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Narrowband imaging with only H-alpha and OIII filters

As light pollution becomes a real issue for me and the good results obtained with my H-alpha filter I decided to start with narrowband imaging in H-alpha (656nm), OIII (501nm) and H-beta (486nm). The color balance is depending on the intensities of the H-alpha, OIII and H-beta lines. Especially the ratio of H-alpha to H-beta is critical to get the color balance right. The H-alpha to H-beta ratio is about 3 to 1 for emission nebulae and 6 to 1 for planetary nebulae, see below graphs.


Source : Harvard University

As an example an ATIK16IC ccd camera which is equiped with the Sony 424 CCD sensor has the following response.



CCD response for a ICX424 CCD chip


From the graph can be concluded that the CCD response for the H-beta and OIII line is appoximately 100% but for the H-alpha line this is only 65%. In order to get a good balans this means that the exposure time needed for H-alpha should be 50% longer than for OIII and H-beta.

Its possible to make narrowband color images with only a H-alpha and OIII filter. The needed H-beta data can be obtained from the recorded H-alpha and OIII data by blending 15-30% of the H-alpha image, depending on the imaged object, into the OIII image. The result will be a synthetic H-beta image. These three narrowband images represent the colordata for the synthetic RGB image, see below table.

Red=H-alpha
Green=OIII
Blue=OIII + X*H-alpha (0,15 < X < 0,30)


© Copyright Rob Kantelberg
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