From: Paul Furman on
Wolfgang Weisselberg wrote:
>
> But with a well placed center pass ND lens you get perfect
> bokeh, see Sony's 135mm STF (Smooth Transition Focus) lens.

That's an interesting one I'd never heard of.
http://www.the135stf.net/index.html

Surprised I never heard of such a filter for screwing on the front.
Probably doesn't work as well unless it's nested inside at the correct
position.
From: Wolfgang Weisselberg on
whisky-dave <whisky-dave(a)final.front.ear> wrote:
> "Wolfgang Weisselberg" <ozcvgtt02(a)sneakemail.com> wrote in message
>> DanP <dan.petre(a)gmail.com> wrote:

>>> So there is no source of light with perfect parallel rays.

>> So you try to deny that stars have perfect parallel rays for
>> the purpose of telescopes and optical lenses on Earth.

> No one heard of gravitational lensing ?

Sure, but Earth's diameter is too small to detect any differences from
parallel rays. You need Earth's orbit as a baseline to detect differences
for close stars.

-Wolfgang
From: whisky-dave on

"Wolfgang Weisselberg" <ozcvgtt02(a)sneakemail.com> wrote in message
news:q2lld7-h5u.ln1(a)ID-52418.user.berlin.de...
> whisky-dave <whisky-dave(a)final.front.ear> wrote:
>> "Wolfgang Weisselberg" <ozcvgtt02(a)sneakemail.com> wrote in message
>>> DanP <dan.petre(a)gmail.com> wrote:
>
>>>> So there is no source of light with perfect parallel rays.
>
>>> So you try to deny that stars have perfect parallel rays for
>>> the purpose of telescopes and optical lenses on Earth.
>
>> No one heard of gravitational lensing ?
>
> Sure, but Earth's diameter is too small to detect any differences from
> parallel rays.

But trh sun isn't.

> You need Earth's orbit as a baseline to detect differences
> for close stars.

But the problem is how to you get two exactly parallel 'rays'.



From: Martin Brown on
On 03/06/2010 09:33, DanP wrote:
> On 2 June, 16:44, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com>
> wrote:
>
>>>> Lenses are imperfect, there's diffraction (and air for us down
>>>> here) and stars are definitively larger than 0, just very far away.
>>>> Stop waffling and try it.
>>> So there is no source of light with perfect parallel rays.
>
> Your previous quote: "stars are definitively larger than 0, just very
> far away." is incompatible with parallel rays.

They are close enough to parallel as makes no difference (apart from our
sun which subtends about half a degree). The largest stars are a few
tens of milliarcseconds across as viewed from the Earth. It is right on
the limits of our best optical telescopes to resolve them. Most stars
are considerably smaller in angular size.

Betelgeuse is just about resolved by the HST it is 20,000 smaller in
diameter than the width of the sun as seen from Earth.

http://www.spaceimages.com/atofbet.html

To all intents and purposes at all ordinary apertures available to
photgraphers and amateur astronomers they are ideal point sources with a
bit of annoying phase screen in front from the atmosphere.
>
>> Both are f/4 at maximum aperture. Does that change your claim?
>> Or does "the amount of light let in depends only of exposure
>> time and f number" hold?
>
> No, both set at f/4 will let in the same amount of light.

That is clearly an insane position to take. What you say is only sort of
correct for an image formed by extended objects that are resolved.

The key point about stars is that they are unresolved.

A star remains unresolved even in the best lenses or telescopes and so
all the light that hits the aperture ends up in the image. The larger
lens captures more light and gives a smaller image Airey disk. The rough
rule of thumb breaks down a bit when the scope is much bigger than 10"
aperture as seeing starts to be the limiting factor and the star images
stay roughly constant angular size after that point.

Astronomers always want bigger apertures to capture more light.
http://www.scopecity.net/LightGrasp.php

It doesn't help on extended objects the laws of physics see to that but
for a point source like a star the bigger aperture always wins.

Regards,
Martin Brown
From: Martin Brown on
On 04/06/2010 13:46, whisky-dave wrote:
> "Wolfgang Weisselberg"<ozcvgtt02(a)sneakemail.com> wrote in message
> news:q2lld7-h5u.ln1(a)ID-52418.user.berlin.de...

>> You need Earth's orbit as a baseline to detect differences
>> for close stars.
>
> But the problem is how to you get two exactly parallel 'rays'.

There are no such things as light rays they are a useful construction
for computing with geometrical optics. The light coming from distant
stars arrives as a wavefront and the shape of the image that is formed
is determined by the shape of the aperture that it goes through.

That is why certain number of leaves of diaphram are preferred. If you
want to see some brutal diffraction effects try putting a small square
aperture mask in front of your longest telephoto lens.

The stars you see at night with the sole exception of the nearby bright
planets are all unresolved and unresolvable with any amateur equipment.
They are as good an approximation to a point object at infinity as you
are ever likely to get. The largest of them subtend angles 1/20000 that
of the sun and most are at least another order of magnitude smaller.

Regards,
Martin Brown