Hydrogen Alpha

I don’t think we know nearly enough about the simplest things to be sure about anything. I was reminded of this when I looked over the data from this nebula. There’s not much to see when looking at it through the red, green, and blue (RGB) visible spectrum of light — or at least, not from this vantage point. An entirely different and vastly more detailed expanse became visible when I captured much longer exposures looking at just an extremely thin slice of the visible spectrum. We base so much of what we believe on what we can see; yet so much is intrinsically invisible to us.


The RGB image looks like this. There's a hint of something there, and if you zoom in you can see that the background stars are being colored red by something. The RGB filters let in a wide spectrum of light, and this includes both light from the nebula and stars, as well as the undesirable light pollution from our own terrestrial home. Capturing a 5-10mins exposure with these wide-band filters in polluted skies is pushing the limits of usable data.

This is where narrow-band filters that focus in on a particular wavelength of light come in to their own. Hydrogen-Alpha, a deep shade of red, is an intriguing wavelength to extract from the night sky. The filter is so narrow — it lets in so little light — that you can achieve over 30 minute exposures even in polluted skies. In fact, even when the moon is out, you can capture Hydrogen-Alpha (Ha) data without compromise.


A single, 20-minute exposure filtering out just the Ha spectrum of light yields this incredible result. That's a whole lot more detail! Of course, all this light was already there — we haven't discovered or uncovered anything new — but rather we're now able to see this by looking for longer at a limited field of view.

The primary image seen at the top of this post is the combined Red, Green, Blue wide-band light combined with the Hydrogen-Alpha narrow-band light representing red. It was composited using the following data:

• 22 x 300-second red exposures
• 17 x 300-second blue exposures
• 17 x 300-second green exposures
• 5 x 1200-second hydrogen-alpha exposures

All imaging data was captured from the back patio of my townhouse in San Jose, 2015. The telescope used was a William Optics Star-71 — a beautiful little 5-element refractor for wide-field imaging — connected to a QSI683 camera on an Atlas EQ-6 mount.