Re: ATM Trying to get rid of tube currents in a Newton telescope

[Bob Scholtz <rscholtz01@sprynet.com>]

I built a 12.5 in classic dobsonian back in the late 80's.  Plywood box, hinged
tail gate (no vents), sonotube tube.  I kept it in the garage which was usually
hotter than the outside ambient temperature.  Before I installed a fan this
scope would not cool down in 4-5 hours.  Tube currents were terrible.  A muffin
fan in the box blowing from the mirror up the tube solved the problem, but I
had to keep it running while observing.  My opinion is that tube currents; box
currents in a truss design, are a lot more common than people think; many folks
don't realize that their image is degraded by "local" seeing.   Two class
designs; Obsession scopes and the Ball Scope have fans as standard equipment.
This is a clue as to how common tube currents are, even in very open designs.

--
Bob

rscholtz01@sprynet.com



Jim Fly wrote:

> I suggest you describe in more detail what you are "seeing" that you
> attribute to tube currents.  Tube currents after 4 hours to cool down seems
> highly unlikely.  Have you had someone else experienced with tube currents
> have a look?  Just "Fuzzy" images is not typically characteristic of tube
> currents.  Tube currents generally cause a "teardrop" smear effect on a
> star that can be disrupted (it looks like smoke") with motion of a free
> hand at the top edge of the tube front (where the warm air escapes).  What
> type of collimation technique/tools are you using?
>
> Regards,
>
>