a commonly known item. Maybe, though, they are in fact just hand-warmers. Sorry about that!

[xpatUSA]

No prob, Cath.

As to Apps, the Govt. Weather Service is pretty good for your particular needs:

Link for my location:

https://forecast.weather.gov/MapClic...0#.VTpMKvAU7To

You can enter co-ordinates at the home page. More interestingly, it can also show graphs and you can delete irrelevant ones, voila:



Typical Gulf Coast values ... 7 am Monday morning doesn't look too good, eh?

[CatherineA]

Hi Ted, I'm not finding anything equivalent for Canada. The information seems to be available here but I need to go to different sources for different variables and the information isn't presented in such a user-friendly way. The Canadian Meteorological Society puts out hundreds of maps for astronomers but it isn't a quick process to find the one you want. In fact, it's a pain.

And since the dew point will vary widely for all sorts of slights changes - a little dip in elevation, slight changes of wind or a nearby structure - perhaps I will always ere on the side of caution when it comes to adding a little heat to the lens. If a dew point calculator based just on temperature and relative humidity tells me that there is a chance of dew forming, then I will just add the hand warmer. I can't see a down-side to that.

[xpatUSA]

Hi Ted, I'm not finding anything equivalent for Canada.

Oops, sorry! As if Location: Ottawa, Canada wasn't a big enough clue ...

And since the dew point will vary widely for all sorts of slights changes - a little dip in elevation, slight changes of wind or a nearby structure -

I didn't know that the dew point will vary widely for all sorts of slight changes. Where did you see that, may I ask?

[Manfred M]

Catherine - have you signed up for the session Roger Miller / Nature Group is putting on this topic at the RA Photo Club on September 18?

I expect Roger and some of the other members would be able to answer your questions.

In terms of dew point issues, this is primarily an issue when the camera and lens temperatures drop below the dew point and condensation forms on and in the camera. I have found it to be a problem in summer shooting when I take my camera out of an air conditioned building and expose it to humid air or in winter shooting where the camera gets quite cold and is returned to warm, humid inside conditions. I can't recall it ever being a issue at this time of year where the temperatures inside and out are not all that different.

[CatherineA]

I didn't know that the dew point will vary widely for all sorts of slight changes. Where did you see that, may I ask?

I looked over my search history to find where I read the words "vary widely" and they were based on what I read in a forum. I have found this quote on the Fred Lossing Observatory site, which is more moderately and better expressed:

Humidity variations can indicate the likelihood of optics and eyepieces dewing.

But dewing is not simply correlated to relative humidity. Dewing tends to happen when the sky is clear, the temperature is dropping and there isn't much wind. Being on a hilltop or in a small valley can make the difference between no dew and dripping telescopes. Unfortunately, the humidity forecast does not have the spatial resolution to know about small hills, valleys, or observatory walls. All of which can reduce dewing.

A sudden spike in the humidity forecast, an hour or so after the cloud forecast predicts a sudden transition from cloudy to clear, when there is no wind, means that ground fog will form.

Also, when the cloud forecast is opaque and the humidity forecast is 95%, rain is likely: a good time to cover the telescopes.


taken from cleardarksky.com

I need to be a bit more careful with my words!

[CatherineA]

In terms of dew point issues, this is primarily an issue when the camera and lens temperatures drop below the dew point and condensation forms on and in the camera. I have found it to be a problem in summer shooting when I take my camera out of an air conditioned building and expose it to humid air or in winter shooting where the camera gets quite cold and is returned to warm, humid inside conditions. I can't recall it ever being a issue at this time of year where the temperatures inside and out are not all that different.

I'm trying to learn more about the dew that can form on a lens while shooting outside. I don't know why that couldn't happen this time of year in Ottawa while shooting at night.

I think I have the info I need now at least for shooting in the Ottawa area but I will talk to other local photographers to see if there is a better way. thank you

[Manfred M]

I'm trying to learn more about the dew that can form on a lens while shooting outside. I don't know why that couldn't happen this time of year in Ottawa while shooting at night.

I think I have the info I need now at least for shooting in the Ottawa area but I will talk to other local photographers to see if there is a better way. thank you

As long as your camera body and lens are warmer than the outdoor temperature, you should not get any condensation issues.

[DanK]

As long as your camera body and lens are warmer than the outdoor temperature, you should not get any condensation issues.

This isn't her issue. This is the issue when walking into the outdoors. It isn't the issue with night photography, which is her concern. The main issue with dew in night photography is temperatures dropping below the dew point. I have had condensation on my camera when doing night photography when my camera was at outside temperature when I started.

Catherine, the dew point is the temperature at which the air is fully saturated with water vapor. Things like wind may make some difference because of evaporation, but wouldn't that just lower the effective dew point a bit? I think that a local prediction of the dew point should be sufficient for most purposes.

There are lots of online weather services, more than have apps. I would just try to find one that has hourly dew point predictions. Dark Sky has it for at least some Canadian locations. I arbitrarily picked Banff: https://darksky.net/forecast/51.1754,-115.5693/uk212/en. Use the drop down menu toward the top left to pick dew point.

[xpatUSA]

As long as your camera body and lens are warmer than the outdoor temperature, you should not get any condensation issues.

That most certainly covers the case where the outdoor temperature and the dewpoint are the same.

Otherwise warmer than the dew-point should do it ...

[xpatUSA]

Humidity variations can indicate the likelihood of optics and eyepieces dewing.

But dewing is not simply correlated to relative humidity. Dewing tends to happen when the sky is clear, the temperature is dropping and there isn't much wind. Being on a hilltop or in a small valley can make the difference between no dew and dripping telescopes. Unfortunately, the humidity forecast does not have the spatial resolution to know about small hills, valleys, or observatory walls. All of which can reduce dewing.

A sudden spike in the humidity forecast, an hour or so after the cloud forecast predicts a sudden transition from cloudy to clear, when there is no wind, means that ground fog will form.

Also, when the cloud forecast is opaque and the humidity forecast is 95%, rain is likely: a good time to cover the telescopes.

Catherine, I found the above quote less than useful, hopping around as it does. In particular, the statement that "dewing is not simply correlated to relative humidity" is quite arguable in that condensation occurs exactly when RH equals 100 percent.

Where does this word "dewing" come from?

[DanK]

I was puzzling over this as well. I was thinking that the presence of wind might slow condensation slightly by disrupting the formation of droplets, but that may have been farfetched. Basically, once a drop in temperature brings relative humidity up to 100%, water has to condense as the air chills further. Would wind matter much?

Clear skies per se have nothing to do with it. However, clear skies are generally a sign of relatively dry air, and dry air cools faster. It's an interesting question: dry air cools faster but has to cool more to reach the dew point, while humid air cools more slowly but doesn't have to cool as much. Which reaches the dew point faster?

[CatherineA]

Catherine, I found the above quote less than useful, hopping around as it does. In particular, the statement that "dewing is not simply correlated to relative humidity" is quite arguable in that condensation occurs exactly when RH equals 100 percent.

Where does this word "dewing" come from?

Thanks for reading the quote so closely. I was just daunted by the apparent complexity of it. You and Dan have me helped a lot with this, thank you.

I don’t know where “dewing” comes from but when I looked it up I found that it is an accepted word for Scrabble.

[xpatUSA]

<> However, clear skies are generally a sign of relatively dry air, and dry air cools faster. It's an interesting question: dry air cools faster but has to cool more to reach the dew point, while humid air cools more slowly but doesn't have to cool as much. Which reaches the dew point faster?

Have a look here: all will be revealed:



I was into that kind of stuff long ago, to do with watches:

https://tcktek.blogspot.com/2007/11/...watch-sir.html

[edit] this might also be relevant:

https://tcktek.blogspot.com/2007/10/...-in-watch.html
.