From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 







It depends on what makes the rings brighter. If brighter rings are due to a
brighter object that you are viewing, then the contrast will not change.

Yes.  If the rings increase in brightness relative to the airy disk, then
contrast suffers.   Since a larger obstruction increases the relative
brightness of the rings, a larger obstruction degrades contrast more.



If an obstruction gives rise to brighter rings, then the resolution of the
instrument will certainly suffer and this will be interpreted as lower
contrast.


Yes, especially true for objects of low contrast such as planetary detail.

What is an " ideal diffraction image" ?

One way I know of for eliminating rings is through apodization, but then
this just increases the size of the central disk - you have just as much
resolution as before - no more, no less.

Yes.  I do not have first hand experience with apodizing masks, but if the
increase of airy disk size is not too dramatic, then it could improve this
discrimination of low contrast detail.

Resolution is the ability of a system to separate close objects.  When
splitting double stars, the airy disk is the image that shows the split,
overlapping airy disks are seen at high magnifications.  If the airy disk
size is reduced for a given magnification, then smaller object separations
can be seen.  A larger aperture reduces the airy disk size.  So does a
larger obstruction.  (see Suiter's work on this)

If by "ideal" you mean an infinitely small central disk with no
rings then I must inform you that such a situation is physically impossible
as this would lead to infinite energy densities (ie a singularity) at the
disk which in nature just doesn't happen.

I agree. I think A. Leonard meant that the rings are responsible for
effects in image contrast.


I think you meant " as the obstruction DECREASES, the Airy disk gets
SMALLER"

No.

Colin

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