Highest magnifications

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.

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.

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.

I'm still impressed by the idea of seeing something at x80 !

I understand that the HST saw a red giant (betelguese?) as a tiny
disk.

btw, i hadn't appreciated how big in the sky andromedia is - i guess
it's because we generally only see the smaller central part.

"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.
If (for instance), you had a 300mm scope, with a 60m focal length, and
designed an eyepiece with only a 0.2mm focal length, you could potentially
get 300000* magnification, but looking at a star, the Airy disk (which is
just under 1 arc second in diameter for this scope in the middle of the
visible wavelengths), would appear about 80 degrees across!.

I'm still impressed by the idea of seeing something at x80 !

I understand that the HST saw a red giant (betelguese?) as a tiny
disk.

btw, i hadn't appreciated how big in the sky andromedia is - i guess
it's because we generally only see the smaller central part.

Best Wishes

(gswork) wrote in message . com...

What is the highest magnification realistically available to amateurs
and the best amatuer scope in realistic seeing.

Hard to say; it depends partly on aperture, partly on optical quality,
but mostly on seeing and on how high *you* want to push the magnification --
or to put it another way, how fuzzy an image you are willing to deal with.

I don't think I've ever gone above 400X with my 12.5-inch scope,
but the seeing here in New England is not especially good. I've
heard of amateurs in Florida, Arizona, or California using well over
1000X on big Dobs. I've recently been reading reports of amateurs
getting spells of incredible seeing while using pro equipment, like
Thomas Cave viewing Mars at 3000X during the 1956 apparition on
a night of perfect seeing at the eyepiece of the 100-inch Mt. Wilson
reflector (http://www.mars.dti.ne.jp/~cmo/198/cave.html).

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.

Imaging star disks is getting to be almost routine with adaptative
optics on the big new telescopes.

btw, i hadn't appreciated how big in the sky andromedia is - i guess
it's because we generally only see the smaller central part.

Probably because you haven't seen it under a dark sky. Yes, it spills
way out of the field of view of most eyepieces in most telescopes.

- Tony Flanders

"gswork" came up with the following after drinking 15 cups of coffee and
then turning his brain loose:


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.

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!


Been reading quite a few of Brian Tung's posts, eh? ;o)


--
Martin
Post your pictures at "Photographs From the Universe of Amateur Astronomy"
Go to: http://home.earthlink.net/~martinhowell for
* Daily space/astro news story updates
* Your astro photos
* One shot solutions
* Unguided astro photography
* Share your observing setup with the world
* Other stuff

gswork wrote:
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!

Your calculations are more or less correct, but they don't take into
account the diffraction of light. This makes just about every star a
disc of noticeable size in most telescopes at reasonably high powers
(usually about 30x per inch of aperture or so, although it varies by
observer). This disc has just about nothing to do with the actual
disc of the star, which is ordinarily at least an order of magnitude
smaller.

The star with the largest known apparent diameter as seen from the Earth
(minus the diffraction effects) is R Doradus, with an apparent diameter
of about 57 milliarcseconds (mas). Betelgeuse, the previous record
holder, has a diameter of about 44 mas--a diameter that was measured as
long ago as the 1920s, by Michelson. (Yes, the same Michelson as in the
Michelson-Morley experiment, and who had an obsession with measuring the
speed of light to ever increasing precision.

R Doradus, by the way, appears about the same size as a golf ball seen
from a distance of 150 km.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

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):

You'd need a top of the line Tasco to get that!

(ducks and runs)

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/
And the Lunar Picture of the Day http://www.lpod.org/
************************************

gswork wrote:
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):

Strictly speaking, you'd be magnifying the airy disk to the apparent size of
the Moon. Brian Tung, in his response, brought up a couple of stars with
apparent sizes in the vicinity of 0.05-arcsecond. You'd need a 100-inch
aperture and incomprehensibly perfect seeing to even have a shot at visually
resolving such an object.

Based on the magnification you're applying to Sirius A, it appears you're
assuming an angular size for that star of 0.006-arcsecond. Resolving this star
visually would require a 1,000-inch (25-meter) aperture and...well, you'd need
to be above the atmosphere for seeing that good ;o)

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
Email: Remove "ic" from .comic above to respond

c (Bill Ferris) wrote in message ...
gswork wrote:
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):

Strictly speaking, you'd be magnifying the airy disk to the apparent size of
the Moon. Brian Tung, in his response, brought up a couple of stars with
apparent sizes in the vicinity of 0.05-arcsecond. You'd need a 100-inch
aperture and incomprehensibly perfect seeing to even have a shot at visually
resolving such an object.

Based on the magnification you're applying to Sirius A, it appears you're
assuming an angular size for that star of 0.006-arcsecond. Resolving this star
visually would require a 1,000-inch (25-meter) aperture and...well, you'd need
to be above the atmosphere for seeing that good ;o)

ah well, the 25M reflector on the moon is beyond my budget at the moment!

"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.

(Tony Flanders) wrote in message m...
(gswork) wrote in message . com...

What is the highest magnification realistically available to amateurs
and the best amatuer scope in realistic seeing.

Hard to say; it depends partly on aperture, partly on optical quality,
but mostly on seeing and on how high *you* want to push the magnification --
or to put it another way, how fuzzy an image you are willing to deal with.

I don't think I've ever gone above 400X with my 12.5-inch scope,
but the seeing here in New England is not especially good. I've
heard of amateurs in Florida, Arizona, or California using well over
1000X on big Dobs. I've recently been reading reports of amateurs
getting spells of incredible seeing while using pro equipment, like
Thomas Cave viewing Mars at 3000X during the 1956 apparition on
a night of perfect seeing at the eyepiece of the 100-inch Mt. Wilson
reflector (http://www.mars.dti.ne.jp/~cmo/198/cave.html).

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.

Imaging star disks is getting to be almost routine with adaptative
optics on the big new telescopes.

this is quite exciting stuff, for soon enough there'll start to be
more evidence of other star's sunspots and other things.

btw, i hadn't appreciated how big in the sky andromedia is - i guess
it's because we generally only see the smaller central part.

Probably because you haven't seen it under a dark sky. Yes, it spills
way out of the field of view of most eyepieces in most telescopes.

Indeed i hacen't, but considering the size i'm now very keen on that
idea!

Sounds ideal for binoculars or perhaps a short tube refractor.