2019-06-01+Sky+and+Telescope

OBSERVING REPORT by Don Ferguson

36 JUNE 2019 • SKY & TELESCOPE

The Dawes Limit

D

istinguishing the binary nature of a star at

the Dawes limit is a stringent test of visual

acuity at the telescope’s limit of resolution, with

atmospheric conditions playing no small role.

Phillip Kane’s “Find Your Dawes Limit” (S&T:

Sept. 2016, p. 30) provided the catalyst for me

to fi nd closure on unresolved double star issues

that had accumulated for decades. His “grand

list of test stars” is outstanding and was almost

up-to-date. I decided to accept his challenge.

The Dawes and

Sparrow Limits

Observers use the ability to separate double

stars as a proxy for a telescope’s resolving

power. Enlarged images of close double stars as

seen in a telescope without a central obstruction

at decreasing separations are simulated at right.

According to the Dawes criterion, resolution is

predicated on the perception of a drop in the

light intensity (dark notches) fl anking the closely

merged star images. In his Catalogue, Dawes

concludes this is the closest that two stars can

overlap and still be recognized as individual

objects. The criterion culminates in his famous

Dawes’ limit equation

The author fi nds that two of his telescopes have somewhat different — lower! —

Dawes’ limits than what theory predicts.

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