California Nebula Face-Off Bortle 2 vrs Bortle 7

Here I compare an emssion nebula captured with narrow band HOO pallates with urban light pollution and wide band dark skies images. Each of these images of the California Nebula, NGC 1499, has exactly the same 5 hours of total exposure in 60 five minute light images.

The same equipment: WO RedCat 250/51 Petzval Refractor, ZWO ASI 533 MC color camera, ASIAIR+ photo controller, and SW AZ-EQ 5 Pro mount. One image is from north of Silver City, New Mexico under Bortle 2 skies using a UV/IR cut filter. The other image is from the Austin, Texas suburbs under Bortle 7 skies with an Optolong L-Ultimate dual 3nm band Ha+O3 filter.

Dark skies, broadband UV/IR cut filter, California Nebula from Silver City

Urban light dome, multi narrow band Optolong L-Ultimate Ha and O3 3nm filter, HOO pallate California Nebula from Austin

Processing was in PixInsight using the WBPP script using GraXpert, BlurXTerminator, StarXterminator, NoiseXterminator, GHS, and Curves plugins. Final exposure and color balance adjustments were done in Photoshop. Similar but not identical processing parameters were used as needed by the different data.

Narrow band filters typically pass 80 to 90% of the in band signal from emission nebula. Broadband sky glow is reduced approximately by the ratio of the total passband to the UV/IR filter pass band (for the Baader UV/IR cut filter 685nm - 420nm = 265 nm). For a dual 3nm passband filter this is a 265/6 or 44x reduction of broadband light. The Bortle scale ranges from a NELM (naked-eye limiting magnitude) of 4.0 for an inner city Bortle 9 sky to 7.6 for a Bortle 1 excellent dark sky site. This 3.6 magnitude range correspnds to 2.52^3.6 or 28 times. This leads us to expect a duction in skyglow with narrow band filters to be comperable to that at a dark sky site. Even with comperable contrast between narrow band emission colors and the sky glow background, there will be substantial differences between images made in the two circumstances.

Sifnificant differences between narrow band bright sky and broad band dark sky images of emission nebulae targets include:

  • Different color (white) balance.
  • Attenuated star brightness
  • Attenuated broadband reflection nebulosity

Although all of these differences are clearly seen in the two images above, it is still a thrill to image colorful emission nebulae under brigh city skies.

Content created: 2023-12-05




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