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Focus - intra and extra - seeing, thermals, glass

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#1 azure1961p

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Posted 01 December 2013 - 03:05 PM

In a reflector - all reflectors really when a focuser is racked out there always appears less thermal/seeing issues while racking the focuser in
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.

Pete

#2 GlennLeDrew

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Posted 01 December 2013 - 05:22 PM

While I wouldn't want to hazard a guess as to the mechanism, it seems to be a manifestation of what is observed during a Ronchi test. At equal distances from focus, inside focus the Ronchi bands are less sharply defined than when outside.

#3 PowellAstro

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Posted 01 December 2013 - 05:28 PM

I've heard Rolland C. Say that inside you are focused on more of the atmosphere. Sounds right.

#4 azure1961p

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Posted 01 December 2013 - 05:40 PM

Thanks guys - it would LOOK that way based on Roland's remarks. The why of it is what puzzles me.

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.


Pete

#5 Mirzam

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Posted 01 December 2013 - 05:46 PM

An object in the near field extends over an appreciable angle so that rays are already slightly convergent when they impinge on the primary. This causes their focus point to be closer to the primary. You are focusing on the upper atmosphere when you rack slightly inward from the infinity focal point. Mirror thermals are better seen as slowly moving shadows superimposed on the focal pattern when racked farther in or out of focus.

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#6 Vic Menard

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Posted 01 December 2013 - 06:33 PM

...You are focusing on the upper atmosphere when you rack slightly inward from the infinity focal point...

:question:
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... :shrug:

#7 Mirzam

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Posted 01 December 2013 - 06:45 PM

You are not actually focusing on the thermals from the mirror. Draw a picture and you will see that convergent rays focus closer than rays coming from infinity.

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#8 Mirzam

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Posted 01 December 2013 - 06:52 PM

You are right. Ray tracing an object closer than infinity gives a divergent pattern.

JimC

#9 azure1961p

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Posted 01 December 2013 - 07:05 PM

So the original premise of racking in is on distant thermals and racking the focuser out is more interior?

Pete

#10 Mirzam

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Posted 01 December 2013 - 07:26 PM

You cannot "focus" on thermals inside the telescope tube. You can only see the projection of shadows caused by differences in the optical properties of a medium. See:

Shadowgraph

JimC

#11 azure1961p

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Posted 01 December 2013 - 08:02 PM

Ok go it. Thanks.

Pete

#12 DesertRat

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Posted 02 December 2013 - 10:53 AM

Pete,

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:

http://www.fpi-proto...greer/index.htm

Glenn

#13 Vic Menard

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Posted 02 December 2013 - 01:23 PM

...In some seeing conditions, moving the focus out a bit on a star will show flying 'dots' corresponding to upper level winds.

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.

Far out of focus may help in the visualization of tube thermals.

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.

#14 DesertRat

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Posted 02 December 2013 - 01:42 PM

Seeing has all kinds of pathologies, from barely detectable to diabolical. I've seen dots, bars, ripples and virtual explosions.

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.

Glenn






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