Approaching the Dawes Limit!

Friday night proved to be somewhat of a new "revelation" for me on the
observation of close visual binaries. The sky was generally quite hazy
and disappointing for just about any kind of serious observing, so at
first I was a bit reluctant to take my scope out.
But I knew that looking at colours of brighter stars and close
binaries would be just about o.k.

When I started to examine the white pair of stars making up Epsilon 2
Lyrae (one of the naked eye stars of the famous 'Double Double') at
300x on my Skywatcher 8-inch, I was quick to spot that they were split
apart with an unusually large amount of room in between... I found it
hard to believe such a view! These two stars are just 2.3" apart in
angular separation and previously always appeared as two blobs of
light in my eyepiece with a hair line thin bit of blackness separating
them.
On this night however, as the summer air was exceptionally hot and
still, the two stars looked *miles* apart! Each component of Epsilon 2
Lyrae had several diffraction rings around it with a circular dot in
the core (the Airy disk) and the cores of the two stars were separated
by a distance that could easily accommodate at least another two
"cores" of the same size. On this basis, I made an estimate that my
8-inch newtonian could have split the two stars had they been as close
as 1.0" or even as close as 0.8" in separation. That would be
*dangerously* close to and approaching my theoretical Dawes Limit of
0.57"... I then turned my scope to the star Mu Draconis, another
binary nearby in the sky and also of approx. 2.3" separation. Again,
the same *miles* apart resolution of the two near-equal brightness
stars.

I then recalled someone telling me back in the Spring that they can
split the star Gamma Virginis when it was just 0.8" apart (this binary
pair is now very close to periastron in its orbit) using an 8-inch. At
that point I thought "Hmmmm.... that's probably because you have a
super-collimated, catadioptric, super-Maksutov class of apochromatic
beast with superior optics costing thousands... way above my league."

How wrong I was. It seems the atmosphere is a BIG decider when it
comes to binary separations and not necessarily always the aperture or
the magnification or the cost or quality of the telescope.

What a "revelation"...

Question: Has anyone ever managed to match Dawes Limit with their
instrument?! Or is that a pie in the sky goal never ever to be
attained?

Abdul Ahad
http://uk.geocities.com/aa_spaceagent/astronomy.html

Alcoholic Astronomers Institute??

AA Institute posted:

Question: Has anyone ever managed to match Dawes Limit with their
instrument?! Or is that a pie in the sky goal never ever to be
attained?

Yes, there are a number of nearly equal doubles which I have been able to
resolve at around the Dawes limit separation. However, because doubles are
rarely equal or of the correct brightness range (and of course, seeing
variations), the Dawes limit figure isn't always achieved (or even always
applicable). Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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"David Knisely" wrote in message

Yes, there are a number of nearly equal doubles which I have been able to
resolve at around the Dawes limit separation.

Isn't Dawes an empirical limit, a rough description of what we are typically
able/not able to resolve? Seems to be quite a few who claim to have
surpassed its "limits".
Ed T.

"Edward" wrote in message
ink.net...

"David Knisely" wrote in message

Yes, there are a number of nearly equal doubles which I have been able
to
resolve at around the Dawes limit separation.

Isn't Dawes an empirical limit, a rough description of what we are
typically
able/not able to resolve? Seems to be quite a few who claim to have
surpassed its "limits".
Ed T.

You can't declare a separation of two point sources who's Airy disks are
touching. This is the idea of the Dawes Limit.

Edward posted:

Isn't Dawes an empirical limit, a rough description of what we are typically
able/not able to resolve? Seems to be quite a few who claim to have
surpassed its "limits".

Yes, but the duplicity of close doubles can be detected at separations which
are a little less than that listed by Dawes. In fact, an elongation of the
star image of a close double star is observable at a separation which is
noticably less than Dawes. I prefer to use the Sparrow criteria which has the
brightness level constant along a line running from the center of one
diffraction disk to the center of the adjacent one (in English units, the
Sparrow limit is about r = 4.47/D, where D is the aperture in inches and r is
the separation in arc seconds). At that separation, the "notching" is still
visible, but perhaps not quite as prominent as at the Dawes level (4.56/D).
Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Stephen Paul posted:

You can't declare a separation of two point sources who's Airy disks are
touching. This is the idea of the Dawes Limit.

Actually, in a way, you can, as the locations of the centers of each of the
diffraction disks are not overlapping but are separated. You can clearly see
that there are two diffraction disks present so the stars are essentually
"resolved", but you cannot declare the double star as "split", as the Airy
disks are touching or overlapping depending on the angular separation and the
aperture being used. All you can say is that the two point sources are
separated, but their Airy disks are merely resolved and not separated. In the
case of star Airy disks significantly overlapping to the extent that there is
no "notching" at the sides but mere elongation, a separation figure can be
very difficult to arrive at, and the stars are "elongated" but not resolved.
This is where I like the Sparrow limit, as, like the Rayleigh limit, it has
some basis in the optics involved and not on a set of observations in a
limited range of apertures and angular separations. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

David Knisely wrote in message ...
Edward posted:

Isn't Dawes an empirical limit, a rough description of what we are typically
able/not able to resolve? Seems to be quite a few who claim to have
surpassed its "limits".

Yes, but the duplicity of close doubles can be detected at separations which
are a little less than that listed by Dawes. In fact, an elongation of the
star image of a close double star is observable at a separation which is
noticably less than Dawes. I prefer to use the Sparrow criteria which has the
brightness level constant along a line running from the center of one
diffraction disk to the center of the adjacent one (in English units, the
Sparrow limit is about r = 4.47/D, where D is the aperture in inches and r is
the separation in arc seconds). At that separation, the "notching" is still
visible, but perhaps not quite as prominent as at the Dawes level (4.56/D).
Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

An obstructed scope may offer an advantage in splitting close doubles,
especially those of nearly equal magnitudes. The central obstruction
redistributes the power from the central dot of the Airy disc to the
first ring with respect to the unobstructed scope. This can give the
obstructed scope an advantage when viewing point sources. Of course
there is a corresponding disadvantage when observing extended targets.

The Physics of the obstructed scope is treated in texts like Suiter
and Rutten & van Venrooij.

Clear skies,

Dennis Persyk
Igloo Observatory Home Page http://dpersyk.home.att.net
Hampshire, IL

New Images http://home.att.net/~dpersyk/new.htm

In article ,
Dennis Persyk wrote:

An obstructed scope may offer an advantage in splitting close doubles,
especially those of nearly equal magnitudes. The central obstruction
redistributes the power from the central dot of the Airy disc to the
first ring with respect to the unobstructed scope. This can give the
obstructed scope an advantage when viewing point sources. Of course
there is a corresponding disadvantage when observing extended targets.


Would the same apply if you placed an obstruction - say a dark disc - in
front of a refractor?

Jochen

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Dennis Persyk posted:

An obstructed scope may offer an advantage in splitting close doubles,
especially those of nearly equal magnitudes. The central obstruction
redistributes the power from the central dot of the Airy disc to the
first ring with respect to the unobstructed scope. This can give the
obstructed scope an advantage when viewing point sources. Of course
there is a corresponding disadvantage when observing extended targets.

The Physics of the obstructed scope is treated in texts like Suiter
and Rutten & van Venrooij.

Well, this might be a bit of an overstatement of any "advantage". While the
diffraction caused by the secondary obstruction does cause a reduction in the
diameter of the apparent "spurious" or Airy disk of a star, the actual amount
of reduction for common central obstruction sizes is slight, and would, of
course, not help with detail in extended objects. It may slightly improve the
ability of the telescope to resolve close double stars but only when the
obstruction reaches a somewhat large size. Indeed, the diffraction disk of a
telescope with a 20 percent central obstruction is only about four percent
smaller than that of an unobstructed instrument. Even a 33 percent central
obstruction would only yield a 10 percent reduction in the Airy disk size, so
for common central obstruction sizes, the "improvement" in effective
resolution would be small at best. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************