From: DanP on
> I guess you could do a thought experiment.
> If 2 lenses gather the same amount of parallel rays along
> their lens axis, they must have the same diameter.  Go from
> there.

Right, at maximum lens aperture bigger lenses gather more light.
Close the bigger lenses aperture to match the small ones and you get
the same amount of light.

DanP
From: whisky-dave on

"Martin Brown" <|||newspam|||@nezumi.demon.co.uk> wrote in message
news:HS6On.18516$%u7.7097(a)newsfe14.iad...
> 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.
But they are photons too.

> 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.
Then that must be for everything we observe.

> 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.
But that is a problem with the viewing rather than the 'ray', 'wavelengh' or
photon viewed.

>
> The stars you see at night with the sole exception of the nearby bright
> planets are all unresolved and unresolvable with any amateur equipment.

So, the planets too were once unresolverable.

> They are as good an approximation to a point object at infinity as you are
> ever likely to get.

True, but that's not the point.


From: Wolfgang Weisselberg on
DanP <dan.petre(a)hotmail.com> wrote:
>> I guess you could do a thought experiment.
>> If 2 lenses gather the same amount of parallel rays along
>> their lens axis, they must have the same diameter.  Go from
>> there.

> Right, at maximum lens aperture bigger lenses gather more light.
> Close the bigger lenses aperture to match the small ones and you get
> the same amount of light.

Please clarify, what exactly do you mean "to match the small
ones"? f/stop? Diameter? Area? What exactly?

-Wolfgang
From: Wolfgang Weisselberg on
George Kerby <ghost_topper(a)hotmail.com> wrote:
> On 6/5/10 7:20 AM, in article 62spd7-tk3.ln1(a)ID-52418.user.berlin.de,

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

> Almost right: Only one.

The sun is a star, but is not part of the collective "the stars"
for the viewing location Earth.

-Wolfgang
From: Pete on
On 2010-06-08 02:50:00 +0100, Wolfgang Weisselberg said:

> George Kerby <ghost_topper(a)hotmail.com> wrote:
>> On 6/5/10 7:20 AM, in article 62spd7-tk3.ln1(a)ID-52418.user.berlin.de,
>
>>> You can't see the stars during the day anyway.
>
>> Almost right: Only one.
>
> The sun is a star, but is not part of the collective "the stars"
> for the viewing location Earth.

It's also the only visible star that's the most difficuly to see during
the night.

--
Pete