Dew Problems (help)

orestis · #1 28-04-2011, 07:14 PM

Hi everyone,

I just came in from an observing session and have seen that both my mirrors on my newt are dewed up.I have the cap off so it doesn't get worse.

Any sugesstions on how to get rid of it.

Thanks in advance
Orestis

jjjnettie (Jeanette) · #2 28-04-2011, 07:48 PM

You can use a hair dryer to dry it. Just aim it down the tube.
It's a good idea to keep an eye on the mirrors during the night. And give them a blast when you see them starting to fog up. Saves it turning into big droplets,

gbeal · #3 28-04-2011, 07:54 PM

If it is a common occurrence, you might have to look at a very slight amount of heating on the edge of the primary, and also on the rear of the secondary.
Gary

The Mekon (John Briggs) · #4 28-04-2011, 08:25 PM

Orestis,

Living in the same area as you do, I have battled dew for over 20years.

I used to use a hair dryer which meant running a 240v ext, cable to the scope. This is not the best arrangement. Then I bought a 12v hair dryer. Used early and often it would work, but also drains the battery quickly.

Now I use Kendricks, and set up at the beginning of a session, these heater straps are great, and the figure of the lens is not affected.

Having a 12" reflector, heater straps may either be expensive or not available for such a big mirror. You could try to make some yourself out of velcro and resistors.

Am not sure if a tube fan would do any good for a reflector, but be sure to place your scope pointing to horizon whenever you take even the shortest break away from the eyepiece. Do not leave it pointing to the zenith - the quickest way for dew to accumulate!

John

mental4astro (Alexander) · #5 29-04-2011, 10:51 AM

Hi Orestis,

Dew. My old friend, and arch enemy.

John's already mentioned the dew strap solution. I've got a few straps rigged up to my big dob. The thing is that the more straps, and the longer they are, they too suck up the big ones from the battery. Yet there is no real way to deal with dewing eyepieces, secondaries or finders unless its with straps. A hair drier is only a short term solution, though a good one to start with. Camping supply stores is where you can find the 12V ones, but these are very hungry beasties too.

240V driers are very fast, but be very careful with them in a dewy enviroment. You need to keep them off the ground & under a box when not in use, and the plug protected so water doesn't get into it.

I've been seeing a few dobs today use small 12V fans to blow air around the primary. SDM Telescopes has been installing these onto their dobs. The bonus is they use much less juice than straps on a big primary. Maybe ask them for a few tips on these.

Another thing that has helped me keep my eyepieces warm when not in use during the night is keeping a hot water bottle in the eyepiece case. You may need to change the water once or twice over the course of the whole night, but a thermos filled with hot water will see you right. The hot water bottle doesn't need much water in it either. Also if you do use straps down the track, the EP heating strap will be more efficient if the EP is already warm to begin with.

orestis · #6 29-04-2011, 06:39 PM

Thanks everyone,

I have yet to use the fan that came with the dob.
I'll try it out soon.

Cheers orestis

The_bluester (Paul) · #7 29-04-2011, 07:47 PM

I will be needing a strap and considered one for the EP as well, but when I was talking to Bintel one of the folks there said that his fix for that was to keep the unused EPs in their caps in his pocket so body heat kept them warm when not in use. He reckoned it usually bought him an hour or so.

I think long term I will need straps for the corrector on my SCT and for the EP as if I took it outside no it would dew up n half an hour and be dripping by midnight. I have been looking into getting resistive heater wire and making my own but I should probably just put my hand in my pocket and buy off the shelf stuff.

barx1963 (Malcolm) · #8 30-04-2011, 12:18 AM

I have 2 Kendrick controllers, 1 a dual channel the other a Digifire 7, straps on all EPs, on the finder (objective and EP), on the secondary and on my green laser pointer. all up 9 heaters. can use a lot of juice from a 12v battery but the trick is to run the EP ones at 30%, the secondary at 10% and the others at 50%. Put them all on when you take the scope out and I can get 5/6 hours of dew free observing.
ONly prob is takes about 10 mins to setup, and a bit loger to pack up, and cables everywhere, but MUCH less frustrating, benn ages since I have had a dew problem.

Malcolm

Waxing_Gibbous (Peter) · #9 30-04-2011, 04:14 AM

Keeping the fan running should do the trick. Dew is a big problem where I am, but I found the constant airflow of even a small fan keeps it at bay.
If you want to go the whole hog, installing an extra boudary layer fan will sove the problem completely.
If this seems to make no sense, try turning your windscreen defroster on. Even with no heat the dew will dissipate in seconds.

orestis · #10 30-04-2011, 07:19 AM

Quote:

Originally Posted by Waxing_Gibbous

Keeping the fan running should do the trick. Dew is a big problem where I am, but I found the constant airflow of even a small fan keeps it at bay.
If you want to go the whole hog, installing an extra boudary layer fan will sove the problem completely.
If this seems to make no sense, try turning your windscreen defroster on. Even with no heat the dew will dissipate in seconds.

Thanks Peter that makes sense.As a flow of air around the mirrors will disallow the settling of dew.So i suppose it acts as a preventative.

Cheers Orestis