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My 8" dob has a 10" ID formatube tube and the 203mm 1:6 ratio Pyrex mirror is held in a UO open aluminum cell. The formatube product is some form of high density cardboard and is very thermally stable and has good insulation properties.
I have an 80mm 12v blower fan mounted in the rear the tube. I sourced this fan from Jaycar and it is rated @ 34cfm (Cat YX-2512). I use this fan to assist with thermal equalisation of the primary mirror, dew protection for the primary mirror and I like to think that the 34cfm flowing through the front of the tube will also assist with keeping dew off the secondary mirror as well. This fan is installed so it is exhausting air from the bottom of the tube and drawing air in from the top. The rationale is that still night air is slightly warmer and less humid at head height as opposed to ankle height and this is therefore the preferable airflow direction from a dew minimisation perspective. This airflow direction also provides for a greater likelihood of a smoother airflow within the tube. I do accept the prefered direction for mirror cooling is the reverse of this with the fan's blowing air on the back of the mirror. However, I find with this cfm rating the 200X35mm mirror temperature stabilises within 15-20 min.
With this scope I have not had any mirror (primary or secondary) dewing problems in 9 years. Since moving to Canberra, during extended observing sessions (6+ hrs) in winter the tube has been very heavily frosted over at times and this has never caused a problem.
I have attached a photo of the fan installation on my dob.
I suspect the dew problem you are having with the primary mirrors in the GSO dobs might have some or all of the following causal factors:
Uninsulated steel tube is cooling very rapidly and possibly "supercooling" below ambient.
Air inside the tube and in contact with the tube walls cools, becomes denser and sinks to the bottom of the tube.
This cold air will flow around the mirror and cause the mirror temp to potentialy drop below ambient which increases the risk of dew forming in humid conditions.
If there is a plate behind the mirror preventing or restricting this cold air from exhausting from the base of the tube it will then pool around the mirror which will further increase the potential for dew formation.
Possible solutions for GSO dobs:
1. Install extraction fan behind mirror.
2. Insulate tube with cork of cloth.
3. Extend tube to a minimum of 350mm in front of secondary
Cheers,
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