From: James Nagler on
On Fri, 04 Jun 2010 13:55:52 +0100, Martin Brown
<|||newspam|||@nezumi.demon.co.uk> wrote:

>The rough
>rule of thumb breaks down a bit when the scope is much bigger than 10"
>aperture as seeing starts to be the limiting factor and the star images
>stay roughly constant angular size after that point.

It is suggested that for more than 99% of the time a 16" dia. optic will
not resolve details any better than the 200" dia. telescope at Mt. Palomar
due to atmospheric turbulence ("seeing"), unless that telescope is used for
observing from high altitudes. It helps to put things into perspective for
those who suffer from aperture-fever when it comes to resolving power.

From: J. Clarke on
On 6/4/2010 8:55 AM, Martin Brown wrote:
> On 03/06/2010 09:33, DanP 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.
>
> They are close enough to parallel as makes no difference (apart from our
> sun which subtends about half a degree). The largest stars are a few
> tens of milliarcseconds across as viewed from the Earth. It is right on
> the limits of our best optical telescopes to resolve them. Most stars
> are considerably smaller in angular size.
>
> Betelgeuse is just about resolved by the HST it is 20,000 smaller in
> diameter than the width of the sun as seen from Earth.
>
> http://www.spaceimages.com/atofbet.html
>
> To all intents and purposes at all ordinary apertures available to
> photgraphers and amateur astronomers they are ideal point sources with a
> bit of annoying phase screen in front from the atmosphere.
>>
>>> Both are f/4 at maximum aperture. Does that change your claim?
>>> Or does "the amount of light let in depends only of exposure
>>> time and f number" hold?
>>
>> No, both set at f/4 will let in the same amount of light.
>
> That is clearly an insane position to take. What you say is only sort of
> correct for an image formed by extended objects that are resolved.
>
> The key point about stars is that they are unresolved.
>
> A star remains unresolved even in the best lenses or telescopes and so
> all the light that hits the aperture ends up in the image. The larger
> lens captures more light and gives a smaller image Airey disk. The rough
> rule of thumb breaks down a bit when the scope is much bigger than 10"
> aperture as seeing starts to be the limiting factor and the star images
> stay roughly constant angular size after that point.
>
> Astronomers always want bigger apertures to capture more light.
> http://www.scopecity.net/LightGrasp.php
>
> It doesn't help on extended objects the laws of physics see to that but
> for a point source like a star the bigger aperture always wins.

I think that you need to investigate the assumptions being made with
those limiting magnitude calculations. Comparing telescopes to
photographic lenses is comparing apples to oranges. Photographic lenses
are used with the primary lens projecting directly on the sensor,
astronomical telescopes almost always use secondary lenses.

From: James Nagler on
On Fri, 04 Jun 2010 09:36:19 -0400, "J. Clarke" <jclarke.usenet(a)cox.net>
wrote:

>On 6/4/2010 8:55 AM, Martin Brown wrote:
>> On 03/06/2010 09:33, DanP 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.
>>
>> They are close enough to parallel as makes no difference (apart from our
>> sun which subtends about half a degree). The largest stars are a few
>> tens of milliarcseconds across as viewed from the Earth. It is right on
>> the limits of our best optical telescopes to resolve them. Most stars
>> are considerably smaller in angular size.
>>
>> Betelgeuse is just about resolved by the HST it is 20,000 smaller in
>> diameter than the width of the sun as seen from Earth.
>>
>> http://www.spaceimages.com/atofbet.html
>>
>> To all intents and purposes at all ordinary apertures available to
>> photgraphers and amateur astronomers they are ideal point sources with a
>> bit of annoying phase screen in front from the atmosphere.
>>>
>>>> Both are f/4 at maximum aperture. Does that change your claim?
>>>> Or does "the amount of light let in depends only of exposure
>>>> time and f number" hold?
>>>
>>> No, both set at f/4 will let in the same amount of light.
>>
>> That is clearly an insane position to take. What you say is only sort of
>> correct for an image formed by extended objects that are resolved.
>>
>> The key point about stars is that they are unresolved.
>>
>> A star remains unresolved even in the best lenses or telescopes and so
>> all the light that hits the aperture ends up in the image. The larger
>> lens captures more light and gives a smaller image Airey disk. The rough
>> rule of thumb breaks down a bit when the scope is much bigger than 10"
>> aperture as seeing starts to be the limiting factor and the star images
>> stay roughly constant angular size after that point.
>>
>> Astronomers always want bigger apertures to capture more light.
>> http://www.scopecity.net/LightGrasp.php
>>
>> It doesn't help on extended objects the laws of physics see to that but
>> for a point source like a star the bigger aperture always wins.
>
>I think that you need to investigate the assumptions being made with
>those limiting magnitude calculations. Comparing telescopes to
>photographic lenses is comparing apples to oranges. Photographic lenses
>are used with the primary lens projecting directly on the sensor,
>astronomical telescopes almost always use secondary lenses.

How many elements in how many groups do you think are behind that main
objective lens in an average camera-lens design?

From: Martin Brown on
On 04/06/2010 14:36, J. Clarke wrote:
> On 6/4/2010 8:55 AM, Martin Brown wrote:
>> On 03/06/2010 09:33, DanP 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.
>>
>> They are close enough to parallel as makes no difference (apart from our
>> sun which subtends about half a degree). The largest stars are a few
>> tens of milliarcseconds across as viewed from the Earth. It is right on
>> the limits of our best optical telescopes to resolve them. Most stars
>> are considerably smaller in angular size.
>>
>> Betelgeuse is just about resolved by the HST it is 20,000 smaller in
>> diameter than the width of the sun as seen from Earth.
>>
>> http://www.spaceimages.com/atofbet.html
>>
>> To all intents and purposes at all ordinary apertures available to
>> photgraphers and amateur astronomers they are ideal point sources with a
>> bit of annoying phase screen in front from the atmosphere.
>>>
>>>> Both are f/4 at maximum aperture. Does that change your claim?
>>>> Or does "the amount of light let in depends only of exposure
>>>> time and f number" hold?
>>>
>>> No, both set at f/4 will let in the same amount of light.
>>
>> That is clearly an insane position to take. What you say is only sort of
>> correct for an image formed by extended objects that are resolved.
>>
>> The key point about stars is that they are unresolved.
>>
>> A star remains unresolved even in the best lenses or telescopes and so
>> all the light that hits the aperture ends up in the image. The larger
>> lens captures more light and gives a smaller image Airey disk. The rough
>> rule of thumb breaks down a bit when the scope is much bigger than 10"
>> aperture as seeing starts to be the limiting factor and the star images
>> stay roughly constant angular size after that point.
>>
>> Astronomers always want bigger apertures to capture more light.
>> http://www.scopecity.net/LightGrasp.php
>>
>> It doesn't help on extended objects the laws of physics see to that but
>> for a point source like a star the bigger aperture always wins.
>
> I think that you need to investigate the assumptions being made with
> those limiting magnitude calculations. Comparing telescopes to
> photographic lenses is comparing apples to oranges. Photographic lenses
> are used with the primary lens projecting directly on the sensor,
> astronomical telescopes almost always use secondary lenses.

So are most modern telescopes - CCD cameras are now commonplace in
amateur astronomy. Professional telescopes have almost always been
operated as cameras with film or plates recording images and now CCDs.
The main effect is that because CCDs have a much better quantum
efficiency than the eye and can integrate for longer the photographic
limiting magnitude is about 3 astronomical stellar magnitudes better.

http://adsabs.harvard.edu/abs/1958LowOB...4...83J
(using Kodak 103a-O plates)

On time exposures it is more a matter of how well you can guide.

The basic rules of optics are that at a fixed focal ratio of say f4

A resolved object like a brick wall if you make the lens 2x bigger the
focal length is also doubled and although it collects 4x the light
because it forms an image that is 2x bigger in linear scale so the
brightness is constant. 4x as much light spread over 4x the area.

An unresolved object is a point so that if you make the lens 2x bigger
it collects 4x the light and dumps it all on the same sized spot. It is
as a result of this that a larger aperture lens or telescope can always
see fainter stars. The only assumption is that the optics are operating
at close to their diffraction limit without excessive aberrations.

This condition is true for good quality telescopes and usually true for
most decent camera lenses when stopped down to f5.6 or slower.

Fast camera lenses used wide open tend to render off axis stars less
than perfectly but should be reasonably good on axis.

Regards,
Martin Brown
From: DanP on
On 4 June, 13:55, Martin Brown <|||newspam...(a)nezumi.demon.co.uk>
wrote:
> On 03/06/2010 09:33, DanP wrote:
>
> > On 2 June, 16:44, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com>

> >> Both are f/4 at maximum aperture.  Does that change your claim?
> >> Or does "the amount of light let in depends only of exposure
> >> time and f number" hold?
>
> > No, both set at f/4 will let in the same amount of light.
>
> That is clearly an insane position to take. What you say is only sort of
> correct for an image formed by extended objects that are resolved.

Well I am not the one to claiming a camera lens .0001mm diameter can
be set to f/4.

Start
Quote--------------------------------------------------------------------------------------------
>> So, talk about lens diameter. Will a lens diameter of 10.000km
>> result in a darker star image than one of 0.0001mm?
> At maximum aperture the 10.000 km will give a brighter picture.
> But set at the maximum aperurte of the .00001mm the results will be
> the same.

Both are f/4 at maximum aperture. Does that change your claim?
Or does "the amount of light let in depends only of exposure
time and f number" hold?
End
Quote--------------------------------------------------------------------------------------------

So, if I shoot stars with 2 different diameter lenses, both set at
same f number, focal length, exposure time and ISO, will the results
be the same?
For the sake of the argument say they resolve the same.

> The key point about stars is that they are unresolved.
>
> A star remains unresolved even in the best lenses or telescopes and so
> all the light that hits the aperture ends up in the image. The larger
> lens captures more light and gives a smaller image Airey disk. The rough
> rule of thumb breaks down a bit when the scope is much bigger than 10"
> aperture as seeing starts to be the limiting factor and the star images
> stay roughly constant angular size after that point.

Thanks, I thought I was actually seeing the size of the stars at pixel
level.

Just curious, would that change at a lower aperture (higher f number)?
I think it would although I see no advantage in it.
It helps to see brighter stars spread over more pixels.

> Astronomers always want bigger apertures to capture more light.http://www..scopecity.net/LightGrasp.php
>
> It doesn't help on extended objects the laws of physics see to that but
> for a point source like a star the bigger aperture always wins.

On telescopes aperture is lens/mirror diameter.
On cameras no one cares about lens diameters and aperture refers to f
stop.

I agree with you on both cases.


DanP