telescope aperture size

In message , Roger Hamlett
writes

"Jochen" wrote in message
...
I have always wondered why they have one central hole for reflectors,
instead of three holes around the edge spaced at 120 degrees.
Admittedely three holes and caps cost a couple of bucks more than one
hole and cap, buth with three holes you would still have access to the
resolution (and hence maginification) appropriate to the full diameter
of your primary, as your telescope is effectively functioning as an
optical interferometer. With one hole, you have completely thrown away
the additional resolving power of the larger primary mirror, as the
image is diffraction limitted by the hole size.

Can anyone throw any light on what seems to me to be a poor design
decision?

You seem to be thinking that a reflector has a 'hole' smaller than the
mirror. It doesn't.

Don't forget that this thread started with a question from someone who
had a telescope in which the hole over the front end _was_ smaller than
the mirror.
Unless someone shows otherwise, I'm going to go on thinking it's a
kludge to cover unacceptable spherical aberration, or some other problem
:-(

Jonathan Silverlight wrote
in :

Don't forget that this thread started with a question from someone who
had a telescope in which the hole over the front end _was_ smaller than
the mirror.
Unless someone shows otherwise, I'm going to go on thinking it's a
kludge to cover unacceptable spherical aberration, or some other problem
:-(

Hi Jonathan,

Well sort of. It's a kludge to cover unacceptable chromatic abberation on
bright objects. See David Knisely's reply.

"Jochen" wrote in message
...
I have always wondered why they have one central hole for reflectors,
instead of three holes around the edge spaced at 120 degrees.
Admittedely three holes and caps cost a couple of bucks more than one
hole and cap, buth with three holes you would still have access to the
resolution (and hence maginification) appropriate to the full diameter
of your primary, as your telescope is effectively functioning as an
optical interferometer. With one hole, you have completely thrown away
the additional resolving power of the larger primary mirror, as the
image is diffraction limitted by the hole size.

Can anyone throw any light on what seems to me to be a poor design
decision?
You seem to be thinking that a reflector has a 'hole' smaller than the
mirror. It doesn't.
How are you going to make the optics for the 'three hole' design?.
The Newtonian reflector, has as much of it's mirror surface as possible
exposed, and then has one part _obtructed_ at the centre. The area
obstructed, and therefore the light loss, is kept as small as possible.
Using instead, multiple seperate 'holes', would reduce the light getting
to the primary mirror. It 'makes sense', if the holes are widely spaced,
and the holes each lead to a seperate smaller mirror, with potentially
cheaper costs for making the three smaller parts, but then the problem is
that each of the seperate telescope assemblies, have to be aligned (and
remain aligned), to the same levels of accuracy as a single mirror. This
is the approach used in the various 'multiple mirror' telescopes, but they
use a lot of work to keep the mirrors precisely aligned (and cost), only
making this worthwhile when the mirror cost is more than that of the
positioning system.
If you have allready made a mirror large enough to cover the three holes,
then you might as well expose as much of it as possible to the incoming
light. You don't 'throw away' the aperture advantage. If you have a 200mm
primary mirror, with a 'hole' just fractionally larger, the ultimate
resolution, is limited by the 200mm diameter, not by the area obstructed
by the primary (perhaps 40mm). The obstruction, slightly takes energy from
the central peak of the Airy disk (when compared to a 200mm scope without
the central obstruction), but for small obstructions, the effect is tiny,
and can actually result in slightly sharper 'edges' to the peak itself
(with the energy being moved from the very centre, but remaining inside
the main 'peak' area, resulting in a slight flattening of the top, and a
sharper fall off).

Best Wishes

"Jonathan Silverlight"
wrote in message ...
In message , Roger Hamlett
writes

"Jochen" wrote in message
...
I have always wondered why they have one central hole for reflectors,
instead of three holes around the edge spaced at 120 degrees.
Admittedely three holes and caps cost a couple of bucks more than one
hole and cap, buth with three holes you would still have access to
the
resolution (and hence maginification) appropriate to the full
diameter
of your primary, as your telescope is effectively functioning as an
optical interferometer. With one hole, you have completely thrown
away
the additional resolving power of the larger primary mirror, as the
image is diffraction limitted by the hole size.

Can anyone throw any light on what seems to me to be a poor design
decision?

You seem to be thinking that a reflector has a 'hole' smaller than the
mirror. It doesn't.

Don't forget that this thread started with a question from someone who
had a telescope in which the hole over the front end _was_ smaller than
the mirror.
Unless someone shows otherwise, I'm going to go on thinking it's a
kludge to cover unacceptable spherical aberration, or some other problem
:-(
The 'general' nature (talking about 'why they have one central hole for
relectors'), suggested that this poster was talking more generally.
Masking down a refractor is suprisingly common (there have been a couple
of reviews published, where the reviewer was suprised at how good the
optical quality seemed to be, until they realised that the scope was
effectively running at a much higher focal ratio, than it appeared).
Masking the edge of the mirror, on a reflector (where there is a problem
with TDE), is also common. However masking a reflector at the front, is
less so, since it many of the aberrations present for 'off axis' stars.
High focal ratio refractors display much less chromatic aberration (this
was the main reason why the early refractors had to be so long), and I'd
be suspicious that a masked down refractor, was being done using an
optical set normally sold for terrestrial use (where CA is less of a
problem in general), and masked down to keep the CA reasonable for
astronomical use...

Best Wishes