Filtered Shot Emission Nebulas in Cygnus This image was made with a light-pollution filter. Most DSLR cameras are not very sensitive to the red light of hydrogen-alpha, but they are not totally insensitive, so you can use them to shoot the brighter emission nebulas. You will, however, need longer exposures to photograph them. Well-made deep-sky and light-pollution filters can help you here. They will filter out other unwanted wavelengths, and pass the important hydrogen-alpha wavelength. This will allow you to take exposures that are about 3 times longer than normal, recording more red emission light.

This area in Cygnus in the heart of the norther Milky Way is full of red emission nebulas and dark nebulas.

Emission nebulas glow from the light of ionized gas. Dark nebulas are clouds of opaque dust and gas that block the light of stars behind them.

The most prominent bright nebula in this picture is NGC 7000, the North America Nebula at upper left.

Deneb is the brightest star in the frame near top center, and Sadr is the next brightest star at lower right.

This image was shot with a 50mm lens at f/2.8 with a IDAS LPS light pollution suppression filter. Although it was made at a reasonably dark-sky site where the Milky Way was visible to the unaided eye, the darkness of the night sky was still compromised by the light pollution from Philadelphia, an urban area with more than 6 million people. The light pollution filter helped filter out the wavelengths from the light pollution and let the wavelengths from the emission nebulas pass through, allowing longer exposures that record more red light.

These types of filters will also help the contrast between emission nebulas and the sky background even at true dark-sky sites with no light pollution because they will filter out the natural airglow.

Normal unfiltered exposures at this site would be about 1 minute at f/2.8 at ISO 1600 to get the sky background up out of the readout noise of the camera. The filter allows exposures about three times longer to reach the same level of brightness of the sky background. Because the emission nebula wavelengths are not filtered, you get three times more red nebulosity compared to the sky background.

This image was shot with an unmodified Canon 1000D (Digital Rebel XS). It shows that you can shoot emission nebulas with an unmodified camera, and that a good filter also helps tremendously in capturing this red light.

Image Data

• Lens / Scope: Nikon 50mm f/1.4 lens with Fotodiox Canon adapter
• Focal Length: 50mm
• F/stop: f/2.8
• Exposure: Stack of two 3-minute exposures (6 minutes total exposure)
• Mount: Orion Sirius polar-aligned German-equatorial mount
• Guiding: None
• Camera: Unmodified Canon EOS 1000D (Digital Rebel XS)
• Mode: JPEG
• ISO: 1600
• White Balance: Custom, set on sky background
• In-Camera Noise Reduction: Off
• Filter: IDAS LPS filter
• Temp: 54F
• Start Time: 3:59 a.m.
• Date: May 31, 2009
• Location: Maxwell, NJ
• Calibration: None
• Processing: Standard in-camera JPEG processing. Stacked in Deepsky Stacker. Contrast increased a bit.