Economical H-Alpha Imaging to Improve Image Contrast *

Many dynamic and interesting astronomical objects glow predominantly in the light of ionized hydrogen. Of particular interest to astrophotographers is the hydrogen-alpha (Hα) emission line at 656.28 nanometers, deep in the red end of the visible light spectrum. Photos taken through filters which isolate this emission line show subtle details which are normally of such low contrast that they are obscured by the broadband light emitted by stars and man-made light pollution, especially from the strongly scattered blue end of the spectrum. The Hα monochromatic image can be used as a luminance channel and combined with a full-spectrum image. This combined image enhances the fine details lost in a conventional color image.

The best way to improve the Hα feature contrast is to use a very narrow-band filter (typically around 5-15nm). But a major problem is that the cost of such a narrow-band filter is very high, especially for large imaging sensors. However, we can still get the benefits of improved contrast by combining less expensive filters to limit the amount of contrast-reducing broadband light which reaches the imaging sensor.

For example, when using a spectrum-enhanced DSLR with the IDAS LPS-V4 astronomical bandpass filter, one can add a deep red filter such as the Kenko R1 filter. In conjunction with the IDAS astronomical filter installed in the spectrum-enhanced DSLR, only a 19 nm band of red light, including the Hα emission line, is allowed to reach the sensor. The IDAS astronomical filter acts to filter out the near-IR component of the image which passes through the Kenko R1 filter.

Kenko R1 + IDAS LPS-V4 filter response plots.

• Canon Camera + camera lens -- 37mm LPS-V4 installed in Canon body for RGB color, and Kenko R1 in front of camera lens for H-alpha shots. No re-focusing is necessary since the Kenko R1 filter is placed in front of the camera lens.


• Scope + any camera -- Use Borg 7519 filter holder with a 52mm LPS-V4 installed on the objective-side threads. For H-alpha shots, insert a Kenko R1 filter using the removable filter holder insert. For parfocal RGB and H-alpha shots, use a photographic skylight (UV) filter which is parfocal with the Kenko R1 filter while taking the RGB shots.

* This document has been updated to describe utilization with the IDAS LPS-V4 filter. The description using older IDAS filters is still available here.