• Messier 21 in Sagittarius, Webb's Cross
    • M20 in Sagittarius, the Trifid Nebula
    Annotated Trifid Nebula
  • Messier 21, or Webb's Cross, in Sagittarius.
  • While not visible to the naked eye, M21
  • is easily seen with small binoculars.
  • Date:  23 June 2025
  • FOV:  cropped to 30' × 20'; the angular size of M21 is about 14'
  • Rotation:  −90.094°
  • Telescope:  Stellarvue SVX102T + SFFX-1 (737mm, nominally 714mm f/7)
  • Guiding:  ZWO ASI120MM mini mono guide camera,
  • Stellarvue F050G 50mm guide scope
  • Computer:  ASIair pro Raspberry Pi
  • Mount:  iOptron CEM40
  • Camera:  ZWO ASI6200MC Pro Cooled (-10°C) Color CMOS with gain 100
  • Pixel size:  3.76 μm
  • Resolution:  1.052 arcsec/pixel
  • Filter:  Optolong UV/IR cut
  • Stacking:  fifty 240 second frames using PixInsight
  • Total Time:  3 hours 20 minutes
  • Processing:  Pixinsight – SPCC, DBE, SPCC, BlurXT, NoiseXT → Linear
  • HTS, GHS, MStr, PixelMath MStr + GHS + HST → Stars,
  • Linear, StarXT, HTS, GHS, MStr,
  • PixelMath MStr + GHS + HST → Starless, HDRMST6PhLm
  • PixelMath ~(~Starless*~Stars) → Blend, Sat, DynCrop/Scale
  • Location:  Cherry Springs State Park near Coudersport, PA