Object of the Week, August 6th 2023 -- The Blue Horsehead Nebula

IC 4592 = Ced 128 = LBN 1113
Reflection Nebula in Scorpius
150' x 60'
RA (J2000): 16h 12m 00s
Dec (J2000): -19° 27' 35"

(I apologize for the very late post this week. I'm currently abroad in India and preoccupied with a bunch of other things before I can get back to astronomy!)

This week, I'm choosing a target that's very popular amongst astrophotographers, but maybe not as well-known in the visual land. There's an obvious reason for this: very low surface brightness.

IC 4592 is part of the rho Ophiuchi complex and lies around the star nu Scorpii. According to Harold Corwin, IC 4592 was discovered by E E Barnard at Lick Observatory. Although he first visually (!!) discovered the region of nebulosity, his formal publication relied on the photograph because the nebula is rather faint to see detail in.

Attachment 5196
DSS2 Color image of the Blue Horsehead
This image has been oriented north down, so it represents what you would see in the northern hemisphere if you were to point binoculars to it when it is on the meridian.

I imagine that the best instrument to see this large and diffuse nebulosity is a rich-field telescope or big binoculars. I had an excellent view of the object through my 25x100 binoculars from a Bortle 3 site with good southern skies in western Arizona. As I scanned the binoculars past nu Scorpii, the brightest region "cut on" like a wall running above the star. The rest of the nebula was marked by faint undulations of light and dark and the absence of stars. The nebula was huge and vaguely triangular in shape.

Attachment 5197
Rho Ophiuchus along with Blue Horsehead in the lower part of the image, by Rogelio Bernal Andreo.

I would wager that this nebula may be visible to the naked eye or in a small monocular, provided the conditions are excellent. From the exceptional skies of the Warner Mountains at 7000ft elevation this past July new moon, I was able to see a general brightness filling the region between Al Niyat and Dschubba in Scorpius. This bright region extended to nu Scorpii, where perhaps it was just a bit brighter. However, this was too weak to confirm as a positive sighting of the nebula. However, this is at 41°N latitude; perhaps from the southern hemisphere, one may have better luck.

As always,
GIVE IT A GO AND LET US KNOW

A difficult object because of scale. My drawing:

Attachment 5198

Mel Bartels

Unfortunately already to low in the sky here. But I get a glimpse with my 8x30 binoculars under very good transparency. The nebula was visible without any problems as a 1°x2° longish object. The VdB's more eastern were also indicated but to small for a 8x magnification.