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


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