From: "Marco Miglionico" 
To: "ATM Archives" 
Reply-To: "Marco Miglionico" 


This is certainly turning out to be a very enlightening discussion. I am
left wondering something though. It is becoming apparent that this is a
very
'grey' area.

The resolving power of a mirror (objctive) is derived from the size of the
airy disc  = 1.22lambda(f)/D set by the size of the objective. Could we be
objective (no pun intended) and derive a formula that would quantize the
effects of diffraction due to secondary obstruction, and therfore clear up
any misunderstanding about how the diffractive effects of the said
obstruction effect the resolving power of the scope? That is a formula that
would show the 'equivelant ' resolving power. It would have to be an
equivelant because the size of the airy disc is already set by the primary
diameter. An 8in scope can thoretically resolve to 0.64arcsec. We would
like a formula that says for example  'with a 20% obstruction this scope
has an equivelant resolving power of 0.8arcsec'.

We can calculate the amount of reinforcement of the first and subsequent
diffraction rings but is it possible to relate this reinforcement to the
degradation of the resolving power?

I am asking this because my knowledge of optical theory is very limited;
but maybe someone has the knowledge and time to do this. Have any
calculations to this effect ever been carried out?

Marco Miglionico.




----- Original Message -----
From: Marco Miglionico 
To: ATM Archives 
Sent: Monday, March 03, 2003 7:37 PM 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
>
>
>
>
>
>
>

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