M4: 7200 ly

NGC 6144: 28,000 ly

This image was pretty much a train wreck. I’ve been wanting to try and get both M4 and NGC 6144 in the frame at the same time (I did it in the past with my DSI IIc camera and ST80 scope with a focal reducer). These lie pretty low on my southern horizon (transit altitude 23°). On top of that, I got the subs as follow-ons to other targets without much manual supervision, and had lots of problems with imaging sequences failing (mainly because of the low altitude and short time above the horizon). Then there were high clouds and haze to contend with (not usually a problem here). Finally, both the R and G subs had horrible sky background between a nearby bright Moon and light pollution and flats did not correct them well. So the final result required so much doctoring that it feels more like a painting (and a very amateur one at that) than like an image. The colors were pretty much guessed at. There should be a lot more going on in the background with the proximity to Antares and the Rho Ophiuchi Complex, but I used short subs to try and get better statistics for sigma clipping (it didn’t work) and had to darken the background much more than I like to try and get rid of the flaws from the background sky glow. This image will make my index go down, but here it is; sometimes you have to take what the data give you. Wikipedia says M4 is one of the closest globulars to Earth and the first to be resolved into individual stars, whereas NGC 6144 is at a good distance. I’ve always found the line of stars crossing the central diameter to be distinctive. I don’t think there are too many places in the sky where you can observe two large globulars this close together, but found a nice article from July 2006 on the Sky & Telescope web site listing this and a number of others I’ll have to try.

Date: 8-17 May 2020

Subject: M4 & NGC 6144

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 + 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: 31x120 L, 30x120 R, 17x120 G, 30x120 B

Stacking: Deep Sky Stacker 4.2.3 (64-bit) dark+flat+bias, κ-σ stacking with κ = 1.5 for L, Median for R, G, B

Processing: StarTools 1.6.394RC: for L & B (separately), cropped, wiped, developed, deconvoluted, and untrack denoised. For R & G, lots of levels and curves to try and pull the histograms down to something that looked like the L & B. Combined L, R, G, & B in StarTools, let it attempt to undo the stretching, then wiped, developed, deconvoluted, ran the color module, and denoised. Back to Photoshop, where I played with the colors and did a lot of layer-masked levels and curves to try to address some of the mess in the background. AstroFrame.