Focus - intra and extra - seeing, thermals, glass
Posted 01 December 2013 - 03:05 PM
seems to show more heat cells and bubbles. Not to be confused with static signs of a rough mirror.
Why the difference on either side of focus for these temperature/seeing effects? What's one showing the other doesn't ?
Thanks in advance.
Posted 01 December 2013 - 05:22 PM
Posted 01 December 2013 - 05:28 PM
Posted 01 December 2013 - 05:40 PM
Glenn - I wonder if that less defined look on the inside is merely a different temperature gradient with in the same tube. The outside focus being still another thermal part of the tube. Its such a convoluted issue really. I was a little surprised and disappointed Suiter doesn't cover this in his book - lest I missed it.
Posted 01 December 2013 - 05:46 PM
Posted 01 December 2013 - 06:33 PM
...You are focusing on the upper atmosphere when you rack slightly inward from the infinity focal point...
I would have thought that the upper atmosphere would focus slightly outward from the infinity focal point:
1/distance to object + 1/distance to focus = 1/focal length
focal length = 10 ft (120-inches)
distance to object = 10,000 ft
distance to focus = 10.01 ft (120.12-inches)
Thermals located closer to the primary mirror would require a negative focus, which I suppose would be akin to creating a virtual image of the turbulence...
Posted 01 December 2013 - 06:45 PM
Posted 01 December 2013 - 06:52 PM
Posted 01 December 2013 - 07:05 PM
Posted 02 December 2013 - 10:53 AM
For focusing on nearby objects, the focuser goes out. Focus on infinity and then aim scope to a nearer object. You must then move out to focus, as Vic's calc above suggests.
Have no idea what Roland said or didn't say, but doubt he would get it wrong.
In some seeing conditions, moving the focus out a bit on a star will show flying 'dots' corresponding to upper level winds. Far out of focus may help in the visualization of tube thermals.
Any further info you can provide, or video you can capture would be very interesting to me. Bryan Greer has some fascinating videos of thermal effects on his site:
Posted 02 December 2013 - 01:23 PM
I often see upper level winds as fast moving bars (aligned with the direction of the winds) moving through a significantly out-focused star image.
...In some seeing conditions, moving the focus out a bit on a star will show flying 'dots' corresponding to upper level winds.
IME, tube thermals are typically much slower moving than upper level winds, with twists and rolls. I believe Rick Singmaster described them as "seething worms". To separate them from upper level image effects, I usually in-focus to resolve boundary layer and tube wall thermal effects.
Far out of focus may help in the visualization of tube thermals.
Posted 02 December 2013 - 01:42 PM
Thermals are indeed slower. I've seen some streamers that are stationary or very slowly moving as well as worm like features crawling not just around the tube but across the field.
Not sure if Pete's original query has been answered. Why would intra vs extra make any difference? Possible filtering out the effects of the atmospheric seeing is a possibility as you report.