Baader Photometric Filters: Dark nebulae appear in a new light

Combination of photometric and RGB filters to highlight dark nebulae

Photographing dark nebulae is a fascinating challenge for astrophotographers, often posing difficulties due to the low brightness and high contrast between celestial objects and dark nebulae. This often requires the use of different techniques and exceptionally dark skies to bring out fine details in the dark nebular structures.

In this test report, we examine the photographic results and the effectiveness of the combination Baader SLOAN/SDSS (ugriz') Filter Set – photometricBaader SLOAN/SDSS (ugriz') Filter Set – photometric Baader SLOAN/SDSS (ugriz') Filter Set – photometric (various versions available) und Baader RGB Filter Set – CMOS-optimized (various versions available) to evaluate its effectiveness in improving contrast and detail, as well as enhancing them in all their colorful glory, while maintaining natural color reproduction.

Equipment used:

All single frames and the final image were taken with a QHY 268M PH, BSI Cooled Medium Size APS-C CameraQHY 268M PH, BSI Cooled Medium Size APS-C Camera QHY 268M PH, BSI Cooled Medium Size APS-C Camera (#1931096, € 2820,-) and the Celestron RASA 11“ as well as a selection of 9 different filters, which were changed manually using the UFC (Univeral Filter Changer) System.

The following filters were used:

The entire system was controlled and monitored by the N.I.N.A. imaging software. Filters were changed manually using the UFC Filter Changer. After each filter change, an automatic focusing motor was used to refocus (always after 90 minutes or sooner if the temperature dropped rapidly) in order to always hit the perfect focus point. Flat frames were taken at the end of the filter session BEFORE each filter change.

Gain settings:

High Gain Mode Gain 0 / Offset 25 (for all wideband exposures)
High Gain Mode Gain 56 / Offset 25 (for narrowband exposures)

Acquisition conditions and software:

The images were taken far from optimal conditions under city skies. With an exposure time of over 70 hours, spread over all filters, the images were taken on every clear night with good seeing conditions between May 2023 and July 2023. The main image shows the reflection nebula Van den Berg (vdB) 152 in the constellation Cepheus together with the very faint and diffuse molecular cloud LDN 1217 (LDN = Lynds Dark Nebula). The object is also known among astrophotographers as "Wolf's Cave Nebula".

Details on filter applications:

Photometric filters to increase contrast:

The use of photometric filters in combination with RGB-Filters led to a significant increase in contrast in the dark nebulae regions. The use of SLOAN g‘, SLOAN r‘ and SLOAN i‘ improved the visibility of fine structures and details, while RGB filters preserved the natural colors.

The three central filters used for the Sloan Digital Sky Survey (SDSS) divide the spectral range into three roughly equal parts: g': 410nm to 550nm, r': 555nm to 695nm, and i': 695nm to 845nm. These filters are therefore the perfect choice for three-color imaging with modern cameras.

Wavelength range of the Celestron RASA 11": The manufacturer specifies the Celestron RASA 11“ with an optimized wavelength range of 400 to 700nm. However, the test with the SLOAN i‘ Filter (695-845nm) clearly shows that a sharp and detailed image is also possible far beyond the fully corrected range. The prerequisite for this is refocusing after each filter change.

Filters for the reduction of light pollution:

The application of the UHC-L / Ultra-L-Booster-FilterUHC-L / Ultra-L-Booster-Filter UHC-L / Ultra-L-Booster-Filter (various versions available) , as well as the IDAS LPS-P2, were used to capture the luminance images. The UHC-L booster filter, which was also specially designed to reduce light pollution, led to a considerable improvement in image quality. The sky appeared darker, which significantly increased the contrast to the dark nebulae. In combination with the LPS-P2 luminance filter, this led to better recognition of fine details and structures.

Use of the f/2 Ultra-HighSpeed H-alpha narrowband filter:

By integrating the narrow-band Baader H-alpha f/2 Ultra-Highspeed-Filters (3.5nm) – CMOS-optimizedBaader H-alpha f/2 Ultra-Highspeed-Filters (3.5nm) – CMOS-optimized Baader H-alpha f/2 Ultra-Highspeed-Filters (3.5nm) – CMOS-optimized (various versions available) certain emission lines could be selectively emphasized. The hydrogen regions, which are rather weak for color filters, could be precisely extracted and re-inserted into the final color image in the subsequent image processing. This helped to enhance the structures around the dark nebula and the H-alpha regions.

Combination of RGB filters for natural colors:

Using RGB-Filters alone tended to reduce the contrast between the sky and dark nebulae. The integration of photometric filters helped to increase this contrast, while RGB filters preserved natural colors and allowed for a balanced color appearance.

Overall appearance and aesthetic gain:

The combination of photometric and RGB-Filters resulted in impressive structures within the dark nebula LDN 1217, emphasizing the fine details and structures while preserving the natural colors, resulting in an aesthetically pleasing and well-balanced image.

Image in full resolution: vdB 152 - Wolf's Cave Nebula (SLOAN g'r'i' + HaLRGB)

Acquisition details:

Bildautor: Yannick Akar
TelescopeCelestron RASA 11“
CameraQHY268M Photo
MountiOptron CEM120
AccessoriesBaader UFC, Baader UFC Tilter, Celestron Focus Motor, Pegasus Astro UPB v2
Total exposure time70.3 hours
H-Alpha f/2 3.5nm
3.25 hours (195 x 60”)
3.15 hours (190 x 60“)
3 hours (180 x 60”)
5.4 hours (325 x 60”)
6.3 hours (380 x 60”)
8.5 hours (340 x 90”)
6.4 hours (385 x 60”)
12,7 hours (1525 x 30”)
21.5 hours (645 x 120”)
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