My first Magazine Sky & Telescope - 02.2019

IMAGE PROCESSING by Timothy Jensen DEEP COLOR Adding narrowband image data, particularly data recorded through a hydrogen-alpha (Hα) ﬁ lter can add faint details and rich color to images taken under less-than-ideal skies. The author took this example of the emission nebulosity IC 434 surrounding the Horsehead Nebula using a one-shot color camera to record color and Hα that were combined in ImagesPlus. Giving Nebulae a A mong the most photographed objects in the night sky are emission nebulae. These expansive ﬁ elds of eye- catching red and magenta nebulosity, with occasional ﬂ ourishes of orange, make for beautiful celestial portraits. However, getting those rich colors isn’t always easy, par- ticularly if you live and shoot from light-polluted locations. While one-shot color cameras like DSLRs or mirrorless cam- eras can capture the entire spectrum of visible light, often their utility is compromised by light pollution and skyglow. One way around this limitation is to image through nar- rowband or other specialized ﬁ lters to increase the contrast in the images. Ultra High Contrast (UHC) ﬁ lters sold by vari- ous manufacturers are a type of combined narrow-bandpass ﬁ lter. They work by passing speciﬁ c wavelengths of light, commonly found in nebulae, while blocking unwanted wave- lengths from most sources of skyglow. Oceanside Photo and Telescope’s new Triad ﬁ lter works in a similar fashion. Individual narrowband ﬁ lters allow only a fraction of the visible spectrum to be seen by the camera. Most commonly used is the hydrogen-alpha (Hα) ﬁ lter that blocks all wave- 66 FE B RUA RY 2 019 • SK Y & TELESCOPE lengths except those surrounding ionized hydrogen emitting light at 656.3 nanometers. Since hydrogen emission is typi- cally the most common light from emission nebulae, this is an excellent ﬁ lter for highlighting details in faint nebulosity. It’s most effective when paired with monochrome cameras, since it only passes a very small portion (often between 3 and 7 nm) of the deep-red region of the spectrum, so images recorded through this ﬁ lter with a color camera (such as a DSLR) appear as a washed-out red. However, it is still possible to use these monochrome images captured with Hα ﬁ lters to enhance an ordinary color image obtained using a one-shot color camera. The trick is to blend the Hα image into a color photograph to bring out the nebula while at the same time maintaining a natural color balance. One common practice is to replace the luminance channel with a Hα image in a color photo. However, this can lead to an odd color bias that will require much work to correct. A better solution is to blend the Hα data into the red channel of an RGB color image. Care must be taken, since the Hα data can overpower the broadband red image. Addition- BOOST Here’s a novel technique to bring out emission nebulosity in your astrophotos.

My first Magazine Sky & Telescope - 02.2019 | Page 68