Approaching the Dawes Limit!

David Knisely wrote in message ...
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?
snip

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).

I should really draw up a list of close binaries with near-equal
brightness components, unless someone knows of a link that already
provides this?

That way we would always have "a star to test by" when it comes to
resolving powers. Ideally, there ought to be at least 3 or 4
equi-brightness binary pairs on view on any given night throughout the
whole year.

A few obvious examples off hand a-

Star: Mags: Separation:
---------------------------------------------------------------
Gamma Arietis 4.8, 4.8 7.8"
Epsilon Arietis 5.2, 5.5 1.5"
7 Tauri 6.6, 6.7 0.8"
57 Ursae Majoris 5.4, 5.4 5.4"
Zeta Bootis 4.5, 4.6 0.8"
Phi Ursae Majoris 5.3, 5.4 0.3"
Phi-2 Cancri 6.3, 6.3 5.1"
42 Ceti 6.5, 6.8 1.6"
Zeta Aquarii 4.3, 4.5 2.1"
Pi Aquilae 6.1, 6.9 1.4"
20 Draconis 7.1, 7.3 1.3"
36 Andromedae 6.0, 6.4 0.9"
[Source: Norton's star
atlas]
----------------------------------------------------------------

Of course by virtue of the narrow separations, the components of some
of the above display relatively "fast" orbital motions, so one has to
check the ephemerides to be precise on the separations at the given
date of observation.

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

David Knisely wrote in message ...
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).

The following are two resources for approaching the problem of the
double-star splitting limit for stars of unequal magnitudes. The
double-star split limit is higher than Dawes' or Rayleigh's limits.
The Haas and Arguelles _Sky & Telescope_ articles both have tables of
suggested doubles between 1 and 5 arcsecs of separation. Personally, I
keep a copy of Chris Lord's nomogram in my observing kit.

1) Chris Lord algorithm and/or nomogram.

Lord, C., BrayeBrook Observatory. (20__). Telescopic Resolution of
Unequal Binaries.
http://www.brayebrookobservatory.org...LICATIONS.html

Haas, Sissy. Jan. 2002. Enjoying Unequal Double-Stars. Sky &
Telescope. 118-121.

2) LADIC (Luis Arguelles Difficulty Index Calculator) "fuzzy-logic"
algorithm

Arguelles, Luis. 2001. Fuzzy-splitting.
http://www.carbonar.es/s33/Fuzzy-spl...splitting.html (web
page with LADIC and Palm pLADIC download links)

Arguelles, Luis. Jan. 2002. Finding your double star limit. Sky &
Telescope. 63-67.

- Canopus

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.

Somehow those sub-arcsecond doubles always seem tighter than you'd
think immediately after looking at 2" to 3" pairs. And they get
exponentially more difficult to split as you get closer to Dawes, with
each tenth of an arcsecond closer making a big difference in the
magnification needed. 0.8" doubles (equal-brightness of course) are
fairly straightforward through my 8" Newt on a good night, but 0.7" is
another level of difficulty, and I've never managed 0.6".

The good thing is, after your eyes get used to doubles this tight,
splitting 1" to 2" pairs is a walk in the park.

Cheers,
Ritesh

(Axel) wrote in message . com...
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.

Somehow those sub-arcsecond doubles always seem tighter than you'd
think immediately after looking at 2" to 3" pairs. And they get
exponentially more difficult to split as you get closer to Dawes, with
each tenth of an arcsecond closer making a big difference in the
magnification needed. 0.8" doubles (equal-brightness of course) are
fairly straightforward through my 8" Newt on a good night, but 0.7" is
another level of difficulty, and I've never managed 0.6".

So it *is* a bit like approaching the speed of light barrier (the
'Einstein Limit')... and some really weird things could start to
happen when you're 99.999% close to Dawes Limit!

I'll tread with caution on any binary that's below 1" separation and
record the momentous results with interest, as it would be a whole new
ball game for me which I thought was probably beyond my equipment.
Thanks for your inspiration.

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