In article
<26d06864-fa4c-44d1-8238-43c27620391e@[EMAIL PROTECTED]
>,
Crown-Horned Snorkack <chornedsnorkack@[EMAIL PROTECTED]
> wrote:
>How many stars can be seen by telescope?
>
>Eye has standard angular resolution of about 1 minute (or larger) and
>can see magnitude 6.
>
>There are estimated to be something of 5000 to 6000 stars up to
>magnitude 6 - over the whole heavenly sphere.
>
>The whole sphere should be something like 40 000 square degrees, or
>150 millions of square minutes. So, out of the 150 million pixels of
>whole sky, 6000 are point sources of light, and the rest, 25 000 times
>more, is empty dark sky.
>
>What is the now resolution of telescopes? 0,1 seconds? 0,01 seconds?
Ground-based telescopes, in a good site, without adaptive optics or
lucky-imaging, get about 0.8 seconds. With lucky imaging you get 0.2
seconds over a field of view of maybe a few square arc-minutes; with
adaptive optics you get 0.05 seconds over a field of view of maybe a
few square seconds.
Hubble gets about 0.05 seconds over a field of view three arc-minutes
on a side.
>And what is the magnitude telescopes can see? +21? +25? +30?
Hubble very long exposures can get to +30. People with very large
amateur telescopes, good drives and CCDs can got to +21 in a
three-hour exposure; below about +25 the fluctuations in the air-glow
start drowning out the stars when working from Earth.
>How many stars (point sources of light which can be seen by telescope,
>resolved from others and believed to be a star or a close multiple)
>can be observed in sky?
I suspect every unobscured star in the Galaxy is in principle
resolvable - lucky-imaging from Palomar gets well below the confusion
limit even in the core of globular clusters. The question is how many
resolvable extra-galactic stars there are; Hubble's resolution is 1
light-year at M31, which is less than the average inter-stellar
distance in the solar neighbourhood, but you'll only be able to get
down to the absolute magnitude of the Sun (24 magnitudes of
distance-modulus between 10pc and M31), so cutting off the whole
bottom of the main sequence will reduce the number of stars
enormously.
http://seds.org/messier/more/m031_g1hst.html
is a Hubble image of a
globular cluster in M31, and I'd be pleased to get something that
sharp of M13 in our own Galaxy from a dark-sky site with the $1000
telescope I've borrowed.
Tom
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