Re: At Last !

#3317

David Galvin
Administrator
2p

Not sure about the black bobs and black streaks but sounds like eye lashes in the way. This really does happen, as for the white disc please read on.

The problem with our  weather is that although it is a wonderfuly clear sky and the stars are shining, you may find that the actual seeing is unsteady. This will result in a washy image with little detail visible. Couple that with tube currents and you will struggle to see any detail.

Try focusing a star such as Polaris or Arcturus until it is a pin point and then take the focus inwards so that star image becomes a blob with rings (it should look like my avatar). You can also take the focus outwards and get a similair result. What you should see, in ideal conditions, would be a steady bullseye type effect with concentric rings.
What you will probaly get is the concentric rings with wavering eddies of air that pass across the light path of the tube. These are, simply, thermal air currents withen the tube and should, given an hour or so settle when the scope comes to equilibrium with the outside temp.
In our out of focus image you may also see flashing bands that move across the field of view which are caused by the bad seeing condions of the atmosphere.
When you try to observe a focused star it may appear to ‘dance’ across the field of view especially if under high magnification. Observing the planets under these conditions can be awful with detail almost invisible, in fact a sky that is slightly hazy can often result in a better image due to the air being staedy.

Video link that show tube currents:
http://www.youtube.com/watch?v=Ed3fYJmgOIo

Bad seeing due to unsteady atmosphere:
http://www.youtube.com/watch?NR=1&v=3SOQ-NUbyUo