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Now, the next BIG contest is ON!

 
 
whisky-dave
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      06-16-2010

"David J Taylor" <(E-Mail Removed)> wrote in message
news:hv75gj$aac$(E-Mail Removed)-september.org...
>> And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will
>> capture more light from a star than a 50mm f/2.8 lens at
>> f/2.8.
>>
>> It's patently obvious that a 200mm f/2.8 lens has a larger
>> diameter than a 50mm f/2.8 lens.
>>
>> -Wolfgang

>
> More light (photons) in a given period, when other things are similar.


Well that makes sense, but I'd have thought it obvious.....
that 200mm f/2.8 lens would have a larger diameter than a 50mm f/2.8 lens.

But what would happen in a parallel universe where the laws of physics are
the same ? would the 200mm taste of strawberries while the 50mm taste like
chocolate.


 
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Pete
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      06-16-2010
On 2010-06-16 13:49:07 +0100, whisky-dave said:

> "David J Taylor" <(E-Mail Removed)> wrote in
> message news:hv75gj$aac$(E-Mail Removed)-september.org...
>>> And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will
>>> capture more light from a star than a 50mm f/2.8 lens at
>>> f/2.8.
>>>
>>> It's patently obvious that a 200mm f/2.8 lens has a larger
>>> diameter than a 50mm f/2.8 lens.
>>>
>>> -Wolfgang

>>
>> More light (photons) in a given period, when other things are similar.

>
> Well that makes sense, but I'd have thought it obvious.....
> that 200mm f/2.8 lens would have a larger diameter than a 50mm f/2.8 lens.
>
> But what would happen in a parallel universe where the laws of physics
> are the same ? would the 200mm taste of strawberries while the 50mm
> taste like chocolate.


Other way around. Can't figure out what a tripod would taste like,
don't think I've eaten anything with three legs. Packaging in my
supermarket suggest some chickens have 12 legs, which is strange: if
they had then how come they got caught?

--
Pete

 
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David J Taylor
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      06-16-2010
"whisky-dave" <(E-Mail Removed)> wrote in message
news:hvah80$fuj$1@qmul...
[]
>> More light (photons) in a given period, when other things are similar.

>
> Well that makes sense, but I'd have thought it obvious.....
> that 200mm f/2.8 lens would have a larger diameter than a 50mm f/2.8
> lens.


Indeed, by definition of f/number.

What some people seem to be having difficulty with is that - given
nominally identical lenses but with everything scaled by four - is that
the light collected would be spread out over a larger area with the 200mm
lens, even though the number of photons per unit area would be the same,
and the implications of that.

Cheers,
David

 
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Martin Brown
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      06-16-2010
On 15/06/2010 13:34, J. Clarke wrote:
> On 6/15/2010 2:10 AM, David J Taylor wrote:
>>> And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will
>>> capture more light from a star than a 50mm f/2.8 lens at
>>> f/2.8.
>>>
>>> It's patently obvious that a 200mm f/2.8 lens has a larger
>>> diameter than a 50mm f/2.8 lens.
>>>
>>> -Wolfgang

>>
>> More light (photons) in a given period, when other things are similar.


And crucially for a star in good focus the light all falls on the same
physical sensor area for a given focal ratio. This f ratio gives a
certain brightness for any extended object irrespective of focal length,
but for an unresolved point source like a star it puts all the light
from the larger aperture onto a same sized spot.

> I don't see Wolfgang's post, but it contains the usual misconception
> about photographic vs astronomical imaging.
>
> The 200 will "capture more light" in the sense that more light will
> enter the lens. However for a given sensor size, a smaller proportion of
> that light will strike the sensor. If you make adjustments so that both
> cover the same field of view, for example by adding two 2x
> teleconverters to the 50, _then_ the 200 will show its "greater light
> gathering", and this will be reflected in the photographic formulae, as
> at that point the 50/2.8 will be functioning as a 200/11.2.


This is true but misleading. The 200mm f2.8 wins out on unresolved
objects because of the physics of diffraction. Camera lenses are just
faster than normally used in astronomy prime focus imaging device. This
is largely because slower optics are easier to make diffraction limited.

Although it is true that astronomical optics tend to be f5 or slower
(with the exception of Schmidt cameras which are about f2 and they are
most definitely *cameras* - no prospect of visual observing). The
Palomar survey Schmidt being one of the early ones:

http://www.nps.gov/history/history/o...o4f.htm#photos

f2.5 with 120" focal length and 48" working aperture capable of
diffraction limited imaging on a 7 degree field of view using 10" or 14"
photographic plates.

> Astronomers generally work with a magnified field to begin with--in
> other words they almost always use some kind of teleconverter--so that
> they are working at effective focal lengths longer than that of their
> primary lens or mirror. In that situation, larger aperture always
> results in a brighter image. Photographers, on the other hand, hardly
> ever use a teleconverter these days, so effect of the diameter of their
> lens is accurately reflected in the f/ratio.


This is nonsensical gibberish. The point about the 200mm f2.8 lens is
that it will form an image of an unresolved point source like a star
with the same linear dimensions as the 50mm f2.8 lens but with 16x as
much light hitting the same patch of the sensor. The gain is *real*
provided that the optics are truly diffraction limited which usually
holds valid for short exposures with working apertures of 4" or less.

Extended objects also get 16x as much light but on an area that is 16x
bigger so that the illumination is the same. Stars are a special case!

Don't take my word for it! Go out and take a few shots of the
constellations with different apertures and focal lengths. Nothing beats
doing the experiment. Guiding will be a problem for modest focal lengths
at just a few seconds so you need to choose very bright stars.

Regards,
Martin Brown
 
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Martin Brown
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      06-16-2010
On 16/06/2010 14:28, David J Taylor wrote:
> "whisky-dave" <(E-Mail Removed)> wrote in message
> news:hvah80$fuj$1@qmul...
> []
>>> More light (photons) in a given period, when other things are similar.

>>
>> Well that makes sense, but I'd have thought it obvious.....
>> that 200mm f/2.8 lens would have a larger diameter than a 50mm f/2.8
>> lens.

>
> Indeed, by definition of f/number.
>
> What some people seem to be having difficulty with is that - given
> nominally identical lenses but with everything scaled by four - is that
> the light collected would be spread out over a larger area with the
> 200mm lens, even though the number of photons per unit area would be the
> same, and the implications of that.


That is true for extended objects, the f ratio tells you how bright the
resulting image will be for any lens - which is why it is so useful.

But it is not true for an unresolved point object like a star. If you
quadruple the aperture at constant f ratio the image of an unresolved
point object stays exactly the same linear size at the sensor but gets
16x as much light delivered to it.

This is a key property of unresolved objects and focal ratios. And it
holds good until the aperture or the exposure time prevents the system
from being diffraction limited (typically >6" and a few seconds).

Regards,
Martin Brown
 
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David J Taylor
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      06-16-2010

"Martin Brown" <|||newspam|||@nezumi.demon.co.uk> wrote in message
newsR5Sn.142419$(E-Mail Removed)...
> On 16/06/2010 14:28, David J Taylor wrote:
>> "whisky-dave" <(E-Mail Removed)> wrote in message
>> news:hvah80$fuj$1@qmul...
>> []
>>>> More light (photons) in a given period, when other things are
>>>> similar.
>>>
>>> Well that makes sense, but I'd have thought it obvious.....
>>> that 200mm f/2.8 lens would have a larger diameter than a 50mm f/2.8
>>> lens.

>>
>> Indeed, by definition of f/number.
>>
>> What some people seem to be having difficulty with is that - given
>> nominally identical lenses but with everything scaled by four - is that
>> the light collected would be spread out over a larger area with the
>> 200mm lens, even though the number of photons per unit area would be
>> the
>> same, and the implications of that.

>
> That is true for extended objects, the f ratio tells you how bright the
> resulting image will be for any lens - which is why it is so useful.
>
> But it is not true for an unresolved point object like a star. If you
> quadruple the aperture at constant f ratio the image of an unresolved
> point object stays exactly the same linear size at the sensor but gets
> 16x as much light delivered to it.
>
> This is a key property of unresolved objects and focal ratios. And it
> holds good until the aperture or the exposure time prevents the system
> from being diffraction limited (typically >6" and a few seconds).
>
> Regards,
> Martin Brown


Yes, I was referring to normal photographic use, not astro-photography.

May I ask - in astrophotography, how important is it that the lens is
diffraction limited, considering how bad atmospheric disturbance can be
during long exposures? Or is there some "optimum" combination of
stacking, exposure time, and nearness to diffraction limits which depends
on the seeing, lens focal length etc. etc. Perhaps a feeling one gains
with experience?

Cheers,
David

 
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Martin Brown
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      06-16-2010
On 16/06/2010 17:22, David J Taylor wrote:
>
> "Martin Brown" <|||newspam|||@nezumi.demon.co.uk> wrote in message
> newsR5Sn.142419$(E-Mail Removed)...
>> On 16/06/2010 14:28, David J Taylor wrote:
>>> "whisky-dave" <(E-Mail Removed)> wrote in message
>>> news:hvah80$fuj$1@qmul...
>>> []
>>>>> More light (photons) in a given period, when other things are similar.
>>>>
>>>> Well that makes sense, but I'd have thought it obvious.....
>>>> that 200mm f/2.8 lens would have a larger diameter than a 50mm f/2.8
>>>> lens.
>>>
>>> Indeed, by definition of f/number.
>>>
>>> What some people seem to be having difficulty with is that - given
>>> nominally identical lenses but with everything scaled by four - is that
>>> the light collected would be spread out over a larger area with the
>>> 200mm lens, even though the number of photons per unit area would be the
>>> same, and the implications of that.

>>
>> That is true for extended objects, the f ratio tells you how bright
>> the resulting image will be for any lens - which is why it is so useful.
>>
>> But it is not true for an unresolved point object like a star. If you
>> quadruple the aperture at constant f ratio the image of an unresolved
>> point object stays exactly the same linear size at the sensor but gets
>> 16x as much light delivered to it.
>>
>> This is a key property of unresolved objects and focal ratios. And it
>> holds good until the aperture or the exposure time prevents the system
>> from being diffraction limited (typically >6" and a few seconds).

>
> Yes, I was referring to normal photographic use, not astro-photography.


Just to stress the point this *only* works for unresolved stars.
Extended nebulae images cannot made brighter with a bigger working
aperture because they have 4x the light spread out over 4x the area. I'm
sure you know this, but other posters here seem horribly confused.
>
> May I ask - in astrophotography, how important is it that the lens is
> diffraction limited, considering how bad atmospheric disturbance can be
> during long exposures? Or is there some "optimum" combination of
> stacking, exposure time, and nearness to diffraction limits which
> depends on the seeing, lens focal length etc. etc. Perhaps a feeling one
> gains with experience?


Its actually still a topic of serious ongoing research!

At 8" and above the diffraction limiting deep sky imaging for amateurs
is less important and the scopes are referred to in a slightly
derogatory way as "light buckets". Planetary observers are more
demanding and their kit tends to be truly diffraction limited on axis so
that on those rare moments of good seeing they have the detail.

For most amateur deep sky imaging a good rule of thumb is that you will
struggle to do any better than 2" arc fwhm (1/1000 solar diameter) which
is equivalent to 3" working aperture. But with devilishly cunning pro
hardware and selective shift and sum of the images with the highest
contrast ground based results on larger apertures can now beat the HST
for small bright targets. The technique is called "Lucky Imaging" and
has revolutionised amateur planetary imaging work. This can be done
small scale with a humble webcam, very good optics and a lot of
patience. Ideally you want good seeing!

http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/

The main effect so far seems to demonstrate that things hitting Jupiter
with a big wallop are more common that we once thought. A couple of
weeks back some of the top amateur planetary imagers caught another
impact (or rather someone else spotted it in his images). And he already
had one notch for finding a thing hits Jupiter event.

http://www.impactlab.com/2010/06/06/...on-on-jupiter/

This stuff is typically right on the limits of the diffraction limit for
the scope and CCD in use and they use specialised software to shift and
add the images to get the optimum result. The good thing is that now
there is usually more than one amateur imager pointed at the same planet
so any transient phenomena can be confirmed after the event.

The main difference between telescope optics and camera lenses these
days is that the scope trades filling the whole field of view with a
good image for perfect diffraction limited behaviour over a smaller
central sweet spot. This is in part an artefact of the most easily mass
produced SCT telescopes which have a slightly curved focal plane.

Regards,
Martin Brown
 
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David J Taylor
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      06-17-2010
"Martin Brown" <|||newspam|||@nezumi.demon.co.uk> wrote in message
news:7B9Sn.37958$(E-Mail Removed)...
[]
> Its actually still a topic of serious ongoing research!


Trust me to ask an awkward question!


[good stuff snipped]

> http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/


[more snipped]

> The main difference between telescope optics and camera lenses these
> days is that the scope trades filling the whole field of view with a
> good image for perfect diffraction limited behaviour over a smaller
> central sweet spot. This is in part an artefact of the most easily mass
> produced SCT telescopes which have a slightly curved focal plane.
>
> Regards,
> Martin Brown


Thanks for that Web site pointer, Martin. Most interesting, and a
demonstration of something which can be done now that digital imaging and
powerful computers are readily available.

BTW: Nikon had/have a "Best Shot Selector" option for some of their
cameras whereby they would take a sequence of images, and select the one
with the largest JPEG file size as the one with the best detail (i.e.
least blur). Just slightly related.

BTW: some years back I had connections with both Cambridge and Edinburgh
astronomy circles. My wife used to bend glass plates to fit that curved
focal plane!

Cheers,
David

 
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John Turco
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      06-30-2010
Better Info wrote:
>
> On Mon, 31 May 2010 05:57:27 -0500, John Turco <(E-Mail Removed)> wrote:
>
> >Ray Fischer wrote:
> >>
> >> DanP <(E-Mail Removed)> wrote:
> >> >On 23 May, 18:50, (E-Mail Removed) (Ray Fischer) wrote:
> >> >> DanP <(E-Mail Removed)> wrote:
> >> >> >On May 23, 3:31 am, (E-Mail Removed) (Ray Fischer) wrote:
> >> >>
> >> >> >> Wrong. Bigger apertures allow higher resolution. That's why big
> >> >> >> telescopes are better than tiny ones.
> >> >>
> >> >> >Telescopes are focused at infinity so that is a different case.
> >> >>
> >> >> ?!?
> >> >>
> >> >> Why is that different?
> >> >
> >> >Because their optics are fixed
> >>
> >> Nope.
> >>
> >> >and you want the biggest lens/mirror>you can get.
> >>
> >> Because bigger means higher resolution.

> >
> >
> >Where astronomical telescopes are concerned, greater light-gathering
> >capability "means higher resolution."
> >
> >Laymen often overstate the importance of magnification, but...if the
> >object in question isn't seen, to begin with, it >can't< be magnified,
> >anyway.

>
> Higher resolution is the result of larger diameters. Greater light
> gathering ability of that larger diameter is just a welcome benefit for
> naked-eye visual observations by humans. Resolution does not depend on the
> amount of light collected. If you put a very strong neutral density filter
> in front of a large 10m dia. telescope, to effectively limit the amount of
> light entering to no more than 1 photon per second, and used a
> high-resolution detector array to measure those photons, in time you will
> still have higher resolution image than could be obtained from a 1m dia.
> telescope with no filter at all.
>
> The Hubble Telescope was trained on a patch of sky where nobody thought
> anything existed, as pure black as the sky can be. Named "The Hubble Deep
> Field" experiment. From a period of 10 days of collecting photons from that
> black region they imaged about 1500 whole galaxies in that empty spot of
> the sky with just as much resolution as they get on bright objects. Image
> brightness has nothing to do with resolution.
>
> http://hubblesite.org/newscenter/arc...eases/1996/01/



All right, then; please, give me additional information. I'd prefer a
telescope with the following capabilities:

planets appear fairly large, in the eyepiece(s)

resolves Saturn's rings distinctly

has electronic "GO TO" functions

What's the absolute cheapest that I could obtain such an instrument
(new, used or refurbished)?

Thanks, for any advice!

--
Cordially,
John Turco <(E-Mail Removed)>

Marie's Musings <http://fairiesandtails.blogspot.com>
 
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Martin Brown
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      06-30-2010
On 30/06/2010 04:10, John Turco wrote:
> Better Info wrote:
>>
>> On Mon, 31 May 2010 05:57:27 -0500, John Turco<(E-Mail Removed)> wrote:
>>
>>> Ray Fischer wrote:
>>>>
>>>> Because bigger means higher resolution.


Only up to a point. There are trick and exceptions where good
observables can be obtained from very large interferometer spacings but
in general the atmosphere limits the effective resolution for long time
exposures from the ground to about what a good 6" scope can do.

Very short exposures of bright objects like planest can be lucky and get
the moments of good seeing. There are shift and add post processing
tricks to make a higher resolution image from the highest contrast
frames in a video stream.
>>>
>>>
>>> Where astronomical telescopes are concerned, greater light-gathering
>>> capability "means higher resolution."


Not quite. Once you get significantly beyond 8" aperture turbulence in
the atmosphere dominates the theoretical resolution gain - except during
very short periods of "lucky seeing". Google that phrase to see how
amateurs using humble webcams can now almost reach professional
standards on the planets with modest apertures and a lot of patience!
>>>
>>> Laymen often overstate the importance of magnification, but...if the
>>> object in question isn't seen, to begin with, it>can't< be magnified,
>>> anyway.

>>
>> Higher resolution is the result of larger diameters. Greater light
>> gathering ability of that larger diameter is just a welcome benefit for
>> naked-eye visual observations by humans. Resolution does not depend on the
>> amount of light collected. If you put a very strong neutral density filter
>> in front of a large 10m dia. telescope, to effectively limit the amount of
>> light entering to no more than 1 photon per second, and used a
>> high-resolution detector array to measure those photons, in time you will
>> still have higher resolution image than could be obtained from a 1m dia.
>> telescope with no filter at all.


Actually no - you wouldn't. The atmosphere in all but the very best
sites limits the effective resolution for time exposures to around 0.5"
arc unless adaptive optics are used to correct the wavefront errors.

>> The Hubble Telescope was trained on a patch of sky where nobody thought
>> anything existed, as pure black as the sky can be. Named "The Hubble Deep
>> Field" experiment. From a period of 10 days of collecting photons from that
>> black region they imaged about 1500 whole galaxies in that empty spot of
>> the sky with just as much resolution as they get on bright objects. Image
>> brightness has nothing to do with resolution.
>>
>> http://hubblesite.org/newscenter/arc...eases/1996/01/


The HST is in space so does not suffer from atmospheric seeing and
yields images consistent with theoretical optical performance for the
aperture (at least it did after COSTAR was added to correct its myopia).
>
>
> All right, then; please, give me additional information. I'd prefer a
> telescope with the following capabilities:
>
> planets appear fairly large, in the eyepiece(s)


That one is hard to do satisfactorily. You are best off with
magnifications upto 20x per inch of aperture (although 40x is sometimes
possible to take advantage of moments of good seeing).

So for a 10" scope at 200x Jupiter which is between 30-50" arc will
only look like a disk 3-5 degrees in diameter or roughly 6-11x bigger
than the moon appears with the naked eye. In an eyepiece this still will
look "small" compared to the 40 degree apparent field of view.
>
> resolves Saturn's rings distinctly


Almost anything at 4" or above will do this, but not at the moment the
rings are almost edge on. See for example:

http://www.curtrenz.com/1024q.html
>
> has electronic "GO TO" functions
>
> What's the absolute cheapest that I could obtain such an instrument
> (new, used or refurbished)?


A secondhand ETX 125 unwanted Xmas bought in February after the novelty
has worn off. Watch eBay or even you local paper for adverts. The LT
series gets you an extra inch or 3 if you will stretch a bit further.

A good 4" refractor would be a bit more - physically larger. And given
where you are posting you probably want to buy a camera adapter to try
your hand at astrophotography and/or wildlife.

If you want something that will work as a long telephoto and can live
with the donut bokeh on highlights and slight softness then the Russian
MTO 1000mm f10 mirror lens is also worth considering. Some specimens are
better than others - mine is from the late 1980's. It will just about
resolve saturns rings. It makes a compact spotting scope too.

There are a few telescope FAQs on the web you could check out too. David
Kniseleys (sp?) is as good as any. The vendors are all trying to sell
you stuff. GPS on a scope is overkill unless you intend to observe from
the back of a moving flatbed truck.
>
> Thanks, for any advice!


NB some (perhaps many) local astro clubs have unused kit (not with goto)
lying around with a shortage of keen observers to use it. If you find
your local astro club they may have loan kit available for a nominal sum
that you can try out and or look through their scopes.

If buying new hold out until they offer the free set of eyepieces deal
and if buying second hand take someone along from your local astroclub
to look the optics over for evidence of using a panscrub to clean them.

A bit of dust and grime hardly afects the image at all, but a bunch of
parallel scractches will do a lot of damage.

Regards,
Martin Brown

PS All my scopes have been pre-owned.
 
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