A (6)

54 AUSTRALIAN SKY & TELESCOPE JULY 2016

probable open cluster Archinal 1. It was discovered

by Brent A. Archinal and introduced in the book Star

Clusters, which Archinal co-authored with Steven

J. Hynes (2003). They note that the cluster contains

about 24 stars within a diameter of 1.5′.

Through my 130-mm refractor at 117×, Archinal 1

is a small misty patch sitting 2.2′ southeast of an

11th-magnitude star. One faint star dwells within the

cluster’s south-southeastern edge, and a couple more

are intermittently visible. I suspected one additional

star at 164×. With my 25-cm scope at 43×, Riddle’s

Trapezium and Archinal 1 share the field of view,

but Archinal 1 looks considerably dimmer than its

neighbour. At 299× there are three stars in a straight

line, tipped slightly west of north, and a fourth star

east-northeast of the line’s centre star. A fifth star is

occasionally glimpsed. Even my 38-cm reflector didn’t

add much to the scene. I only spotted a few very faint

stars among the brighter quartet.

Where are the rest of those 24 stars? Richard

Harshaw managed to tease out a bunch of them with

his 28-cm Schmidt-Cassegrain. He advises, “Folks,

hit it with 400× or more. At that point a wonderful

sprinkling of star glitter emerges from behind the four

main stars.” I know I’ll give it a try. Will you?

Those who like a difficult challenge can try for

the exceptionally faint planetary nebula Yer kes -

McDonald 16 (YM 16), 30′ north of Archinal 1.

Through my 38-cm reflector at 79×, this phantom

glow appears roughly 5′ across. It’s defined mostly by

diaphanous rims east-northeast and west-southwest

of each other, like a set of parentheses. The west-

southwestern rim is shorter and more tenuous. A

barely perceived glow fills the area between them.

The nebula is ensconced in a wedge of 13th- and

14th-magnitude stars. The star at the wedge’s point

is northwest of the nebula, and the three stars of the

arc lie off the opposite side. YM 16 is visible with a

narrowband or hydrogen-beta filter, but an O III filter

is detrimental. I prefer a filterless view.

When preparing this tour, years after visiting YM

16, I found that my logged description and eyepiece

sketch don’t tell the whole story. Images show that

this planetary nebula is actually quite oval. A large

portion of YM 16, southwest of what I’d detected, was

completely invisible to me. This extends the nebula

to a length of about 6′. I’d love to hear from you if you

spot any part of this elusive nebula. ✦

Left: North is up in this wide-

angle view that takes in Yerkes-

McDonald 16 (YM 16), Archinal

1 and Riddle’s Trapezium,

described in the text.

Below: Don Goldman captured

the elusive nebulosity of YM 16

through a 40-cm Ritchey-

Chrétien reflector; this image

combines 6 hours of 3-nm H-α

and almost 11 hours of 3-nm

O III exposures.

YM 16

YM 16

Archinal 1

Riddle’s Trapezium

POSS-II / STSCI / CALTECH / PALOMAR OBSERVATORY

SUE FRENCH

says she is

“an amateur

astronomer

in the original

senseofthat

word amateur

—Idoitforthe

love of it.” She

welcomes your

comments at

scfrench@nycap.

rr.com

Targets