Startrail pictures/problems with dew

[josh_cooper]

I am interested in taking some time lapse pictures of stars but am

concerned about condensation. The last time I tried to take pictures

at night for a long period of time was this past winter. I got back

to my car and my camera was covered with a layer of frost which built

up during the 2-3 hours I was out.

 

I would like to take pictures of the stars (camera might be outside

for 6-8 hours) but I don't want to have a bunch of dew build up on

the camera.

 

Can anyone tell me how to predict the appearance of dew, that is, is

there a way I can tell before going outside if dew will be a

problem? Does anyone have tips to minimize the affects of dew (I

don't want to hold an umbrella above the camera all night long). Is

dew a problem serious enough to warrant my concern, or can I simply

go back to the tent, wipe off the moisture, and go on my way?

[imaginator]

It really depends on how much moisture is in the air, and temperature of lens relative to air temp. I did some a few weeks ago and was very lucky... low humidity, not too cold, and a mild breeze kept the lens clear all night. On other nights, like during the meteor shower in November, I had to keep swapping lenses, putting the unused one in my pocket to keep warm, but this isn't an option for really long shots of 6 hours, so I've been checking into the systems that astronomers use: Check out this <A HREF="http://www.kendrick-ai.com/dewremover.html">commercial unit</A> They have all sizes (2 inch should work) but the controller is expensive... might be able to make your own instead) or you could try making the whole thing yourself (that's what I'm going to try) Check out this method for making a <A HREF=http://www.seds.org/billa/dp/dewpower.html">homemade setup</A> for a large telescope. A much smaller one would be easy and inexpensive to make. Basically some resistors wired to a couple batteries, and mounted around the lens barrel. Also add a "dew shield" (like a lens hood) for better protection (simple as taping on a black cardboad tube) For shorter star-trail shots, just bring two lenses (I use a 28mm and a 50mm) and use one until it dews, then swap to the other, wipe the dew off and keep in pocket to warm for next swap.

[imaginator]

Woops, second link failed... try <A HREF="http://www.seds.org/billa/dp/dewpower.html">this</A>

[hans andersson]

Hi Josh

I do only know the situations we have in Sweden, but here will the frost be on the topside of the equipment. It will fall down and hereby just cover surfaces that is accesible from abowe. This make that by using a kind of topcower can help. Ofcause this will not help if you have to direct the camera stright up. If I had to have it stright up I had to first keep the lens so warm that it not frees and that it was warmer than the sorounding and hereby the condence want be on the lens. This work if it is not underfreesen damp. If it is that you also need a kind of fan to keep it away from the frontlens.

The heating can be either s small infraheating equipment, but they are normally quite big, or that you arrange some resistors with reflectors on a rebuild lenshud, and feed it with 12V from a auto battery or equal. It will be enough if it give you abowe 25 - 30 W.

If this help you, I'm happy.

Regards Hans A

[larry h.]

Josh, this is totally amateur weather advice from a totally amateur photographer and astronomer. If you are not aware of it already, the key thing you need to understand on the Weather Channel is the Dew Point. Most people know Relative Humidity, but don't realize RH is a function of temperature; it varies as the temp rises & falls during the day and night.

 

Dew Point, on the other hand stays relatively constant through 24-hr cycles. It is the temp at which dew will start to form if the real temp falls below it. So watch your forecast to see what the low temp and the Dew Point are for a given day. If the Dew Point is below the forecasted low temp, you will most likely get little or no dew.

 

Now, in the southern US (humid subtropical climate), this happens about once every blue moon. Which means we almost always have dew. Incidently, Dew point is helpful to figure out how uncomfortable you will be during the day, also. DP of 70 degrees F is uncomfortable; DP of 75 F is oppresive and 80 F is unbearable. This is true when it's 70 F (real temp) with 100% relative humidity, too.

[bill_speaks]

Check out this link. Scroll down the page where there is a device like a blow drier.

 

http://www.hfo.org.uk/Anti%20Dew/antidewingsystem.html

 

Bill