From: "Frank Q" 
To: "Marco Miglionico" ,
        "ATM Archives" 
Reply-To: "Frank Q" 


Hi All

May I suggest that someone tonight take their telescope out, put a few
different sized cardboard obstructions in front of it, look at Sirius,
Pollux  or whatever is up there and tell us what they found. I would but I
don't have a telescope yet. Lots of people with lots of hopefully different
telescopes - the results should be interesting!

Cheers

Frank Q


----- Original Message -----
From: "Marco Miglionico" 
To: "ATM Archives" 
Sent: Thursday, March 06, 2003 6:51 AM Subject: Re: ATM Diffraction due to
secondary sizing.


>
> 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
> >
> >
> >
> >
> >
> >
> >
>

--- BBBS/NT v4.00 MP