Messier 20 SHO and SHORGB (by mouse-over)
These are large images and may take a while to upload to full resolution

OTA: CDK24” f/6.5
Camera: Moravian C3-61000 Pro, 2x2 binned at capture
Observatory: Heaven's Mirror, Chile

Exposures:
R: 12 x 600 sec
G: 11 x 600
B: 12 x 600
H: 14 x 900 sec
O: 17 x 900
S: 17 x 900
Total Exposure time used: 17.8 hours
Image Width: 31 arcminutes

Processing: PixInsight, Topaz, Radiant, custom scripts for image weighting, extraction of emission-line intensities, and star replacement
A single pixel at the distance of M 20 (5,200 lyrs) spans 7.6E+16 km

These two images rest on a rigorous method for quantifying the emission-line intensity recorded through narrowband filters, which do not discriminate between the actual emission-line photons and those from the background continuum. To my knowledge, no other method used in amateur astrophotography does this separation by a theoretically sound method. Having isolated the emission line component, I created two images. The first is SHO only, with a color mapping approximating the true emission-line colors. The second, which shows on the mouse-over, is RGB, augmented by 1x the SHO emission lines. (The second image also can be viewed by clicking the thumbnail to the right of the main image.)

=11.0ptThe processing involved star removal and replacement, again using a custom script that assures the star colors retain their calibrated colors using a method called "Alpha Compositing." One that is difficult to reveal fine details in this nebula is that an outer gas shell surrounds it and that shell acts like a haze or fog and softens both the color contrast and, consequently, the detail. The effect has been reduced by selectively boosting the midrange of selected colors most affected by haze. Because of the fine detail in these images, I have published them at their full resolution, and I hope this does not spawn too many problems in viewing them.

Interesting observations arise from comparing these two images. Both images display almost the same level of detail in the dark clouds within M 20, but those in the narrowband image appear more conspicuous and, to me, have more fine detail. However, I believe this appearance devolves from the attributes of human vision, namely, that our red-sensing cones are larger in size and far less abundant than our rods, and those red cones participate to a greater level in our perception of the SHORGB image.

A second observation is more problematic. Below the central circular cloud of M 20 lies a blue region, which shows more detail in the SHORGB image than in the SHO image. Both images were processed quite similarly, and no masking was used, making the reason for this difference may be because I have simulated the H-beta in the SHO image, assigning 1/3 of the H-alpha value to it and thereby over-printing the structure of the OIII channel. That downside may have been redeemed by boosting the bluish colors near the center of M 20.

In any case, I hope you find these two images interesting, each in itself and in their contrasts.

Alex Woronow