If you are having a dew problem with your dob and in the interests of simple solutions first, FWIW, you might wish to consider the following before investing in a heated secondary:
1. A general rule of ATM thumb for solid tube dobs is the tube should extend beyond the secondary for at least the diameter of the tube. In your case, assuming the tube is in the order of 330mm diameter you might like to consider making a dew shield from corrugated cardboard, closed cell high density foam etc to ensure the secondary has adequate dew shielding. The advertising photos for the 300mm GSO dobs look like the tube would be lucky to extend more than 150mm past the secondary. I suspect a simple (and cheap) dew shield will probably fix your problem in all but extreme conditions.
2. I understand the GSO dobs have steel tubes. I have read reports of owners lining the inside of these tubes with a thin layer of cork or felt to provide some insulation and assist with the prevention of "cold air" tube currents and resultant " potential overcooling and dewing of the optics. I have experienced dewing of both primary and secondary mirrors in a steel tubed Newtonian during winter in the Central Highlands on Victoria but have never seen dew on the optics of my Sonotube/Formatube dob in very similar conditions here during Canberra winters. This dob has a corrugated cardboard dew shield that extends the tube 450mm in front of the secondary, and this is for a 250mm diameter tube.
3. Heated secondaries have to be carefully managed as they have the potential to introduce significant heat plumes into the optical path. The Protostar web site has some interesting video footage of this effect.
4. Getting electrical power to the secondary can be an interesting exercise as well. To avoid increasing the diffraction from the secondary support structure by running wires along the secondary spider arms, it is preferable to use two of the secondary vanes as conductors. However for this to work, at least one will have to be electrically isolated from the steel tube and from the other spider arms.
Without trying to sound smart don't observe close to zenith on dewey nights and make sure your scope is level between sessions. Also the dew shield discussed above should help any dewing of the primary as well.
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
I use my fan sometimes to cool my mirror down (when I am impatient).
Originally I noticed that I had turbulents in the tube when I left the fan on.
BUT!! I have since noticed that when I am viewing and the slightest dewing starts on the secondary, I turn the fan back on again and the dew does not affect either mirrors.
I think the original turbulents are caused my temperature variations inside the OTA. When stabilised to ambient temps the turbulents dissappear and the mirrors do not fog because of the slight draught blowing up the tube. I was going to make a dew sheild but found that the fan stops the dewing. And we have fog most nights here. My OTA and base get dripping wet but inside the OTA is dry. I am suprised at how long these Duracell 'AA' batteries last! Apart from using the fan for cool-downs I have also left the fan running for 4+ hours many times and they refuse to go flat. I am still going to make a dew sheild anyway, but for now, the fan works a treat. I usually pack up because I get too wet, not the optics! LOL. Funny but true.
I do have a dew sheild on my finderscope. It works great.
Last edited by ballaratdragons; 14-04-2005 at 02:08 AM.
Originally posted by iceman Rob I'm aware of that to avoid dew, but it's inevitable over the course of the night to be pointing closer to upwards.
I was hoping something like a fan would help to stop the mirror dewing up? (or slow it down at least)
Mike,
Like I said I was not trying to be too smart in my reply but if we are having a discussion on dew we need to include some of the basics so people new to astronomy can learn some of the more simple methods to avoid dew short of fan installations,etc.
Maybe I could have said it better but that was the intention.
yeah i have had my fan running all night (observing) but i still had a SMALL amount of dew on my secondry... mind you it was damn near raining.. just ask the boys that came up !
I use the same set up as stringscope (Fans exhausting out the rear of the scope) When things get damp at a star party, I',m usally one of the last to pack up. Unless you observe in severly cold conditions like I some times do, this works very well. I have a piece of surpluss nichrome heater wire coiled behind the secondary hooked up to 12V for backup, when temps get below freezing.
