12" Dob Secondary Mirror Fogging HELP!!

[Tamtarn]

Because of the weather situation at the moment we're experiencing occasional fogging of the secondary mirror.

We've investigated the Kendricks Dew Guard and apparently you have to dismantle the mirror and it's attachment to fit the Dew Guard but not having any experience in dismantling any part of our Dob as yet we would rather some simpler sollution if there is such a thing ! Also we're technically challenged so Sheeny's DIY is out of the question.

We have a Kendricks Controller which we've used with our Kendricks Dew Heater on our ETX 125.

Would a Dew Shield help ?

Hoping the IIS experts can come up with some suggestions.

Barb & David

[janoskiss (Steve H)]

A large (90+mm) baffled fan behind the primary blowing air up the tube would help. But do use one large fan instead of multiple smaller fans like I did (for reasons I'll post about another).

[astronut (John)]

Barb& David, The secondary on my LB12" succumbed to the same problem.
I have home made dew heaters for the e/p and Finderscope but before I go pulling the secondary apart, I've made a dew shield for the front. This should stop the rapid loss of heat from the mirror and hopefully holding off the dew. It extends 38cm out from the front.
This is a lot cheaper than the alternative. Cheers, John

Originally Posted by Tamtarn

Because of the weather situation at the moment we're experiencing occasional fogging of the secondary mirror.

We've investigated the Kendricks Dew Guard and apparently you have to dismantle the mirror and it's attachment to fit the Dew Guard but not having any experience in dismantling any part of our Dob as yet we would rather some simpler sollution if there is such a thing ! Also we're technically challenged so Sheeny's DIY is out of the question.

We have a Kendricks Controller which we've used with our Kendricks Dew Heater on our ETX 125.

Would a Dew Shield help ?

Hoping the IIS experts can come up with some suggestions.

Barb & David

[astro_nutt]

Try making a dew shield out of black paper from a stationary supplier, use masking tape to secure to the top of the scope...good luck!!

[rmcpb (Rob)]

Yeh, a combination of a dew shield that extends 1.5x the diameter of the primary in front of the focuser and a baffled fan blowing up the tube from behind the mirror should be a great help.

I don't understand how a secondary heater does not cause problems with thermals coming off the secondary as it is warmed so I would tend to try the low tech solutions first.

[Merlin66 (Ken)]

The amount of heat required is next to nix ( check out some of the discussions on MAPUG) there's not enough heat to cause thermals etc.
Remember the metal telescope tube around the eyepiece will aggrevate the problem. Cork lining would be good.

[Tamtarn]

Originally Posted by janoskiss

A large (90+mm) baffled fan behind the primary blowing air up the tube would help. But do use one large fan instead of multiple smaller fans like I did (for reasons I'll post about another).

Steve our Dob has a 70mm cooling fan already fitted do you think this will solve the problem. Or should we try to source a larger fan.

We haven't bothered to use it as a cooling fan because we place the dob outside well before viewing to give it plenty of time to adjust to outside temperature.


David

[Gargoyle_Steve (Steve)]

I have occasionally suffered pretty heavy moisture on my secondary, as in forming large droplets - if I'm at home I run an extension cord out, plug a hair dryer in, pull the eyepiece and 2" to 1.25" adaptor out of the focuser and blow warm/hot air directly in for a minute or 2. Bingo - moisture gone! Have not seen any appreciable degrading of image while secondary is still warm, and it's a hell of a lot better than the "coma" around everything with a wet mirror.

I have used the same dryer to get rid of view-destroying moisture off my primary too by holding the hair dryer down the tube (pushing it alongside and past the secondary. Again a few quick minutes of hot air (highest fan speed on dryer to get the air down there, working on a simple extension tube at present) to let the warm/hot air evaporate the moisture - 5 mins or so to let the tube air currents normalise again - back to viewing.

Until I get around to making a secondary heater that hair dryer will work just fine for me, and I don't think I can make an effective primary heater - 10 inches is just too big to handle that way!

[Tamtarn]

Thanks guys for the info. Lots of simple sollutions in a short space of time

Will try the hair dryer Steve. Had thought of making a dew shield but didn't know if it would effect the secondary at all. Now we know it does thanks to the other guys we'll make a dew shield as well.

Great work guys

Barb and David

[janoskiss (Steve H)]

David, Re fans. Every little bit helps, but the factory fitted 70mm installation might not be enough to keep the dew away for much longer. It is unbaffled and probably fine to help with cooling but too small to keep the dew away for any length of time.

As our resident imaging guru Bird demonstrated for us a number of times, the temperature usually keeps dropping throughout the night and you need at least a fan so the mirror can keep up. Otherwise you will get suboptimal performance from the mirror.

But in any case, certainly the longer the scope sits outside the more likely it is to collect dew. So quicker cooling with a fan is desireable.

[slice of heaven]

Originally Posted by janoskiss

A large (90+mm) baffled fan behind the primary blowing air up the tube would help.

Sealing the bottom is necessary for the 70mm to work reasonably. Starkler posted an example of his method that's worth a look. A variable speed added will help so you can run the fan at the lowest speed necessary for cooling or keeping dew at bay.

But do use one large fan instead of multiple smaller fans like I did (for reasons I'll post about another).

I'm up for a new fan/fans as the original has developed a rumble at low speeds. Can you expand on this please Steve before I tackle it.

[janoskiss (Steve H)]

Slice, when you have more than one fan (I have three on the back of my 12"), they will always run at slightly different speeds. This creates some interesting but highly undesirable effects, with the air currents in the tube pulsating at the low "beat" frequencies generated by the difference in speed between the fans. This is the same effect that musicians use to tune string instruments. You can see it as dancing star images they form little rotating ellipses that dance to the "beat". OTOH a single fan (on my 8") has no noticeable effect on the views even in the best seeing at over 300x. (Btw, I do have foam for mechanical insulation of the baffling from the tube in both cases.)

[slice of heaven]

Thanks Steve
One single large fan will do me fine then.

[Starkler (Geoff)]

Originally Posted by slice of heaven

Starkler posted an example of his method that's worth a look. A variable speed added will help so you can run the fan at the lowest speed necessary for cooling or keeping dew at bay.

Yep here.

With the fan mechanically isolated from the OTA, I havent found vibrations to be an issue and hence no need for a speed control.

[Tamtarn]

Hi Guys

Just a quick word of thanks for all your info on cooling fans to help stop secondary miror fogging up.
Have purchased 120mm Computer fan and will commence Geoff's baffled fan mods tomorrow, replacing fitted 70mm fan

David