From: Martin Brown on
On 07/06/2010 13:28, whisky-dave wrote:
> "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.

Doesn't help you when the optics are close to diffraction limited then
the wave nature of light is very important and cannot be ignored. This
is usually the case for most decent camera lenses at f5.6 or slower.
>
>> 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.

Yes. A star in focus just gives you the idealised point spread function
for the imaging system you have in front of the CCD detector.
>
>> 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.

No. It is *intrinsic* to the physics of the problem. A finite size of
aperture necessarily gives a point spread function that is the Fourier
transform of its shape. Bigger aperture samples more of the wavefront
and resolves finer angular detail (all other things being equal).

A disk is as good as it gets and gives the classic resolution equation

1.22.lambda/D

Where D is the diameter of the lens and lambda is the chosen wavelength.

Short wavelengths give a slightly higher resolution with the same 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.
>
> So, the planets too were once unresolverable.

And they are a pretty good approximation to point sources where camera
lenses are concerned. My 1000mm will show Venus and Saturn clearly.
>
>> 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.

It is *exactly* the point. They are good enough as point sources for all
terrestrial telescopes below about 2m diameter.

Regards,
Martin Brown
From: Gill Collins on
On Tue, 8 Jun 2010 12:30:05 +0100, Pete
<available.on.request(a)aserver.invalid> wrote:

>
>If you know of any software that would show me the twilight times, I'd
>really appreciate it.

You might try "Kalendis".

http://individual.utoronto.ca/kalendis/
http://individual.utoronto.ca/kalendis/kalendis.htm

I've been using it for many years, and in more recent years to create
full-year TXT file lists of sun/moon rise/set, lunar-phase, twilight times
to read back on my CHDK cameras using the EVF or LCD display, right from
the SD card using CHDK's built-in text reader. (Save a month or year of
data as comma or tab delimited file then import into a spreadsheet program
to select which required columns to print.) This way I have a list for any
latitude I might be at or heading to, right in my hands in my camera. It
can be important information if you are hoping to catch that full-moon
peeking above the horizon during sunset (or sunrise). Or if you want to
have a new(er) moon illuminated by earth-shine with its bright sliver of
sunlit portion in part of your frame. There's no sense heading to that
summit for a spectacular photograph if the players and stage-hands for
lighting aren't in the house that day.

For a sample of its HTML output format (how it looks when imported into a
spreadsheet as well from its .csv or .tab files):

http://individual.utoronto.ca/kalendis/exports/twilight/Greenwich/jun.htm

For a good page of information on "twilight"

http://individual.utoronto.ca/kalendis/twilight/index.htm

From: Peter on
"Pete" <available.on.request(a)aserver.invalid> wrote in message
news:2010060812300565633-availableonrequest(a)aserverinvalid...

> Tell me about it! I enjoy taking photos during twilight. I can't find
> information about light levels for my region so I have to guess when to
> set out on a trip. During most of the year I end up getting too cold
> before I've finished the shoot. I've had three successful trips during the
> last 12 months - I've learnt a lot and want to use that knowledge to take
> better shots in the future.

IMHO Pre sunrise and twolight are the best times. the difference being that
in the morning the light gets worse, after prime, whatever that is.


>> Ever hear of "The Land of the Midnight Sun"?
>
> Yes. For me, it would be very difficult to both get there and survive
> there, but I'd love to go.


Did that last summer. The running joke there is teenagers say to their
parents: "don't worry, I'll be home before dark."


--
Peter

From: George Kerby on



On 6/7/10 8:50 PM, in article 87k0e7-bva.ln1(a)ID-52418.user.berlin.de,
"Wolfgang Weisselberg" <ozcvgtt02(a)sneakemail.com> wrote:

> 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.
>
That's like asking if infinity is farther than the moonies. Nice try!

From: DanP on
On 8 June, 02:53, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
wrote:
> DanP <dan.pe...(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

f/stop.

Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.

Set them both to f/2.8 or smaller and you get the same light.


DanP
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