Our nightly nemesis - The very air we breath!

[bond19]

Not sure if it belongs here but thought you might like watching this....

http://bit.ly/1v93K1r

[michael.h.f.wilkinson]

Nice one. Looking at the images of out of focus stars in my SCT when it has not cooled down properly can also show the kind of vortices seen here (albeit of a much gentler nature).

[Stu]

That is excellent, nice find.

How exactly is it done Michael?

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[michael.h.f.wilkinson]

There is an explanation here:

http://en.wikipedia.org/wiki/Schlieren_photography

[Paul73]

Very clever.

Demonstrates why we need to let scopes cool.

Paul

[DeepSkyBagger]

Very nice - just see what happens when you use a hair dryer to clear away the dew!

[chiltonstar]

I've done the demo with a bare hand and a glass of sherry......

Shows you why you need to pick up your scope using a handle or wearing insulating gloves though, doesn't it?

Chris

[Peter Drew]

The optical interference soon passes. We use a hairdryer to remove dew from our 16" SCT, this displays the same effect as the video. The optics revert to normal in less than 5 minutes. It would take quite a time to warm up a mirror to the point where it would need the traditional cool down time. The video also demonstrates why you don't want someone with a bald head standing underneath your telescope.     

[John]

With dobs you are standing right beside the tube so I guess body heat can enter the light path more easily than a refractor, SCT or mak-cassegrain when your body is behind the light path. On really cold nights I've noticed air currents issuing from my bare hand and into the light path when I place my hand on the scope tube. Thye stop almost as soon as my hand is removed from the tube. A good reason to be well insulated, apart from keeping oneself warm.

[Stu]

With dobs you are standing right beside the tube so I guess body heat can enter the light path more easily than a refractor, SCT or mak-cassegrain when your body is behind the light path. On really cold nights I've noticed air currents issuing from my bare hand and into the light path when I place my hand on the scope tube. Thye stop almost as soon as my hand is removed from the tube. A good reason to be well insulated, apart from keeping oneself warm.

I've noticed exactly the same John, it's amazing to watch the currents appear as your hand touches the tube.

I have thought of putting 'Dob knob' on the side so I can move the scope without touching the tube. Easy enough to do

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[DeepSkyBagger]

'Dob knob' Brilliant! Practical and amusing.

[chiltonstar]

If I rotate my Mak in its rings to get the finder in a better position, the diff pattern shows "flaming" on one side for about five minutes or so if I don't use gloves.

For the same reason, I don't use a mains hair dryer to blow dew off the front corrector, but use a v.low powered system with a nightlight in a small tin as a heat source (and handwarmer) and a tiny rechargeable fan to gently blow warm air across the side of the tin towards the front of the optics.

Chris

[kev]

At least with a dob you are to the side of the light path. On a conventional newt with non rotating head you can be under it. That really mucks up the air currents, especially if you breath deeply.

Sent from my iPhone so excuse the typos!