From: "Frank Q" 
To: "CSC" ,
        "Atm" 
Reply-To: "Frank Q" 


Hi All

Pls see comments among the text below

Cheers

Frank Q

----- Original Message -----
From: "CSC" 
To: "Atm" 
Sent: Tuesday, March 04, 2003 11:19 PM Subject: RE: ATM Diffraction due to
secondary sizing.


>
> Marco:
>
>   An extended object you are viewing is composed of mosaic of airy disks,
> each with it's diffraction ring system.  When fine detail is observed,
let's
> say a narrow bright line on Jupiter, something with a width near the size
of
> the airy disk, the image will be a fine line of superimposed airy disks
with
> surrounding diffraction rings.  The diffraction rings fall outside the
line
> of fine detail, superimposed on the dimmer adjacent area, diminishing
> contrast. The brighter the ring, the greater the loss of contrast.
#######################################################

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.

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.


>
> According to Arthur Leonard in his treatise on unobstructed TCT scopes, he
> says for 0% obstruction, the ratio of light intensity between the airy
disk
> and the first ring is about 11.6:1 .  At 25% diameter obstruction it is
> 4.6:1.
>
> The ideal diffraction image would be no rings, only the airy disks.

#################################################

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.

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.

>
> Note that as the obstruction increases, the Airy disk gets SMALLER, so
> resolution for point sources like double stars improves. But this
> overlapping of bright diffraction rings reduces the contrast of fine
detail,
> and for subtle low contrast detail, it is especially injurious.
####################################################

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


>
> I have not seen a Schmidt-Cass or Mak that will outperform a medium-long
> focus Newtonian, given the same optical tolerances and allowing the
> Catadioptric scopes enough cool down time.
>
> There is no difference in size of airy disk with a brighter star.  There
> are more rings visible, and stray light and other effects make it
difficult
> to see the airy disk in the glare.  I was just out collimating some scopes
> for the next CMAS star party and used a 1.5" red xmas ball 300 yards away
to
> catch the sun glint.  The solar image had numerous bright rings and a
small
> unstable airy disk, but a tiny dim glint off the wire hanger showed a
> perfect airy disk with 2 beautiful diffraction rings.
>
> Good stuff, keep it coming.
>
> Colin
>
> -----Original Message-----
> From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Marco
> Miglionico
> Sent: Monday, March 03, 2003 2:38 PM
> To: ATM Archives
> Subject: ATM Diffraction due to secondary sizing.
>
>
>
> Hello all discerning optical geniuses.
>
> I have a brain teaser on optical systems. It has been annoying me all day.
> In Texereau's book on telescope making he describes the varying effects of
a
> central obstruction in a newtonian telescope. 10% by diameter is not
> noticable, 20% is noticable but not objectionable, 30% is unrecommended
for
> planetary/lunar observation and 40% reserved for wide field deep sky
> photography.
>
> Reasons cited for these figures is the effect on diffraction. With
> progressive increases of central obstruction, more energy is planted in
the
> first diffraction ring  (when looking at a point source) thereby
increasing
> the airy disc to a size deemed unnacceptable for use on a telescope
intended
> to look at the planets/moon.
>
> Here is the problem though. Commercial telescopes of the cassegrain
veriety
> often have central obstructions approaching 37% by diameter which Texereau
> may well have baulked at the idea of. There is however no question about
> these telescopes' ability to resolve very fine detail on the planets. They
> are in fact among the top performers. So my question is (in the words of a
> true potiticain) who is almost right and who is almost wrong?
>
> This subject may touch upon the debate waged recently about what the
minimum
> size of detail seen in a scope is. I think the outcome of this was that
> details well below the theoretical resolving power of the scope have been
> attested to.
>
> Another thing that confuses me is that there seems to be no relationship
> (written down) between the size of diffraction disk and the brightness of
> the point source of light.(A star). I am presuming then there is none!
>
> How does the strengthening of the first and subsequent diffraction rings,
> due to increases obstruction size, (when looking at a star) effect the
> amount of detail seen when the scope is aimed at a planetary object.?
Could
> it be that airy discs and diffraction only apply to point sources of
light,
> when talking about the ability to see detail on the moon/planets based on
> how the scope performs on a star test?
>
> My thinking is that the dimmer an object (eg a star) the less visible are
> the first and subsequent diffraction rings. That being the case, if a
planet
> was treated as being the source of a (not quite) infinite amount of very
dim
> point sources of light, then only the central airy disc would be visible.
Am
> I right? If I am right then central obstructions up to the point where the
> second ring becomes as bright (containing as much energy) as the central
> airy disc. My thinking behind this is that at almost infinitely low
dimness
> the point where the first diffraction ring becomes visible is the point at
> which the energy is split 50/50 between the airy disc and the first
> diffraction ring. This would happen at a central obstruction of roughly
55%.
> Question mark. (i may be writing nonsense here). It has often been said
that
> the size of the central obstruction should not be overemphasised. I would
> love someone to clarify these points!
>
> Stop falling asleep. Wake up. You have the right to delete my ramblings. I
> think it's quite an interesting subject though!
>
> Marco Miglionico
>
> www.geocities.com/telescopiman
>
>
>
>
>
>
>
>

--- BBBS/NT v4.00 MP