"Roger Hamlett" wrote in message ...
"gswork" wrote in message
om...
Using a bit of mathematics i have been looking into degrees,
arcseconds and so on.

Having verified the moon to be .5 degrees, and the sun to be about the
same - and this agreeing with books - i was happy that i was doing the
right thing.

When getting say x2 magnification does that mean we see the moon as if
it was 1 whole degree? it ought to follow.
Yes.

I used the same calculations on nearby stars to fully appreciate why
we just can't see them as disks through telescopes, i roughly
estimated that seeing Sirius A as a disc roughly the size of the moon
would require x300,000 magnification (but i may have that wrong):

360 / 2Pi =57.3 (used later)

Moon degrees = 57.3 * arctan(diameter/distance) (thats roughly
3476/384000)
= just over 0.5 degrees

Sirius A degrees = 57.3 * arctan(1390000km/8.6ly)
(ly=9,460,530,000,000km)
= 0.000001645 degrees
multiply that by 300,000 to get just under .5 degrees!

but please do correct any errors, it's a first bash at this.
It's about right. Quicker just to think of a AU, as about 499 light seconds,
and Sirius as about 8.6 light years away, and 1.7* the size of the Sun.

What is the highest magnification realistically available to amateurs
and the best amatuer scope in realistic seeing. A really huge mirror
in space, or an even bigger one built on the moon, would perhaps get
to the point of being able to see nearby stars as discs.
The problem is that you can magnify as much as you want, but the information
actually 'available', s limited by the size of the Airy disk, even before
the atmosphere gets involved. You could build a telescope to give 300000*
magnification quite easily (I'm suprised that some of the 'supermarket'
scopes have not advertised this!...), but all you see is the Airy disk
magnified to a huge size. Since the size of the Airy disk is directly
related to aperture, this provides the ultimate 'limit' on being able to
tell that something isn't a 'point', but has size.
Get yourself a copy of 'aberrator'. This is a superb program, that allows
you to see how the resolution of a scope changes with aperture, and various
'faults'. Though it is limited (it won't let you define a 20m telescope for
instance), you will get a good idea of how the discernable 'point' changes
with aperture and magnification.

Thanks for the tip, i hadn not heard of aberrator, i can use that to
get sensible predictions for smaller scopes.