Mak cass internal dewing.

[Doley68]

Hello all,

Had a good night observing Jupiter last night but it was extremely damp and dew was as bad as I can remember. So much so that the primary mirror of my mak fogged up. 

I have a dew heater band for the corrector plate which did its job as always.

My scope is kept in an observatory with a pouch of dessicant inside the focuser.

Did anybody else have this trouble last night, and what could be a solution.

Chris.

[JamesF]

I had similar problems wth my SCT last night.  The primary didn't look stunning and the internal face of the corrector had more dew on it than the outside.  I assume it's because warm air gets trapped inside the OTA and as the corrector and mirror drop in temperature the moisture in the warmer air inside tends to settle out on them.  My OTA had been outside many hours at this point too so it wasn't lack of cooling time, though it might be down to rapid changes in ambient air temperature I guess.  I'd not be surprised if that were to happen in the winter.  Today it was pleasantly warm in the sunshine here, but under a clear sky the temperature can drop very quickly once the Sun sets.

The neatest solution I've seen to this is a vent tube that fits inside the baffle tube (not a tight fit -- 20mm would be plenty) and either pumps air in or draws air out of the OTA.  The gap between the tube and the baffle then allowing air to travel in the opposite direction.  Obviously this isn't going to work whilst actually using the scope, but if the air inside is close to ambient when you start observing there should be far less trouble.

Short of cutting some slots in the OTA to allow the air to escape (not to mention allowing spiders and other creepy-crawlies in) I'm not aware of another solution.

James

[Doley68]

James,

I think ventilation slots would be the best solution as you say. Perhaps with removable covers. You would only remove them when you think necessary. A bit of warm air from a hairdryer could be wafted through them if needed also.

Why don't manufacturers do this, it sounds a reasonable idea to me.

Chris.

[brantuk]

You do in fact get vents with computer controlled fans on the back end of Edge series scopes. Not seen them on any of the smaller Sct's and Mak's though. The only solution I've used for Sct primary dewing is a big dew band round the back of the scope. Not used it in heavy dew though. Last night was moist and the temperature was up and down like a yoyo.

[Stu]

I get the same problem sometimes with my Mak. I normally run the built in fans all the time, and if the air is dewy then I guess that just chucks moisture into the tube causing the fogging up. I have tried a dew strip around the back of the scope too and that does seem to help.

The other thing I've thought of doing is some form of cap over both vents, or at least the entry vent which contains dessicant bags to remove as much moisture as possible.

There are nights when very heavy dew does make the Mak very tricky to use.

Cheers,

Stu

[Zakalwe]

I had the same problem with my C11 SCT last night. The scope was in the obsy for a couple of days, so cooling wasn't a problem. I'm loathed to put a dewband around the primary...after all aren't tube currents to be avoided? I do use a dewband and dewshield for the corrector.

[JamesF]

This evening I've been looking at the photo sequence starting here with a view to using it as the basis for a DIY SCT cooler of my own:

http://www.flickr.com/photos/averen1/3549549922/in/photostream/

Can't work out what on earth is going on in that first image though.  It looks like a standard 32mm-ish solvent weld union with some other hacked-about fitting glued inside.  I just can't think what that second fitting is at all.

I've read that genuine Dyson vacuum cleaner filters (not the cheap knock-offs) are the things to use for filtering the incoming air.

James

[JamesF]

You do in fact get vents with computer controlled fans on the back end of Edge series scopes. Not seen them on any of the smaller Sct's and Mak's though. The only solution I've used for Sct primary dewing is a big dew band round the back of the scope. Not used it in heavy dew though. Last night was moist and the temperature was up and down like a yoyo.

Do the Edge scopes perhaps need them because the flattener lenses in the baffle tube effectively seal  the entire OTA, I wonder?

James

[hobsey]

Would it work if after use, take the diagonal out and place the opening near a dehumidifier to draw the moisture out?

[Stu]

Would it work if after use, take the diagonal out and place the opening near a dehumidifier to draw the moisture out?

I've had to dry mine out afterwards by carefully pointing warm air from a hair dryer up the baffle tube. Still took a long time. The problem is prevention during use.

I know that tube currents are an issue, but I think with very gentle heat around the primary end you can keep it just above dewing without too many currents. Always a balance e though.

I do also think that the key is making sure only dry air gets into the tube when cooling it so any form of desiccant cap when storing and a desiccant 'filter' when cooling may help. I've even considered placing a bag of desiccant inside the tube, behind the primary to see if that helps but haven't gone that far yet!!

Stu

[Alien 13]

Hi i have been reading this thread with great interest and yes i dont own a MAK/SCTbut  i do have a background in the aerospace industry and have been involed with environmental testing of products.

I have a couple of ideas that might help, warming the air inside the tube does remove the frost but you end up with a tube full of air with more moisture than you started with, im my industry we would purge the tube with dry nitrogen or argon but dont know if there is anything comercially available to do this.

The second idea is to have a peltier device inside the tube or thermally bonded to it that acts as a dew magnet, so you still get dew/frost but not on the optics, this would only need to be a couple of degrees below the ambient temp to work so minimising any tube currents.

Alan

[Zakalwe]

Hi i have been reading this thread with great interest and yes i dont own a MAK/SCTbut  i do have a background in the aerospace industry and have been involed with environmental testing of products.

I have a couple of ideas that might help, warming the air inside the tube does remove the frost but you end up with a tube full of air with more moisture than you started with, im my industry we would purge the tube with dry nitrogen or argon but dont know if there is anything comercially available to do this.

The second idea is to have a peltier device inside the tube or thermally bonded to it that acts as a dew magnet, so you still get dew/frost but not on the optics, this would only need to be a couple of degrees below the ambient temp to work so minimising any tube currents.

Alan

Its a good point about warm air containing more moisture than cold air. I currently have my SCT sitting in the lounge with the secondary removed to allow it to warm up and dry out. The problem is that the air inside the house will be warm and moist.

Purging with argon is interesting. On a SCT the baffle protrudes quite a way into the OTA, so the argon gas would settle around the primary as it is heavier than air. That could be well worth trying. You could also "pour" some argon into the dewshield and remove the need for dew-straps.

A small bottle of welding gas and a regulator might do the trick?

http://www.hobbyweld.co.uk/?page=38&t=Pure+Argon

[Mr Spock]

Heating a scope is a bad idea - especially if you have spent hours waiting for it to cool down! What you need is to draw the warm and moist air out of the tube, allowing it to cool.

Get a nosepiece, preferably with a flange (like a T-mount adaptor) and preferably 2". Build a box from mdf - doesn't have to be deep - around 120mm square. Leave one top of the box open and attach a 120mm computer fan, drill a hole in the bottom and glue the nosepiece in. Attach the fan to a battery so the fan draws air out of the scope. As there's a gap between the back plate and the baffle tube it will draw air from around the primary.

The above will also assist with cooling the scope to start with. Dew is heat exchange so if you are getting dew it means your scope is radiating heat.

[NickK]

Storing the scope is what I would be concerned with if the scope has high amounts of water vapour inside. If it's a sealed unit (except for EP) then storing it with a vent open in a sealed box with desiccant may be the answer to reducing the water vapour inside. Then fit the kit once with the warm scope and don't take it off outside!

I remember changing the optical train at an SGL star party - big mistake.. the back lens inside the scope dewed up (back of the Petzval flattener). Learnt my lesson!

[hobsey]

Build a box from mdf - doesn't have to be deep

I like this idea but rubbish with wood work.

Are you using MDF just as an example, can you use anything? Reason I ask is, could you use say like a tuppa wear tub to cut down on labour?

[Mr Spock]

You can use anything as long as you get a good seal round the edges.

[NickK]

Hi i have been reading this thread with great interest and yes i dont own a MAK/SCTbut  i do have a background in the aerospace industry and have been involed with environmental testing of products.

I have a couple of ideas that might help, warming the air inside the tube does remove the frost but you end up with a tube full of air with more moisture than you started with, im my industry we would purge the tube with dry nitrogen or argon but dont know if there is anything comercially available to do this.

The second idea is to have a peltier device inside the tube or thermally bonded to it that acts as a dew magnet, so you still get dew/frost but not on the optics, this would only need to be a couple of degrees below the ambient temp to work so minimising any tube currents

Alan

You could.. in theory make a moisture trap.. by pumping the air from the tube, over a chilled heatsink, then back into the OTA. If the moisture drops off the HS it could be drained out too. That way the peltier stays outside of the tube. It stops any chance of water dropping onto the optics. For a box dehumidifier after the session - put two heats sinks on - draw the moist air out of the box, pass over the cold heat sink (cold enough to condense but not to freeze), let the condensed moisture drain out a collecting tray underneath.. then pass the cold air over the hot side heat sink. The warm dry air returning will then pick up more moisture from the scope.. 

The problem is usually the front optical surface and with the wind in the UK I'd have thought that the argon would get blown away easily.

[Jove]

Dew is heat exchange so if you are getting dew it means your scope is radiating heat.

Presumably this means that its best to leave a scope out to cool from long before dew point is reached, to ensure it is close to thermal equilibrium with the air?

[Mr Spock]

I used to have a Mak. It would take up to three hours to cool properly. If it went from room temperature to a cold night with the temperature dropping it almost never cooled enough.

[NickK]

You could have a chiller box by using a pelt - both dehumidify and chills the scope down