This is a very practical accessory which I've found actually works (!) in day-to-day use. It is NOT designed to remove huge quantities of water, but rather to absorb the condensation which unavoidably occurs with changes in temperature as the scope is moved (for example) from the cold night air into a warmer room. It is equally valuable for maintaining a dry enviroment within the OTA when the scope is kept outside in an un-heated observatory.
Why should you bother about moisture in the OTA? - well, it's not so much the water (which is harmless) but rather the nasty substances that are dissolved in it. Oxides of nitrogen and sulphur are present in the air (abundant in urban environments) and these dissolve in the water to form corrosive solutions. It's these that will attack your mirror coatings over time. If you remove the water then these gasses have nothing to dissolve in and so can't exert their corrosive effects - you coatings will therefore last longer.
The perforated tube is lined with fine filter paper which prevents any dust from the dessicant finding it's way into the OTA, it's then filled with dry (blue) silica gel. As water is absorbed the gel turns from blue to pink - whereupon it is saturated. The gel can be regenerated by placing it in a warm oven, or on a hot radiator to drive off the water (it will turn blue again) and it can be re-used. Sufficient silica gel is supplied to refill the tube twice so the cap will never be out of action.
The colour of the gel can be viewed through the lens on the back of the dustcap, so you can quickly assess the condition of the air inside the OTA at a glance without removing it.
I am working on a design to fit refractor tubes. Whilst these do not have mirror surfaces Flourite elements are very sensitve to condensation. Even if the glass elements are all hard coated, removing the water vapour will reduce the accumulation of stains and prolong the interval before cleaning is necessary. The only problem with refractors is that with 1-1/4" draw-tubes there is limited diamter for the silica gel tube - this necessitates a rather longer dustcap than I would have liked. 2" draw-tubes are no problem.
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