Sky & Telescope - June 2019 - 40

Observing Report

0.7

Fully
separated

0.6

Ephe

0.5

Touching
meris

Summary

Elongated
Notched

0.4

Round, not
yet observed

0.3
0.2
2016

2016.5

2017

2017.5

2018

2018.5

2019

2019.5

Date
p OBSERVATIONS OF STF 2173 The author has been
observing STF 2173 since September 2016, when the two
components were fully separated. He has been following their
ever-decreasing separation and projected the March 2019
result to be of a fully round image.

Separation (arcseconds)

2.5

The double stars that showed Dawes' limit resolutions
signiﬁcantly less than 4.56″/a were HWE 28 and 52 Orionis
with the 3½-inch and 52 Arietis and STF 2173 (March 2018
observation) with the 7-inch telescope.
I couldn't discern elongation or other image irregularities
with smaller separations than those of STT 410 and BU 311.
Their separation values were at the lower limits of distinguishability for the 3½-inch and 7-inch telescopes, respectively.
Neither value approached the r/2 established by Sidgwick.
Not only did the larger telescope halve the Dawes resolution of the smaller telescope (0.5″ vs. 1.0″), but this effect
was replicated when investigating the strictly subjective lower
distinguishable limits, the Sparrow limit (0.43″ vs. 0.87″).
My personal Dawes' limit (rp) for both telescopes is
rp =

1.5

My Dawes
3½-inch

My Sparrow
3½-inch

0.5

Lim

My Dawes
7-inch

it
My Sparrow
7-inch

0
0

100

150

200

250

300

350

Diameter of telescope (millimeters)
p PERSONAL DAWES' LIMITS The graph above summarizes the author's findings. The open square is the average of
the separations of HWE 28 and 52 Ori, while the open circle
is the average of 52 Ari and STF 2173. The solid square and
solid circle represent the Sparrow limit for the two telescopes,
derived from STT 410 and BU 311.

Separations for this study were, by necessity, obtained
piecemeal from various double stars at various positions in
their orbits. But in 2016, the companion of STF 2173 was
positioned in its 46.4-year, grade-1 orbit (https://is.gd/usno_
orb6) to offer a seamless investigation from fully separated
stars to Dawes' limit to unseparable in a three-year period.
I observed STF 2173 in September 2016 as two stars with
visible dark sky (0.63″) between the components. A year later,
the yellow stars were just touching with an interpolated gap
of 0.54″. The pair reached a separation of 0.50″ in March 2018
and, when observed one frosty morning before dawn, displayed
a notched, gold inﬁnity symbol with the smaller lobe pointing
towards the vicinity of 140°. In early October 2018, I could still
distinguish an elongated image with the components separated
by about 0.44″. Soon, the fast-moving stars will merge beyond
the distinguishable limit of my 7-inch telescope, then pass into
the daylight sky. At the time of writing, the next available viewing of the round image of STF 2173 will be in March 2019.
I called this endeavor "The Lazy Astronomer's One-Star
Dawes Limit and Beyond Determination."

J U N E 2 019 * SK Y & TELESCOPE

3.5″
.
a

When a is given in millimeters, the constant is 89″. My
value of Dawes' constant is considerably (23%) smaller than
the conventional 4.56″. The signiﬁcance is debatable.
Next August, I will celebrate my eighty-fourth birthday.
Perhaps it's time to note at what point stars are nicely separated and enjoy the images and not worry about egg shapes
or fused nuclei or other subtleties. There are few sights
lovelier than a double star that looks like matched diamonds
suitable for a Bond ﬁlm. The star STF 3050 (Σ3050) in
Andromeda comes to mind.

¢ DON FERGUSON is a retired chemist whose interests include planetary nebulae, the history of astronomy, Enceladus,
and - his first love - double stars. S&T published his article
on planetary nebulae in April 1995. You can email Don at
dferg28571@aol.com.

DON FERGUSON (2); G R A PH PA PER: Z A K0 0 /
DIGITA LVISION V ECTORS / G E T T Y IM AG ES

Separation (arcseconds)

Not only did the larger
telescope halve the Dawes
resolution of the smaller
telescope, but this effect was
replicated when investigating
the strictly subjective lower
distinguishable limits, the
Sparrow limit.

https://www.is.gd/usno_orb6 https://www.is.gd/usno_orb6

Sky & Telescope - June 2019

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