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Viewing by eye versus astrophotography



 
 
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  #1  
Old October 4th 08, 03:49 PM posted to sci.astro.amateur
Peter Webb[_2_]
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Posts: 927
Default Viewing by eye versus astrophotography

I know this question is almost meaningless ... but perhaps not completely.

When it comes to "seeing" detail, what improvement can you get by
photography over the human eye? The human eye is presumably better at
optical separation, but CCDs for minimum magnitude.

For example, I have heard that you can start seeing the spiral structure of
galaxies at about a 8 - 10" telescope aperture; if you are doing
astrophotography can this be seen with a lower aperture scope?


  #2  
Old October 4th 08, 05:58 PM posted to sci.astro.amateur
Chris L Peterson
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Posts: 10,007
Default Viewing by eye versus astrophotography

On Sun, 5 Oct 2008 00:49:22 +1000, "Peter Webb"
wrote:

I know this question is almost meaningless ... but perhaps not completely.

When it comes to "seeing" detail, what improvement can you get by
photography over the human eye? The human eye is presumably better at
optical separation, but CCDs for minimum magnitude.

For example, I have heard that you can start seeing the spiral structure of
galaxies at about a 8 - 10" telescope aperture; if you are doing
astrophotography can this be seen with a lower aperture scope?


When it comes to deep sky astronomical objects, the eye is vastly
inferior to modern electronic imagers, both in terms of resolution and
sensitivity. The main factor is sensitivity. While electronic detectors
have better QE than the eye (that is, they record a higher percentage of
photons), the main improvement comes from their ability to integrate
over long periods. The eye only "sees" a photon for about 100ms, so
outside that period there's no additive effect. A CCD or similar sensor
can accumulate photons for any length of time. This makes them, in
effect, many orders of magnitude more sensitive that the eye.

Also, the resolution of the eye is poor for dim objects. In fact, it is
very poor- even bad telescope optics are unlikely to impact the quality
of DSO viewing. In contrast, an electronic detector can be set up with
an objective of the proper focal length in order to achieve resolution
limited by the optics or the seeing, whichever is finer. This is only
possible visually with very bright objects (the Sun, Moon, and bright
planets), where high magnification can be used without losing too much
light.

Electronic detectors also achieve higher resolution than the eye or film
because of their independent pixels. This results in a nearly flat MTF.
While the eye and film have a resolution that varies with contrast,
electronic detectors do not, until the Nyquist sampling limit is
approached.

Electronic detectors have much greater dynamic range than either film or
the scotopic eye.

Of course, electronic detectors are capable of recording color in DSOs,
which is largely impossible visually.

One place where there can be an advantage to visual observation is with
the Moon and planets. That's because the short sampling time of the
eye/brain - a disadvantage for DSOs- works in our favor to help freeze
seeing effects. However, this advantage is increasingly overcome by the
use of lucky imaging techniques- stacking fast video frames based on
individual frame quality. This can produce equally good resolution to
the eye, with a substantial improvement in color and dynamic range.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #3  
Old October 5th 08, 04:20 AM posted to sci.astro.amateur
David Nakamoto
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Posts: 183
Default Viewing by eye versus astrophotography

Chris L Peterson wrote:
On Sun, 5 Oct 2008 00:49:22 +1000, "Peter Webb"
wrote:


I know this question is almost meaningless ... but perhaps not completely.

When it comes to "seeing" detail, what improvement can you get by
photography over the human eye? The human eye is presumably better at
optical separation, but CCDs for minimum magnitude.

For example, I have heard that you can start seeing the spiral structure of
galaxies at about a 8 - 10" telescope aperture; if you are doing
astrophotography can this be seen with a lower aperture scope?


When it comes to deep sky astronomical objects, the eye is vastly
inferior to modern electronic imagers, both in terms of resolution and
sensitivity. The main factor is sensitivity. While electronic detectors
have better QE than the eye (that is, they record a higher percentage of
photons), the main improvement comes from their ability to integrate
over long periods. The eye only "sees" a photon for about 100ms, so
outside that period there's no additive effect. A CCD or similar sensor
can accumulate photons for any length of time. This makes them, in
effect, many orders of magnitude more sensitive that the eye.

Also, the resolution of the eye is poor for dim objects. In fact, it is
very poor- even bad telescope optics are unlikely to impact the quality
of DSO viewing. In contrast, an electronic detector can be set up with
an objective of the proper focal length in order to achieve resolution
limited by the optics or the seeing, whichever is finer. This is only
possible visually with very bright objects (the Sun, Moon, and bright
planets), where high magnification can be used without losing too much
light.

Electronic detectors also achieve higher resolution than the eye or film
because of their independent pixels. This results in a nearly flat MTF.
While the eye and film have a resolution that varies with contrast,
electronic detectors do not, until the Nyquist sampling limit is
approached.

Electronic detectors have much greater dynamic range than either film or
the scotopic eye.

Of course, electronic detectors are capable of recording color in DSOs,
which is largely impossible visually.

One place where there can be an advantage to visual observation is with
the Moon and planets. That's because the short sampling time of the
eye/brain - a disadvantage for DSOs- works in our favor to help freeze
seeing effects. However, this advantage is increasingly overcome by the
use of lucky imaging techniques- stacking fast video frames based on
individual frame quality. This can produce equally good resolution to
the eye, with a substantial improvement in color and dynamic range.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


On this last point, about planets and the Moon, I'll concede that with
the Moon you'll see more with the unaided eye through most telescopes
than with imaging, but my own experience is that you see more with
stacking images from a video taken with good optics and good seeing than
with the eye through the same telescope, perhaps twice the resolution,
although I admit I've not measured it objectively yet. Basically, the
web or video camera with medium to high resolution and selectively
stacking images seem to push the resolution of what you can see closer
to the theoretical limits of the optics. It at least pushes them a bit
further than what the seeing might allow through the telescope with the
unaided eye.

Just my humble opinion.

--- Dave
  #4  
Old October 5th 08, 05:01 AM posted to sci.astro.amateur
Chris L Peterson
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Posts: 10,007
Default Viewing by eye versus astrophotography

On Sun, 05 Oct 2008 03:20:19 GMT, David Nakamoto
wrote:

On this last point, about planets and the Moon, I'll concede that with
the Moon you'll see more with the unaided eye through most telescopes
than with imaging, but my own experience is that you see more with
stacking images from a video taken with good optics and good seeing than
with the eye through the same telescope, perhaps twice the resolution,
although I admit I've not measured it objectively yet.


I agree. There might be some advantage to visual planetary astronomy
with poor seeing, or certain kinds of seeing. But even that is giving
way to video imaging, I think.

My point, really, was that planetary and lunar viewing is probably the
only case where visual observation and imaging are at least in the same
ballpark so far as comparisons go. With everything else, imaging is
unarguably far beyond visual, even where the imaging aperture is much
smaller than the visual.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #5  
Old October 5th 08, 07:25 AM posted to sci.astro.amateur
David Nakamoto
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Posts: 183
Default Viewing by eye versus astrophotography

Chris L Peterson wrote:
On Sun, 05 Oct 2008 03:20:19 GMT, David Nakamoto
wrote:


On this last point, about planets and the Moon, I'll concede that with
the Moon you'll see more with the unaided eye through most telescopes
than with imaging, but my own experience is that you see more with
stacking images from a video taken with good optics and good seeing than
with the eye through the same telescope, perhaps twice the resolution,
although I admit I've not measured it objectively yet.


I agree. There might be some advantage to visual planetary astronomy
with poor seeing, or certain kinds of seeing. But even that is giving
way to video imaging, I think.

My point, really, was that planetary and lunar viewing is probably the
only case where visual observation and imaging are at least in the same
ballpark so far as comparisons go. With everything else, imaging is
unarguably far beyond visual, even where the imaging aperture is much
smaller than the visual.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Total agreement. Through a 5 inch SCT, Omega Centauri was a glow with
some stars visible. 30 seconds later with a monochromatic imager, it
"resolved" into hundreds of stars. First light for that camera too.

--- Dave
  #6  
Old October 5th 08, 03:17 PM posted to sci.astro.amateur
William Hamblen
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Posts: 343
Default Viewing by eye versus astrophotography

On Sun, 5 Oct 2008 00:49:22 +1000, "Peter Webb"
wrote:

For example, I have heard that you can start seeing the spiral structure of
galaxies at about a 8 - 10" telescope aperture; if you are doing
astrophotography can this be seen with a lower aperture scope?


You can photograph this with a telephoto lens.

Bud
  #7  
Old October 5th 08, 04:31 PM posted to sci.astro.amateur
Chris L Peterson
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Posts: 10,007
Default Viewing by eye versus astrophotography

On Sun, 05 Oct 2008 09:17:01 -0500, William Hamblen
wrote:

On Sun, 5 Oct 2008 00:49:22 +1000, "Peter Webb"
wrote:

For example, I have heard that you can start seeing the spiral structure of
galaxies at about a 8 - 10" telescope aperture; if you are doing
astrophotography can this be seen with a lower aperture scope?


You can photograph this with a telephoto lens.


I imaged M51 once using a pinhole! Not beautiful, but somewhat more
structure, and far more contrast, than I've ever seen visually through a
telescope.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #8  
Old October 5th 08, 04:42 PM posted to sci.astro.amateur
Paul Schlyter[_2_]
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Posts: 893
Default Viewing by eye versus astrophotography

In article ,
Peter Webb wrote:
I know this question is almost meaningless ... but perhaps not completely.

When it comes to "seeing" detail, what improvement can you get by
photography over the human eye? The human eye is presumably better at
optical separation, but CCDs for minimum magnitude.


The major advantage with photography is reproducibility. In a few
seconds (or a few hours, depending on what exposure time you use)
you get an image which later can be closely examined any number of
times by anyone. Visual observations o.t.o.h. can only be performed
"here-and-now" by the observer himself, one cannot later re-create the
same visual observation. True, you can draw, or describe, what you
see - but that's always a matter of interpretation which never can be
repeated later by someone else on the same visual observation.
Perhaps the best way to describe it is that visual observations are
"volatile".

The major advantage of visual observations is pleasu it's always
more pleasant to see something live than to see a photograph or video
of it.

And that's why visual observations are so popular among amateur astronomers
(who observe for the joy of it) while at the same time it's virtually
never used by professionals (who are demanded to produce reproducible
scientific results).


For example, I have heard that you can start seeing the spiral structure of
galaxies at about a 8 - 10" telescope aperture; if you are doing
astrophotography can this be seen with a lower aperture scope?




--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stjarnhimlen dot se
WWW: http://stjarnhimlen.se/
  #10  
Old October 5th 08, 07:26 PM posted to sci.astro.amateur
Thomas Womack
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Posts: 206
Default Viewing by eye versus astrophotography

In article ,
Chris L Peterson wrote:
On Sun, 05 Oct 2008 15:42:46 GMT, (Paul Schlyter) wrote:

The major advantage with photography is reproducibility.


I would call that _a_ major advantage, not _the_ major advantage.
Advantages depend on intent, and reproducibility may not necessarily be
a goal.

The major advantage of visual observations is pleasu it's always
more pleasant to see something live than to see a photograph or video
of it.


Again, this depends on intent and personal taste. I've never seen any
object directly through a telescope (with the possible exception of
Saturn) that gave me anywhere near the satisfaction of seeing an image
appear on my screen, as the result of my own imaging effort. Without
imaging, I might not bother to own a telescope at all.


I agree that, for planets, galaxies and nebulae, imaging is
unbeatable.

I've not seen images that capture the contrasty nature of a star
cluster seen through a large scope, basically because no screen
technology that I know of can do the transition between space-black
and star-white in the space of no more than the eye can see. Some
planetarium shows come close.

I have seen Omega Centauri through a 36" Dobsonian (yes, I know this
is something of a best case for visual observation), and that is much
more spectacular than any picture of a globular that I've encountered,
even from Hubble or VLT-adaptive-optics.

There's something of the same for the Moon, the blackness of the
shadows of the mountains versus the light bright enough to make you
screw up your eyes a bit (even in my pathetic 4") reflected from their
peaks.

On the other hand, I've also felt a reasonable sense of achievement
from taking some technically awful wide-field photos with a 70/2.8
lens and a DSLR and using them to rediscover Ceres. Not sure it's a
delight I can convey it words, but it did feel good to see that moving
dot ...

Tom

 




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