NavList:

   

Reply

On 23 Feb 2005 at 3:02, Frank Reed wrote:

> And yet, the eye is STILL an optical system first. The input to all that
> detection, transmission, and processing power is still just light passing
> through an aperture. The resolution limit imposed by diffraction is very
> strong. Yes, you can tease out *some* details at levels a little below the
> resolution limit but there's precious little to work with.

In video microscopy it's common to go beyond the diffraction limit and
pick out features (but not "detail") that should be too small to see. This
works because of motion (video microscopy provides 30 frames/sec).
Although on a still photo one would not be able to distinguish features
from artifacts, persistance and motion give additional clues that allow
one to discriminate. Artifacts don't obey Newton's laws, for example.

An example of the odd behavior of the human visual system can be seen when
viewing MRI scans. When each image in a stack is looked at as a still
image, anatomical detail can be very difficult to pick out. When the
observer is allowed to pan back and forth (using the image slices as a
video stream) then the apparent resolution of the images increases by
about an order of magnitude. Somehow the brain puts all this detail into
the moving image that's not really there (except that it is there--i.e. if
you're familiar with histological sections of the same tissue, the MRI
detail matches the genuine anatomical detail). I have no idea how this
works, but it is amazing to witness.

> And you [Alex] concluded:
> "In the case of measuring star distances with a sextant, I can see two
> stars as one dot, but still understand that this dot is somehow
> "imperfect".  This really happens when the distance is about 0.5' (with my
> scope). But still I somehow can measure to 0.1' in many cases."
>
> What's the magnification of your scope again? If it's 7x, then on the
> retina, 0.5 minutes of separation is magnified to 3.5 minutes and 0.1
> minutes is magnified to 0.7 minutes. So if that's what you're seeing,
> there's no surprise here. This is just what you should expect from normal,
> correctly  focused imaging resolution. With a quality sextant and a good 7x
> telescope,  you should expect to get 0.1' accuracy repeatably. But can you
> do this when you  un-mount the scope?? If you can reach that accuracy with
> your unaided eye, then  this would qualify as a hyperacuity task. It's
> worth testing.

Before I had a scope I was able to get an index error consistently within
0.2' by overlaying stars using just a sight tube. But I'm pretty sure I
couldn't see any change in the image over a much larger range. Rather I
would note an approximately equal separation on both sides and then just
try to center the tangent screw in the middle of these positions. This
worked because my sextant has very little backlash. Once I got a
telescope, the index error remained the same (but was easier to test).

Ken Muldrew.

   

Reply