4.3.2020 Foam dewshield:

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In the afternoon, during a cloud waiting session, I made an aluminized foam dewshield for the 6". Two layers of grey, camping mattress foam were taped quite tightly onto the main tube first. This was to build up the diameter and provide a firm base and extra insulation. The diameter was now just beyond the full cell diameter and well clear of the 150mm lens aperture. The matt black foam has a "machined" moulding pattern. Which helps to avoid grazing reflections. I can paint it matt black inside if really needed.

I made the dewshield itself, the full width of the aluminized mattress. By careful sizing and internal taping of the seam, it was able to slide comfortably over the foam base layers. While still remaining straight and true without sag. This allowed enough clearance for the D-ERF cell's thumbscrews and a dew heater band around the objective cell nose. I shall add another heating band around the D-ERF when it is back in place for solar.

I'm hoping the considerable thickness of closed cell foam [30mm+] will help to avoid dewing when I open up in the morning. Hopefully aided by the 20cm thick coverage of the main tube behind the objective. The lens had repeatedly steamed up on its rear face. Which was very difficult to reach without making a door in the main tube.

The objective enjoys a minimum, 6" long dewshield when it is pushed right back to the saddle. As shown in the image [above right.] When extended forwards it provides plenty of protection. Fully according to classical refractor standards. Where up to 2.5 x aperture was once the norm. Only the modern demand for ultra short OTAs has undermined the dewshield as a protective barrier to dew. I still need to make a foam plug to close off the open end and provide further protection from overnight frosts. Proof of the pudding tomorrow if we get the promised sunshine.

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