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Mysterious tube currents

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I've only had a few chances to try out my new scope because of the recent bad weather, and as it's a 12" F5.3 reflector there are a few things to get used to (my other scope is a 4.5" F4). A couple of times when looking through the new scope I'd noticed that when I defocused a star to make it fill a large part of the FOV I could see what appeared to be either smoke or bubbles blowing through the view from one side of the image to the other. I assumed that this had something to do with tube currents, so I set out tonight to see how this effect depended on how long the mirror had been cooling and whether the fan was on or off etc. However, I quickly discovered the source was at the other end of the tube.

I have a habit of holding the tube near the top with my left hand whilst observing (it's a dobsonian) and the patterns that I could see were being produced by heat from my hand! This is good news because it's a simple problem to solve. In retrospect I suppose it's a silly thing to do, but I'd be interested to know if anyone else has seen this effect.

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I'd heard of tube currents, but I had no idea that just a finger on the side of the tube could produce such a visible effect.

I also have a heater on the secondary but that doesn't seem to generate any noticeable effect.

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