M13 rose at the right time to allow some reasonable data acquisition there before moving on to do more narrowband collection on the Trifid Nebula (a longer-term project I’m slowly getting data for), and as the Moon was fairly bright this bright target seemed a reasonable choice. I did not get much real luminance data because I chose to do the luminance last, when M13 was nearest the meridian, and ran out of time. So this is my first attempt to use some synthetic luminance, created by combining the stacked R, G, and B in the usual way and then extracting L from that. In Photoshop, I layered this synthetic L at 40% opacity over the stack of the few subs taken with the L filter, and used the result for luminance. A bit of curves, levels, saturation, and additional layer-masked deep space noise reduction applied.

Date: 12 May 2017

Subject: M13, Great Hercules Globular Cluster

Scope: AT8IN + High Point Scientific coma corrector

Filters: ZWO 31 mm diameter unmounted L, R, G, B

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

Guiding: Orion Thin Off-axis Guider + DSI +PHD 2.6.3.2 (Win 10 ASCOM) using predictive PEC algorithm

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

Acquisition: Sequence Generator Pro 2.6.0.21

Exposure: 15x120 R, 15x120 G, 15x120 B, 4x120 L

Stacking: Neb 4.1.5, flats & darks, trans+rot align, Nebulosity 1.5σ stack and align.

Processing: Created a synthetic luminance by color combining the stacked R, G, and B subs (enhancing R about 10% to get a neutral background) in Nebulosity, then extracting luminance using Nebulosity. In Photoshop, layered this at 40% opacity over the stack of subs shot through the L filter. StarTools 1.4.327: Stretched and deconvoluted the luminance, “magic” star size reduction (1 pixel). Stretched, color balanced, desaturated (a little), and deconvoluted RGB in StarTools. Layered luminance over RGB in Photoshop CC 2014. Carboni’s Astronomy Tools deep space and space noise reduction. Applied GradientXTerminator medium/high. AstroFrame.