Re: ATM Re:Interpretation of Star Test

["Alan Levin" <ailevin@ix.netcom.com>]

I apologize for the late and perhaps redundant post:

sumner & birkmann wrote:
> 
> You're right, it is outside of focus where I notice the blobby
> image.  I tried the same thing with our clubs 30" but the image was
> steady and symmetric inside and outside of focus.  The 30" does have
> an open tube while the coulter does not.  Maybe thats the
> difference?
> 
> Mark

Mark,

If the 30 inch looks symmetric on either side of focus, it is well
figured.  I don't buy the "you are focusing on the upper atmosphere"
argument. This would only hold at one precise out of focus position if
you were focusing on turbulence at a particular level.  Variations in
the atmosphere should effect both sides of focus the same way if the
distribution of light on either side of focus is the same without
considering the atmosphere.

My hypothesis, admittedly based on limited data, is that turbulence
has more impact on one side of focus when the system is under or over
corrected.  Since most commercial large optics are undercorrected,
they are more sensitive to turbulence outside focus.  If you find an
overcorrected optic (a turned down edge is in a gross sense like
overcorrection), it will be more sensitive inside focus.  This is
based on looking at the longitudinal slices of under corrected
apertures in Suiter's Star Testing Astronomical Telescopes, and tests
on my own telescopes.  

In an undercorrected optic, the pattern outside focus has broader more
diffuse rings whose boundaries get harder to distinguish when you add
the tubulent noise of the atmosphere.  Since you are closer to your
constrast limits on detecting the ring structure, enough turbulence
puts you below threshhold and you just see a blob.  Inside focus you
will have narrower more distinct rings with more dark space between
them. Even though turbulence will make the rings look a bit noisy or
distorted, you will still distinguish the ring structure easily.  If
the optics are overcorrected, the argument is the same except that
inside and outside are switched.  If it is perfectly corrected, then
the turbulence will have the same effect on either side of focus.

Since we have many home made telescopes here which are more prone
overcorrection, I'm guessing that some of us see turbulence inside
focus and others see turbulence outside focus.  If someone has a
system that they know from independent tests is overcorrected, I would
be interested to here how turbulence effects it.  Also, wathcing the
mirror as it cools provides interesting data since the figure of the
mirror will change.  These experiments are easy unless you have
continuous perfect seeing <G>.

I have seen the argument here before that better optics are less
sensitive to seeing.  My more specific suggestion  is that better
corrected optics are less sensitive to seeing.  It might also be
interesting to see if an  apodizing mask that suppresses the ring
structure could supress the seeing over certain ranges of spatial
frequency.  This might be a better way to go than stopping down a
large aperture.

Good Observing, 
Alan