Dew Shields/heaters

[astroturf (Bryan)]

Hello all
I'm looking at buying either a dew shield or a combined heater shield
Is the heater necessary considering a lot of observing is done from Sydney
Would a shield on it's own do the job to a satisfactory degree

Bryan

[jase (Jason)]

The use of a heater is not really related to your location. It comes back to the temperature dew point to keep dew off the front objective or corrector plate (SCTs). The shield will assist in reducing stray light entering the OTA. If you are doing a lot of work close to zenith, then the shield will not be very effective at reducing dew. In such cases, both heater and shield would be recommended.

[sheeny (Al)]

G'Day Bryan,

What sort of scope do you have? I would suggest that if you have a refractor or SCT and you don't want to miss any observing/imaging sessions due to dew, then you'll need both.

The problem with dew is it depends on the relative humidity and the drop in temperature at night. Sydney can get pretty humid at times, so I would expect dew on a reasonable number of clear observing nights.

Al.

[montewilson (Monte)]

Hi Bryan,

Everything below is correct.

I would however venture to say that an ounce of prevention is worth a pound of cure with dew.

The dew shield is a good idea. It reduces the amount of cold sky that your lens is exposed too that is why they work even though your car bonnet might be dripping in water.

In the short term get a shield. They are cheap and help protect from stray light too. When you are not using the scope (such as going inside for a coffee) cover it. You will be surprised how little dew you will suffer from doing this on a normal night wth a couple of hours observing.

This is not infalible and you will get dew if the air is humid and your scope is cold. If you are likely to have all nighters then you should use heaters.

It is a whole new discussion. I made mine using Nichromium wire which is available at Dick Smith or Jaycar. There are a few steps involved and if you are interested I might get motivated to do an article for IIS about how to do it with pictures.

Suffice to say, I don't recommend paying ultrabucks for heaters and controllers, save your money for cool stuf like eyepieces instead .

[Doug]

G'day all,
I have an AstroZap dew shield that I use on the LX200. It draws about 1.8A @ 12v and does not need a controller as the wattage is so low that conductive losses prevent any alarming temperature build up. Of course I monitored it with a thermocouple probe till I was assured of this fact.
Some nights I have had to run it at about 15v to keep the corrector plate clear; but not often.
HTH,
Doug

[Geoff45 (Geoff)]

Originally Posted by astroturf

Hello all
I'm looking at buying either a dew shield or a combined heater shield
Is the heater necessary considering a lot of observing is done from Sydney
Would a shield on it's own do the job to a satisfactory degree

Bryan

You could check out the home made dew heater in the "Projects and Articles" link. It works well, is cheap, and easy to put together.
Geoff