skyandtelescope.com • JUNE 2019 39Putting 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 inter-
stices. These three stars mirrored the sub-Dawes-limit resolu-
tion 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 fi ts 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 refl ecting telescope on a Dobsonian mount dis-
tinctly 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.
The effect a small difference in magnitude has on star
images is also demonstrated by ovate 51 Aquarii (Δmag =
0.2) and peanut-shaped 59 Hydrae (Δmag = 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. Hav-
ing 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 RA Dec. Mag(v) Size/Sep PA (°) Year Observed Comments
STF 2244 17 h 57.1m +00° 04′ 6.6, 6.9 0.64′′ 101 2017 Separated
BU 395 00 h 37.3m –24° 46′ 6.2, 6.6 0.60′′ 118 2 018 S epar ated
52 Ari 03 h 05.4m +25° 15′ 6.2, 6.2 0.50′′ 258 2017 Classic, notched
51 Aqr 22 h 24.1m –04° 50′ 6.5, 6.6 0.46′′ 31 2016 Ovate
59 Hya 14 h 58.7m –27° 39′ 6.2, 6.8 0.46′′ 14 2016 Peanut-shaped
BU 311 04 h 26.9m –24° 05 ′ 6.7, 7.1 0.43′′ 157 2016 Ovate?
BU 932 13 h 34.7m –13° 13 ′ 6.3, 7.3 0.43′′ 66 2016 Peanut-shaped?
Angular sizes and separations are from recent catalogs. When necessary, the author interpolated to refl ect closest observation year. Right ascension and declination are
for equinox 2000.0.
STF 2173: Separations Using a 7-inch Telescope
Object RA Dec. Mag(v) Size/Sep PA (°) Year Observed Comments
STF 2173 17 h 30.4m –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.
It is a point of considerable interest to determine
the separating power of any given telescopic aperture.
Having ascertained about Y ve and thirty years
ago, by comparisons of the performance of several
telescopes of very dif ferent 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.
—Rev. W. R. Dawes, Catalogue of Micrometrical
Measurements of Double Stars (1867)