Avoiding dew inside a large refractor.

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Wednesday: 33F. Checking through the telescope without an eyepiece, yesterday, showed a large mist patch in the center rear of the 7" objective. It seems like madness having to warm a telescope for solar observation. It takes a long time for inner dew to lift when there is no heat in the sunlight thanks to the solar filter. Removing the filter might not help either. Though I have avoided doing so for safety reasons.

Overnight, the front of the objective lens is only partially protected by the full aperture, solar foil filter. The dewshield is always capped overnight with a large saucepan lid lined with a thin disk of foam. The foil filter has an aluminium housing and the saucepan lid is also of metal. All this metal only increases the area of conductivity in the unheated and well ventilated observatory.

I'm not sure if there would be any advantage to having an insulating foam or polystyrene plug directly in front of the lens. The 10" diameter aluminium dewshield is also in contact with the heavy lens cell. With the 7' long x 8" diameter steel tube immediately behind it. A bit of a no-win situation without added heat close to the lens cell?

I have no desire to spend hundreds on a commercial dew heater band and control box. So I think I'll try a sleeve of camping mattress foam on the upper main tube and dewshield first. Just to see if it helps to insulate the area around the objective lens.The temperature differences to avoid dew are quite small.

I may even try a computer fan to circulate air in the tube. This had been on the back burner to see if it helped instrument seeing in hot weather. The problem is deciding which end to have the fan and whether to suck or blow. It would require a large hole be cut in the tube. Finding a new [secondhand] tube in decent cosmetic condition may be difficult if I make a mess of it. A door behind the objective could be useful for [careful] cleaning. It could carry a small computer fan.

11.00. The sun is teasing again with milky light through thin cloud. Wish me luck.

11.15. 33/36F The edges of shadows of the slit on the observatory wall are not sharp even now. I could get a sharp edge on the sun's disk but little else. I have removed the solar foil filter in an attempt to clear away any dew inside the obj. lens.  The sun is steadily brightening so there may still be hope. I found enough grey foam for the upper end of the tube but not the dewshield yet. This stuff is as cheap as chips so I may buy another roll. It's a shame it marks so easily.

12.00. 34/36F sky still milky as the wind picks up and blows inside the dome. Still no definition!

13.15. No change in image clarity. Went in to have lunch.

14.30. No improvement in image clarity so I gave up for today. The sky never showed any blue even overhead.

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