From: LOL! on
On Fri, 4 Jun 2010 17:04:09 -0700 (PDT), DanP <dan.petre(a)gmail.com> wrote:

>
>Only laser light is parallel. Stars have an angular size (larger than
>0) so they cannot have parallel rays.

"Coherent light" doesn't mean parallel light rays. All lasers are rated in
milliradians (mRad) of divergence (or convergence). There is no such thing
as an absolute 100% collimated laser. The parallelism of the nearest
starlight (not counting the sun) would far surpass anything that can be
created by human hands.

You have no idea how funny this is watching them trying to explain the
merest basics of optics and light to you. You *can* find the shutter button
on a camera, I hope? (not)

LOL!

From: Ray Fischer on
DanP <dan.petre(a)gmail.com> wrote:
>On 3 June, 23:09, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
>wrote:
>> DanP <dan.pe...(a)hotmail.com> 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.
>>
>> And yet the rays are parallel. �Fancy that.
>
>Only laser light is parallel.

I think you'll find that lasers are much less parallel than is
starlight.

> Stars have an angular size (larger than
>0) so they cannot have parallel rays.

If you want to get anal then there is no such thing as parallel rays
since there will always be some error.

>But I can concede that is practically true for short focal lengths.

Even for long.

It's not hard to calculate. Take a star of diameter 1,000,000 miles
(largish, but not very). Put at a distance of five light years, or
5.9e12 miles. One divided by the other, inverse sin, and you get a
divergence maximum of 9.740283e-06 degrees, or about 0.17
microradians.

--
Ray Fischer
rfischer(a)sonic.net

From: DanP on
On 5 June, 06:52, rfisc...(a)sonic.net (Ray Fischer) wrote:
> DanP  <dan.pe...(a)gmail.com> wrote:
> >On 3 June, 23:09, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
> >wrote:
> >> DanP <dan.pe...(a)hotmail.com> 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.
>
> >> And yet the rays are parallel. Fancy that.
>
> >Only laser light is parallel.
>
> I think you'll find that lasers are much less parallel than is
> starlight.
>
> > Stars have an angular size (larger than
> >0) so they cannot have parallel rays.
>
> If you want to get anal then there is no such thing as parallel rays
> since there will always be some error.
>
> >But I can concede that is practically true for short focal lengths.
>
> Even for long.
>
> It's not hard to calculate.  Take a star of diameter 1,000,000 miles
> (largish, but not very).  Put at a distance of five light years, or
> 5.9e12 miles.  One divided by the other, inverse sin, and you get a
> divergence maximum of 9.740283e-06 degrees, or about 0.17
> microradians.

Fair enough. I thought the star covers a number of pixels due to its
size
Martin cleared that one.


DanP
From: Wolfgang Weisselberg on
whisky-dave <whisky-dave(a)final.front.ear> wrote:
> "Wolfgang Weisselberg" <ozcvgtt02(a)sneakemail.com> wrote in message
>> whisky-dave <whisky-dave(a)final.front.ear> wrote:

>>> 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 can't see the stars during the day anyway.

>> 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'.

The solution is infinitely far away objects. Stars are
sufficiently far away for any single telescope.

-Wolfgang
From: George Kerby on



On 6/5/10 7:20 AM, in article 62spd7-tk3.ln1(a)ID-52418.user.berlin.de,
"Wolfgang Weisselberg" <ozcvgtt02(a)sneakemail.com> wrote:

> whisky-dave <whisky-dave(a)final.front.ear> wrote:
>> "Wolfgang Weisselberg" <ozcvgtt02(a)sneakemail.com> wrote in message
>>> whisky-dave <whisky-dave(a)final.front.ear> wrote:
>
>>>> 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 can't see the stars during the day anyway.
>
Almost right: Only one.