Observing the Horsehead Nebula

2013 November 29

Callum Potter

Perhaps one of the biggest challenges for the visual observer is trying to see the Horsehead Nebula in Orion. It’s a bit of a routine target for imagers, requiring nothing particularly special – a DSLR on a short focus refractor on a driven mount is probably enough to capture it, with not too long an exposure needed, especially if stacked. You don’t even need a modified DSLR as the accompanying image from Graham Relf shows.

The Horsehead is a dark nebula overlaying the bright nebula IC 434. E. E. Barnard catalogued it as B33 in his list of dark nebulae published in a 1919 paper in the Astrophysical Journal, where he notes ‘Dark mass, diam 4′, on nebulous strip extending south from zeta Orionis’. IC 434 is the bright nebula, a region of hydrogen gas ionised by the nearby star.

Every winter season for the past few years I have been trying to see the Horsehead visually, but with no success. This year I have invested in an H-Beta filter, hoping this will be the magic bullet. Of course a good sky at a dark time of the month is needed too, so fingers are crossed for some good observing weather this winter.

You might wonder why use an H-Beta rather than an H-Alpha filter. H-Alpha light is a spectral line centred on 656nm, and although it is the strongest emission line of ionised hydrogen, our eyes are not very sensitive to the deep red. The H-Beta line is about three times fainter but is emitted at around 486nm which is much better for our night vision. So for regions where HII emissions dominate an H-Beta filter should give the human eye a better, higher contrast view.

A couple of other things make visually observing the Horsehead tricky. It is quite close to zeta Orionis, and glare from this star can hide it. It is best to try and keep the star just out of the field of view, or use an occulting bar. Also the nebula is rather small, just a 4′ notch – a careful balance of magnification and field size will help.  (Image above by Bob Winter.)

The H-Beta filter is so commonly used to view B33 that it has become known as the ‘Horsehead filter’, and while there is not a huge number of objects it is effective on, here are a few more suggestions for the winter months:

– The California Nebula, NGC 1499, in Perseus is a large HII region – around 2´1°;

– The Flaming Star Nebula, IC 405, associated with the star AE Aurigae;

– The Cocoon Nebula, IC 5146, in Cygnus;

– Barnards Loop, Sh2-276, in Orion might also reveal itself. You could try the filter with the unaided eye. There may be some difficulties getting the orientation right, and reflections may make the filter less effective – a solution to this is to fit the filter into a blackened tube, so that the filter is held steadily, and the reflections minimised.

It can be interesting to try applying the filter to other objects, which might reveal a different perspective – for example M42 and M43.

If you do manage to view the Horsehead this winter, or any of these other objects, visually with or without an H-Beta filter, please send your observations to the Deep Sky Section, and I’ll report in a future Journal on your results. Of course images of any of these objects would be appreciated too.

Callum Potter, Director, Deep Sky Section