From: whisky-dave on

"Martin Brown" <|||newspam|||@nezumi.demon.co.uk> wrote in message
news:GloPn.23481$%u7.19312(a)newsfe14.iad...
> On 07/06/2010 13:28, whisky-dave wrote:
>> "Martin Brown"<|||newspam|||@nezumi.demon.co.uk> wrote in message
>> news:HS6On.18516$%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.

Interesting although the word clearly can be misleading.
here's a photo of Venus I took with my G10.
http://www.flickr.com/photos/whiskydave/3466971234/
I can sill see it's a planet rather than a star.

>>> 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.

But first you must be sure you're looking a point source of light,
my G10 can resolve Venus as not a star.
The suns gravatational effect is enough to 'bend' light, so any light we
observe from earth is affected by gravity, mostly it's not important, but
when talking of exactly parallel rays well there's really no such thing as
light bends around the universe
and is 'bent' by gravity which changes slightly through space and even ion
the earth we can detect changes due to local densities.

And itsn't the definition of flat in optics to be less that 1/4 wavelengh of
the light
used to observe the object, which in the case of visable light means the
Earth is flat because the Earths surfacwe curves less than the 1/4
wavelengh. IIRC
So it';s all about the actual words we use, so yes according to
opticians/optical engineers the Earth can be considered flat, and they sell
spectacles ;-)


From: Martin Brown on
On 09/06/2010 13:59, whisky-dave wrote:
> "Martin Brown"<|||newspam|||@nezumi.demon.co.uk> wrote in message
> news:GloPn.23481$%u7.19312(a)newsfe14.iad...
>> On 07/06/2010 13:28, whisky-dave wrote:
>>> "Martin Brown"<|||newspam|||@nezumi.demon.co.uk> wrote in message

>>>> 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.
>
> Interesting although the word clearly can be misleading.
> here's a photo of Venus I took with my G10.
> http://www.flickr.com/photos/whiskydave/3466971234/
> I can sill see it's a planet rather than a star.

Actually you can't. Venus is so very much brighter than the illuminated
moon surface that it is massively over exposed and spread into several
adjacent pixels. Think brilliant white cloud for Venus and dark basalt
or tarmac for the moons surface - both in direct sunlight but Venus
somewhat closer to the sun and so more brightly illuminated.

You need a roughly 3" aperture and 15" focal length to get Venus clearly
resolved. See for example the images of the moon and venus conjunction
of 19 May 2007 complete with varying exposures.

<http://www.trivalleystargazers.org/gert/moon_venus_20070519/moon_venus_20070519.html>

Actually done with a 4" aperture which is a bit overkill.
>
>>>> 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.
>
> But first you must be sure you're looking a point source of light,
> my G10 can resolve Venus as not a star.

When overexposed it gives a bigger blob for a point object. That is not
resolved. To get an idea of how much detail would be visible on the moon
if you had resolved Venus take a look at the URL above.

There is an outside chance to photograph Venus with a really high
quality 2" working aperture, a suitably long focal length and a very
steady tripod when it is at its largest apparent size - when it will
look crescent shaped. But that is not what you have recorded.

It is easier with binoculars. A good pair of 10x50s will just show Venus
at thin crescent phase and Jupiters satellites

> The suns gravatational effect is enough to 'bend' light, so any light we
> observe from earth is affected by gravity, mostly it's not important, but
> when talking of exactly parallel rays well there's really no such thing as
> light bends around the universe
> and is 'bent' by gravity which changes slightly through space and even ion
> the earth we can detect changes due to local densities.

The extent to which the sun bends light rays is barely measurable even
with the best professional optical gear available. Modern measurements
rely on VLBI radio astronomy.
>
> And itsn't the definition of flat in optics to be less that 1/4 wavelengh of
> the light
> used to observe the object, which in the case of visable light means the
> Earth is flat because the Earths surfacwe curves less than the 1/4
> wavelengh. IIRC

I don't know who sold you that one. Diffraction limited is sometimes
considered to set in when optical surfaces are within lambda/4 of the
ideal perfect shape. But most serious optics are more like lambda/10.

> So it';s all about the actual words we use, so yes according to
> opticians/optical engineers the Earth can be considered flat, and they sell
> spectacles ;-)

Spectacle makers may well think the Earth is flat. They must find new
members of the flat Earth society from somewhere.

Regards,
Martin Brown
From: bugbear on
whisky-dave wrote:
>
> Interesting although the word clearly can be misleading.
> here's a photo of Venus I took with my G10.
> http://www.flickr.com/photos/whiskydave/3466971234/
> I can sill see it's a planet rather than a star.

In that image (at 768x1024) the moon
is around 39 pixels across,
so roughly speaking (the moon is around 31 minutes of arc)
we're at a pixel per arc-minute.

Venus varies from 10 - 69 arc *seconds*,
so Venus should be around a single pixel.

Well, on the image, Venus is a bit fuzzy,
but even the central fully exposed area
is 8 pixels.

I don't that image resolves Venus very well.

BugBear
From: Doc Johnson on
On Wed, 9 Jun 2010 13:59:10 +0100, "whisky-dave"
<whisky-dave(a)final.front.ear> wrote:

>Earth is flat because the Earths surfacwe curves less than the 1/4
>wavelengh. IIRC

A bad mix of apples and oranges but a fun concept if the surface were
perfectly smooth, and then only applicable to one small portion of the
earth at a time.

Though I know one section of earth where it's even flatter, than that
bad-analogy. A section of North Dakota. I surmise it has to do with the
continental plains slowly rising to meet the Rocky Mountains further out
west--this slow geological rise negating the curvature of the earth. When
traveling through N. Dakota in 1979 to go photograph the total eclipse, I
was driving all night just the night before--to catch up with the event
next morning. I had been noticing that when I would first spot a car's
headlights in the distance on the flat and straight roads (at 2-3 am
there's so little traffic too), that it seemed like an astoundingly long
time before our cars would pass each other. The duration being so long is
what first caught my attention. When driving at night and you see another
car's headlights you instinctively expect it to pass in an acceptable
amount of time, even if it was sitting still on the side of the road in
front of you. You can almost judge if a car is standing still or traveling
toward you just by how long it's taking to meet those oncoming lights. This
expected time delay becomes almost instinctual, not worth thinking about.
Something was highly amiss that night. I decided to time just how long it
took to meet up with some oncoming headlights (prompted by the
environmentally induced boredom too).

I was speeding along that night at 70-75 mph in order to be there on time
for the solar spectacle. From the moment I first noticed the tiny glint of
another oncoming car's headlights on the distant horizon (and looked down
at the clock on the dashboard to note the time), until the time we passed
each other, it took (drum roll please) ... 35 MINUTES. There was no break
in seeing those headlights during this time so I know they were coming from
the same vehicle.

If the other car was traveling toward me at a similar rate of speed this
means that I spotted another car about 20.4 to 21.8 miles away. Keeping in
mind those headlights are only a couple feet from the ground and being seen
from the viewing height in the average car-seat.

Knowing that the distance of the horizon on a calm ocean as seen by a 6 ft.
tall person from a standing height is only 3 miles away, it puts that
section of N. Dakota's flatness into perspective. If viewing the ocean's
horizon from a seated position knock off about another half mile or so,
it's about 2 to 2.5 miles away. If wanting to see the ocean's horizon 21.5
miles away you'd have to do so from a viewing height of about 308 ft.

That one stretch of road to Minot, N. Dakota should be in a record-book
somewhere as the flattest spot on earth. (Until another area can be found
to best it.)


p.s. The eclipse photographs turned out spectacular. Using my OM2n I got
some nice images of bright crimson prominences, among many other unique
images. Localized events like crescents of light projected on snow-drifts
by pin-holes formed between shadows of hands and fingers, etc. (I engaged
some nearby kids on that impromptu viewer's-hill-gathering to help, and to
help entertain and educate them.) Though I had no idea that it would get so
dark that changing a roll of film during totality would be as difficult as
it was, my most frantic film-change ever. A light overcast sky that morning
(which at first concerned me but only added to the event) also afforded
some specialized solar-eclipse effects. The light high-altitude haze acting
as a mild rear-projection screen had relayed waves of rainbow colors in the
sky just moments before totality approached. Then during totality the whole
sky seemingly inverted in an instant, as if seeing the universe in
negative. The darkest spot in the sky was the sun. It was astounding, right
to the very core of your DNA.

Afterward I stopped off at a large nearby truck-stop for an early lunch.
About 300-400 others were there. (Minot had become an eclipse destination
that morning.) You could have heard a toothpick drop in that large
dining-hall. Nobody was talking during their whole meal. Only hushed and
solemn whispers when placing orders or paying at the register. As if they
were all put in their place for once in their small lives. Now knowing with
absolute certainty for the first time in their lives that they cannot be in
control of everything. They were all genuinely awe-struck by what had just
occurred, as was I. I can only imagine how fearful someone would have been
if they didn't know what had just taken place. Here it is 31 years later
and I still get goose-bumps reminiscing about it. It's as if all throughout
the evolution of life on earth that DNA has never quite been able to adapt
to the solar eclipse experience. Your DNA doesn't know how to react to it.
I understand the "eclipse chaser". Experiencing totality is a mental,
emotional, and physical rush, unlike any other. That one in Minot that
morning, with the high-altitude rear-projection screen unveiling another
awe-inducing layer of its effects, even more so.





From: Doug McDonald on
On 6/9/2010 11:17 AM, Doc Johnson wrote:

>
> Though I know one section of earth where it's even flatter, than that
> bad-analogy. A section of North Dakota. I surmise it has to do with the
> continental plains slowly rising to meet the Rocky Mountains further out
> west--this slow geological rise negating the curvature of the earth. When
> traveling through N. Dakota in 1979 to go photograph the total eclipse,

I've only seen one total eclipse of the sun, in Costa Rica in, I believe, 1991.

This eclipse was teh beginning of the eclipse craze; the travel company I went
with was oblivious to it happening before I asked them if they could
do a pre-existing trip "add-on" ... which they did, getting one
other customer. After that, they have done lots of eclipse trips.

This was awesome ... it lasted 7 minutes. I had a 600mm mirror lens
to photograph it. This was on a camera on a small mini-tripod about
a foot high, which turned out to be quite adequate. The thing of note was that
the feet of the tripod were slightly awash in the Pacific Ocean. The beach
was packed with eclipse-gazers (this beach, Manuel Antonio, is the most
popular in Costa Rica.)

Doug McDonald
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