Moist

Dew shields ready to go
(note the skewers in the largest shield)

Due to current levels of night time humidity and high ambient air temperatures for this time of year in the UK, my scope, finder scope and binoculars objective lenses and all my camera lenses have been dewing up! The problem is caused by the radiation of heat from my astro equipment.  On clear nights glass and painted metal lose heat by radiation out into the Cosmos. This radiation continues until a balance is achieved between heat loss through radiation to space and heat gain from the local environment by convection and conduction. Unfortunately, this balance is often achieved when the objective or camera lens is at a lower temperature than the surrounding air.  This becomes a real problem when the lens temperature is lower than the dewpoint temperature of the air.  Dew point is the temperature at which the air becomes saturated and can no longer hold as much water as a vapour.  Consequently, when the temperature of lens drops below the dewpoint of the air, a film of condensate is deposited across the surface of the optics. The night's astronomy is over unless you have a low temperature  12 volt hair dryer to hand.

The best approach is to prevent dewing up before it occurs!  There are two main methods for achieving this:

1. Hi-tech:  Attach 12 volt heaters on the outside of the telescope in the vicinity of the objective lens or mirror. Main disadvantages being cost of the kit and the heavy use of battery power.
2. Low-tech:  Attach thermally insulating dew shields around the relevant parts of telescopes, camera lenses and binoculars.  Main advantage is that,. although reasonably expensive to buy, they are easy and cheap to make.

As the weather has been awful, I decided to use my free time to make dew shields for all my kit that has, in the past and from time to time, dewed up .

For about £6, the discount store Wilkinsons sell matt black carpets for the rear of estate cars. The material is easy to cut to the appropriate size with a craft knife. If the carpet is rolled with the backing on the outside the furry side provides a flocked surface which prevents any stray reflected light causing optical problems.  The carpet can be stuck together using superglue or contact adhesive to form insulative thermal cylinders . I finished and waterproofed  the outside backing material with a layer of black duck tape.

As I have a refracting telescope my photographs of stars show no diffraction spikes.  Diffraction spikes
are an optical effect of reflecting telescopes, the light being diffracted by the 'spider' holding the reflecting telescope's secondary mirror which reflects starlight to the eyepiece.  Diffraction spikes, although artificial  can add to the beauty of an image.  I have pierced the dew shield for my 127mm refractor to enable the optional insertion of two matt black stained softwood barbecue meat skewers, which should provide the same diffraction effects as a 'spider'.

If the rain stops, the clouds part and the wind drops I will test how well all this DIY kit works!

Example of Diffraction Spikes
(Credit Possum Observatory: John Drummond)