A drawing of the Great Nebula in Orion (M42) as seen through a H-beta filter. Images and copyright Harry Roberts ©, all rights reserved
Christmas/New Year time is “giving time”; and no deep sky target “gives” like Orion’s M42 – even under suburban skies.
Despite “folk-lore” that Hydrogen Beta filters only work on the Horse-Head nebula and the Pelican nebula – I’m happy to report that, at least under urban skies with a bigger ‘scope, they transform the view of M42, showing details normally invisible except at dark sites
The logic goes like this: M42 shines in emission, in several narrow bands –little white light is produced. The main emission bands are H-alpha, OIII and H-beta. H-alpha shines three times more strongly than H-beta – but the human eye can’t see H-alpha (except for very bright sources). OIII shines much brighter than H-beta too, but it seems to do so as a generalised glow over the whole nebula – and the improvement in nebula detail appears minor in OIII.
By contrast, H-beta is close to human peak sensitivity, and hi-lights the hydrogen ionisation areas – I was astonished by the new detail visible. Dark areas outlining the nebula, as well as the “Fishes Mouth” and “Fish-Hook” clouds, were starkly black. And the great swathes of ionised nebulosity that show so well in deep-sky photography were now conspicuous – particularly on M42’s west side (right in Fig). Almost the “whole” of the nebula, including the faintest northern fringes, were seen in the Astronomik H-beta filter – although the faintest wisps surrounding bright star Iota Orionis were not seen– but will be captured I think in future sessions.
Caution: we need a bit more aperture with H-beta: it only passes a third of the available light compared to OIII (or invisible H-alpha) – and with the H-beta filter on the 3” ‘scope the view is just too dim; and I’m yet to try the 4”. Also H-beta slightly suppresses the bright Huygenian region where OIII dominates.
We may review the detailed structure of M42 (Ed. willing) in a future piece on summer’s extraordinary nebula; but based on brief usage an H-beta filter is the perfect “stocking-filler” for an astronomical Christmas, exploring the greatest deep-sky object of them all – Orion’s M42.
Clear skies for the New Year!
Harry Roberts is a frequent contributor to this blog and a member of the Sydney City Skywatchers.
2 responses to “Harry sees more of the Great Nebula in Orion through a Hydrogen-beta filter than in white light”
we are dying to know what the bright light star is we have seen for the last two nights from my place in newtown looking towards the east ?
Hello Debbie. You do not indicate the time of night, but you probably mean the planet Venus that currently is rising in the east just before 3 am AEDT each morning. Venus is always bright and is the third brightest object in the sky after the Sun and the Moon.