4100 ± 200 ly

I have not been particularly happy with my prior images of M20. It’s difficult to image from my location because it is low on my southern horizon (transit altitude 27°) so I don’t get a lot of good imaging time, but fortunately this is a fairly bright target so I can get away with a bit less time. Of course this is one of the iconic astrophotography targets so I very much want to get a decent image. For this attempt, I collected data over several nights while dodging cloudy weather. I wanted to get HSOLRGB, and I still don’t have the RGB, but have enough to put the narrowband data together. I’m glad to have L. I don’t think I could use Hα for L because Hα doesn’t pick up the blue reflection part of the nebula. OIII gets some of that but shows less detail in the emission part. So L is important here. The two renditions here are LSHO as provided by Annie’s Astro Actions and LHSO as built by me. As usually seems the case, the Hubble palette seems to bring out more detail. I’m looking forward to seeing how RGB looks; I suspect the dust will show up better with the B filter, and I like RGB stars. I tried composing everything in StarTools from linear data, but that did not work as well as processing and optimizing each of the channels separately and then combining the color layers in Photoshop.

Date: 27-29 May 2020

Subject: M20, Trifid Nebula

Scope: AT8IN+High Point Scientific Coma Corrector

Filters: ZWO 31 mm diameter unmounted L, Hα, OIII, SII

Mount: EQ-6 (EQMOD 2.000j)+PEC

Guiding: Orion Thin Off-axis Guider + ASI120MM-mini +PHD 2.6.8 (Win 10 ASCOM)

Camera: ASI1600MM-Cool, -20 °C, Gain 139 Offset 21

Acquisition: Sequence Generator Pro 3.1.0.479

Exposure: 49x180 L, 22x300 Hα, 22x300 OIII, 24x300 SII

Stacking: Deep Sky Stacker 4.2.3 (64-bit) dark+flat+bias, κ-σ stacking with κ = 1.5

Processing: StarTools 1.6.394RC: Separately optimized L, Hα, OIII, and SII, using pretty much the same procedure for each. Software binned 2x2, cropped, developed (~92%), HDR (reveal all), deconvoluted slightly (2 pixels), and untrack denoised. In Photoshop, combined layers with Annie’s Astro Actions, then applied the L. Some levels adjustments. AstroFrame.