Sky & Telescope - June 2019 - 39

It is a point of considerable interest to determine
the separating power of any given telescopic aperture.
Having ascertained about Yve and thirty years
ago, by comparisons of the performance of several
telescopes of very different apertures . . . I examined
with a great variety of apertures a vast number of
double stars . . . in order to ascertain the separating
power of those apertures, as expressed in inches of
aperture and seconds of distance.

Putting My Telescopes to the Test -
the 7-inch
The 7-inch telescope showed STF 2244 (Σ2244) with dark
sky between its components instead of a fused dumbbell of
merged stars. Similarly, STF 2173 (Σ2173; September 2016
observation) and BU 395 showed darkness in their interstices. These three stars mirrored the sub-Dawes-limit resolution trend established by the 3½-inch telescope.
However, in September 2017 I decided there was no dark
sky between the stars of STF 2173. Instead, they appeared to
be just touching. I may have witnessed the two star images
merging during the six-week observation period, as initially I
imagined that I could see a small gap between them. Try as I
might, there was no relating these stars (0.54″ separation) to
the Rayleigh criterion.
Another matched (Δ mag = 0.02) 6th-magnitude double
star that ﬁts the Dawes criterion nicely is 52 Arietis. My
observation from January 2017 with the 7-inch telescope
yielded a classic Dawes' resolution image at an estimated
separation of 0.5″. An observation by another astronomer
from approximately the same time undertaken using a
10-inch f/5 reﬂecting telescope on a Dobsonian mount distinctly showed two separate components (see https://is.gd/
Dawes52Ari). Unlike in the image from the 10-inch scope,
I didn't see any space between the symmetrical pair. The
notching convinced me it represented Dawes' limit.

-Rev. W. R. Dawes, Catalogue of Micrometrical
Measurements of Double Stars (1867)

The effect a small difference in magnitude has on star
images is also demonstrated by ovate 51 Aquarii (Δ mag =
0.2) and peanut-shaped 59 Hydrae (Δ mag = 0.6). Again, both
were easily distinguished.
BU 311 had a listed separation of 0.43″ and was deemed
close to the lower distinguishable limit. At times, its image
seemed ovate but kept regressing to a round image. Having identical separation, BU 932 yielded the same spotty
results. Each suggested a lower limit of distinguishability
for the 7-inch telescope.

Separations Using a 7-inch Telescope
Object

Dec.

Mag(v)

Size/Sep

PA (°)

Year Observed

Comments

STF 2244

17h

RA
57.1m

+00° 04′

6.6, 6.9

0.64″

101

2017

Separated

BU 395

00h 37.3m

-24° 46′

6.2, 6.6

0.60″

118

2018

Separated

52 Ari

03h 05.4m

+25° 15′

6.2, 6.2

0.50″

258

2017

Classic, notched

51 Aqr

22h

24.1m

-04° 50′

6.5, 6.6

0.46″

2016

Ovate

59 Hya

14h 58.7m

-27° 39′

6.2, 6.8

0.46″

2016

Peanut-shaped

BU 311

04h

26.9m

-24° 05′

6.7, 7.1

0.43″

157

2016

Ovate?

BU 932

13h 34.7m

-13° 13′

6.3, 7.3

0.43″

2016

Peanut-shaped?

Angular sizes and separations are from recent catalogs. When necessary, the author interpolated to reflect closest observation year. Right ascension and declination are
for equinox 2000.0.

STF 2173: Separations Using a 7-inch Telescope
Object
STF 2173

RA
17h

30.4m

Dec.

Mag(v)

Size/Sep

PA (°)

Year Observed

Comments

-01° 04′

6.1, 6.2

0.63″

142

Sept 2016

Separated

0.54″

139

Sept 2017

Just touching

0.50″

138

Mar 2018

Classic, notched

0.44″

134

Oct 2018

Elongated

0.40″

130

Mar 2019

Round image

Angular sizes and separations are from the author's interpolations. The last value is a projection. Right ascension and declination are for equinox 2000.0.

sk yandtele scope.com * J U N E 2 019

https://www.is.gd/Dawes52Ari https://www.is.gd/Dawes52Ari http://www.skyandtelescope.com

Sky & Telescope - June 2019

Table of Contents for the Digital Edition of Sky & Telescope - June 2019