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Contains:  Sh2-274
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Sh2-274, Medusa Nebula, SHO & HSO + LRGB stars, Feb 2020, 


            David Dearden
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Sh2-274, Medusa Nebula, SHO & HSO + LRGB stars, Feb 2020

Technical card

Dates:Feb. 21, 2020Feb. 22, 2020Feb. 24, 2020Feb. 25, 2020

ZWO Blue 31 mm: 20x120" (gain: 139.00) -20C bin 1x1
ZWO Green 31 mm: 20x120" (gain: 139.00) -20C bin 1x1
ZWO H-alpha 7nm: 55x300" (gain: 139.00) -20C bin 1x1
ZWO Luminance 31 mm: 60x120" (gain: 139.00) -20C bin 1x1
ZWO OIII 31 mm 7 nm: 20x300" (gain: 139.00) -20C bin 1x1
ZWO Red 31 mm: 20x120" (gain: 139.00) -20C bin 1x1
ZWO SII 7nm: 17x300" (gain: 139.00) -20C bin 1x1

Integration: 11.7 hours

Avg. Moon age: 14.63 days

Avg. Moon phase: 2.33%

Bortle Dark-Sky Scale: 5.00 job: 3303452

RA center: 7h 29' 1"

DEC center: +13° 15' 14"

Pixel scale: 1.719 arcsec/pixel

Orientation: 89.528 degrees

Field radius: 0.682 degrees

Resolution: 2268x1740

Locations: Mapleton Lateral Observatory, Mapleton, Utah, United States

Data source: Backyard


1500 LY

I saw I had not tried this target in 4 years and was looking for something that would work well in narrowband, so thought I’d try the Medusa Nebula. I was fighting my focuser trying to get the tension properly adjusted, so had a hard time getting good focus on the first subframes I took. Not fully satisfied with the early result, I decided to integrate some more and I’m glad I did. I took LRGB data primarily intending to use it for star colors and was interested to see that the nebula itself is pretty dim; the narrowband filters helped a lot, especially Hα although the nebula showed up in all 3 filters. Processing was also a struggle this time. I found I needed to do levels & curves on the nebula after adding an Hα luminance layer but before using a “lighten” combine to add in the LRGB stars. I processed both an SHO Hubble palette version and a HSO version; once again, I think the chemical differences in the nebula show up better in the Hubble version so that’s the one I prefer. It’s interesting that Wikipedia describes this as a planetary nebula. If that’s indeed correct, it’s an unusually structured one.

Date: 21-25 Feb 2020
Subject: SH2-274, Medusa Nebula
Scope: AT8IN+High Point Scientific Coma Corrector
Filters: ZWO 31 mm diameter unmounted 7 nm Hα, SII, OIII, L, R, G, B
Mount: EQ-6 (EQMOD 2.000j)+PEC
Guiding: Orion Thin Off-axis Guider + ASI120MM-mini +PHD 2.6.7 (Win 10 ASCOM)
Camera: ASI1600MM-Cool, -20 °C, Gain 139 Offset 21
Acquisition: Sequence Generator Pro
Exposure: 55x300 Hα, 17x300 SII, 20x300 OIII, 60x120 L, 20x120 R, 20x120 G, 20x120 B
Stacking: Deep Sky Stacker 4.2.3 (64-bit) dark+flat+bias, κ-σ stacking with κ = 1.5.
Processing: StarTools 1.6.387: For Hα, SII, & OIII each, software binned 2x2, cropped, developed, HDR “reveal all”, deconvoluted, and untrack denoised (grain equalization). Combined L, R, G, & B in StarTools, adjusting for exposure times. 2x2 software binned, cropped, developed, HDR “reveal all,” color module with heavy saturation boost and that barely made the color show up. Untrack denoised (grain removal). In Photoshop, used Annie’s Astro Actions to do SHO Hubble combination, attempting to overlap the histograms for the 3 colors as much as possible. Did the same using the RGB action to do HSO. For both SHO and HSO, applied Hα as luminance and did some levels & curves adjustments. For the LRGB, used Astronomy Tools’ “increase star color” and “decrease star size,” then combined with the SHO and HSO using “lighten.” More levels and curves. AstroFrame.



David Dearden
License: None (All rights reserved)


  • Final
    Sh2-274, Medusa Nebula, SHO & HSO + LRGB stars, Feb 2020, 


            David Dearden
  • Sh2-274, Medusa Nebula, SHO & HSO + LRGB stars, Feb 2020, 


            David Dearden


Description: This is the HSO version, but I think I prefer the SHO.

Sky plot

Sky plot


Sh2-274, Medusa Nebula, SHO & HSO + LRGB stars, Feb 2020, 


            David Dearden