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