The Horsehead Nebula…from L.A.
- Telescope: Stellarvue SVA130T-IS
- Mount: Losmandy G-11 with Gemini 2 controller
- Autoguiding: Yes
- Optical Configuration: 0.72x field flattener & reducer (f/5)
- Filter: 12-nm narrowband H-alpha
- Camera: Canon 60Da
- Light Frames: 12, 4-min. exposures
- Calibration: 13 darks, 100 biases, 30 flats
- Exposure Time: 48 min. (12 x 4 min.)
- ISO: 1600
- Pre-Processing: PixInsight
- Processing: Photoshop CC (processed in a single red channel and flattened)
- Imaging Location: Los Angeles, Calif.
Photographing wispy deep-sky objects (DSOs) in the light-polluted skies of Los Angeles is not for the faint of heart. L.A. was an early adopter of LED streetlights; so night has now become day, ablaze with broadband sky glow. But those gossamer DSOs can be photographed under such conditions, as the image above of the Horsehead Nebula and Flame Nebula proves. The trick is to use a narrowband filter in front of the camera sensor, which blocks all wavelengths of light except the ones of interest. In this case, the wavelength of interest is the red emission line of singly ionized hydrogen at 656 nm…aka, H-alpha. The Horsehead and Flame nebulae both emit at the H-alpha wavelength, so the filter allows their “voices” to be “heard” above the raucous roar of urban light pollution.
This image makes use of a narrowband H-alpha filter with a full-width half-max transmission window of 12 nm. It is a proof-of-concept shot intended to flush out the process and nail down some of the parameters (see bulleted details above). Wind and the meridian precluded taking more light frames, which would have further enhanced the quality and detail of the image. During the imaging session, a very bright LED streetlight gleamed at full intensity about 50 yards away, and the full moon (two days past full) was rising in the east. Yet, the results speak for themselves. It can be done, and done well.
Stay tuned for more narrowband images to come on this website.
