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Observing Report

38 JUNE 2019 • SKY & TELESCOPE

I used my venerable 3½-inch and 7-inch Questar tele-

scopes for this project. Working high power for the instru-

ments is 160× and 320×, which I can double using internal

Barlow lenses if conditions permit. The conventional separa-

tion at the Dawes limit for each telescope is 1.3′′ and 0.65′′;

these values are approximately the separations of 53 Aquarii

and STF 2244 (Σ2244), respectively.

Putting My Telescopes to the Test —

the 3½-inch

In 2016, since 53 Aquarii approximated the separation

encountered at the Dawes limit for a 3½-inch telescope,

the star pair should have presented a notched, elongated

Airy disk. Instead, I saw two stars separated by ample dark

sky. Ditto for its doppelgänger, Kappa^1 Sculptoris. With a

separation of 1.1′′, STF 749 (Σ749) also showed an expanse

of darkness between its components. These stars gave me my

fi rst evidence that a sub-Dawes-limit resolution was possible

with this telescope.

There are doubts on the binary nature of 24 Ophiuchi.

It doesn’t have a calculated orbit and probably is an optical

double, but its bluish components certainly invite inspection.

Early in this study, I saw 24 Oph as two touching stars.

On the other hand, HWE 28 (Howe 28) is a bona fi de

physical double and showed a darkly notched, oblong image

at 260× during moments of better seeing at its declination of

almost –36°. The WDS separation was listed at about 1.0′′.

One of the rare doubles whose attributes (6th-magnitude

components with Δmag = 0.04) almost exactly meet the

Dawes criterion is 52 Orionis. It displayed the symmetrical,

notched, oblong entity associated with Dawes’ limit at an

estimated separation of about 1.0′′.

pTHE TELESCOPES The author determined the Dawes limit of two

telescopes, a 3½-inch and a 7-inch Questar.

Separations Using a 3½-inch Telescope

Object RA Dec. Mag(v) Size/Sep PA (°) Year Observed Comments

53 Aqr 22 h 26.6m –16° 45′ 6.3, 6.4 1.33′′ 78 2016 Separated

Kappa^1 Scl 00 h 09.4m –27° 59′ 6.1, 6.2 1.31′′ 258 2017 Separated

STF 749 05 h 37.1m +26° 55′ 6.5, 6.6 1.11′′ 319 2017 Separated

24 Oph 16 h 56.8m –23° 09′ 6.2, 6.3 1.02′′ 304 2015 Touching stars

HWE 28 13 h 53.5m –35° 40′ 6.3, 6.4 1.01′′ 315 2017 Classic, notched

52 Ori 05 h 48.0m +06° 27′ 6.0, 6.0 0.99′′ 222 2015 Classic, notched

14 Ori 05 h 07.9m +08° 30 ′ 5.8, 6.7 0.95′′ 286 2017 Peanut-shaped

STF 2 00 h 09.3m +79° 43 ′ 6.7, 6.9 0.92′′ 15 2017 Ovate

STT 410 20 h 39.6m +40° 35′ 6.7, 6.8 0.87′′ 42016 Elon gated

16 Vul 20 h 02.0m +24° 56′ 5.8, 6.2 0.85′′ 127 2016 Round image

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.

While not separable with this telescope, 14 Orionis (Δ (^) mag
= 0.9) gave a peanut-shaped image while STF 2 (Σ2; Δ (^) mag =
0.2) presented an ovate image. Both were easily distinguished.
The lower distinguishable limit for the 3½-inch telescope
may well have been defi ned when I compared STT 410
(OΣ410) and 16 Vulpeculae. The pale blue, elongated image
of STT 410 (0.87′′ separation) was distinctive at 260×, while
the image of 16 Vul with 0.85′′ separation remained mostly
round and showed only an occasional hint of duality.