Argon Purging of Closed Tube Scopes

[glend (Glen)]

I have been using Argon purging on my cold finger modified Canon DSLR for about a year now and it has proven a very effective way to prevent internal condensation on the sensor and camera internals (the T-Adaptor is capped with a Baader filter so the camera is a closed aperture). Argon can not transport water vapour in the way that plain air does, and it is heavier than air so will not percolate away as fast as some other gases, and it will displace air in a closed vessel. It has occurred to me that for any closed tube scope, Argon filling may assist with the prevention of internal optic fogging. My Skywatcher MN190 is a closed tube scope and could easily be filled with Argon, and I am going to try this out at some point. Of course external glass surfaces like the Corrector would still need a heat strap but the internal mirrors should not be at risk of fogging on even the most humid cold nights - provided the Argon can be held in the tube without leaking out. As stated above,being heavier than air Argon tends to stay in vessels, simply sealing the rear of the scope would prevent the loss of the gas provided you don't 'tip' it out the focuser. Argon gas is readily available in any Bunnings that sells welding equipment, and the 1 litre compressed disposable bottles are cost effective. Of course over the course of some days it would probably leak out of the tube but if you had a long winter imaging session planned it could be very useful for extension of the session without worry about internal mirror surface fogging. I am not sure of how it would go in SCTs, but would welcome the discussion re other closed tube designs.

[Camelopardalis (Dunk)]

Glen, while they might be closed tube scopes they are usually not airtight - I only speak with experience with my SCTs of course.

Despite their reputation, I've never had a case of internal fogging - with one exception...a particularly humid Sydney evening when I left my Tempest fans on when using my C11 HD, the fans drew the moist air into the tube and my dew heater was insufficient.

It'd be an interesting experiment of course, but I'd suspect the external surface of the corrector plate is the one you need to worry about most, and an oversized dew shield (by length) can go a long way...

[glend (Glen)]

Originally Posted by Camelopardalis

.... I'd suspect the external surface of the corrector plate is the one you need to worry about most, and an oversized dew shield (by length) can go a long way...

Your right of course Dunk, and that's why I use a heater strap around the corrector and a long dew shield. I have yet to have a problem with the primary personally but threw the idea out there for discussion.

[Chris.B (Christopher)]

Interesting idea. It may help with stopping mould internally forming as well. If the gas does leak during the night you may get a slight focus shift. Argon would have a different refractive index to air.

[glend (Glen)]

The refractive index of Argon is 1.0003032, for Air it is 1.000293, a vacuum is 1.0000.
If you can detect a focal shift over a refractive difference of 0.0000102 then you might notice it.

[OzEclipse (Joe Cali)]

Glen,

I agree that the refractive index difference won't affect focus. But why Argon, nitrogen gas is cheaper and just as effective.

Joe

[glend (Glen)]

Originally Posted by OzEclipse

Glen,

I agree that the refractive index difference won't affect focus. But why Argon, nitrogen gas is cheaper and just as effective.

Joe

Simply because I use it in my camera now and have had no problems with it.

[bugeater (Marty)]

Is it that argon can't carry water vapor or simply that you are purging with dry gas?

[glend (Glen)]

Originally Posted by bugeater

Is it that argon can't carry water vapor or simply that you are purging with dry gas?

Look I realise it's a rather bizare suggestion, and many people will laugh about it but it works well for my DSLR. Re the gas choice:

Yes, the longer answer is that Argon cannot carry water vapour, and it is heavier than air or air components like Nitrogen so it will not just rise out of a contained vessel unless disturbed. For me it is also easy to get in disposable cylinders from most any Bunnings where it is sold as a shielding gas for welding purposes (all you need is a cheap disposable bottle regulator and a length of clear hose to inject it). Explore Scientific has been using it to fill its eyepieces for many years, so that's a form of endorsement.
When I use it in my camera the camera is enclosed in a plastic ziplock bag (with a cutout for the T- adaptor which has a Baader filter screwed in the front to seal the camera aperture and adaptor area). The bag cutout is duct taped to seal it against the adaptor. I don't try to pressurise the bag or camera, rather just allow the Argon to displace the air. Pressurisation would not work anyway as it would force the gas to find a way out through the tape seal or around tapped screws in the tube (tubes are not pressure vessels) - better to just have an equalised membrane.
As far as a scope is concerned, leaks would be a problem in most of them, especially those like refractors which had rear focusers which would allow the gas to drain out under gravity through the draw tube gaps, etc. A scope with a front mounted focuser would hold the gas better, provided that the rear can be sealed, and in some that would be hard. I believe with Mak-Newts it could work, but not RCs or SCTs which have rear mount focusers which would leak. With the Mak-Newt you could just put a plastic bag around the rear section of the tube and duct tape around it to seal off the back, the Argon should be held there for a fairly long imaging run as long as you don't tip the scope to the point where the Argon could run out any focuser gaps. I will be trying it with my Mak-Newt.