Coma Sizer 2.4 web
29P/Schwassmann-Wachmann · 2026-04-08 12:56 UT · SSTAC (MPC-O58)
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COMA DIAMETER  (isophote 5σ above sky, σ = 1.42 ADU/px)
  Diameter          : 54.0 px = 77" = 1.29'
  Range (sky ×1–1.5): 1.14' – 1.29'

PHOTOMETRY FOR m1  (aperture = coma diameter)
  Aperture          : D 54.0 px  (r 27.0 px)   D 77.2"
  Sky annulus       : r 292 – 336 px   (D 584 – 671 px)   r 417" – 480"
  Sky chosen by     : least error for Tycho (stars +0.009, coma-in-sky +0.050 mag)
  Tycho, Diameter   : Inner 54.0 · Middle 584 · Sky 87
  Tycho, Radius     : Inner 27.0 · Middle 292 · Sky 44
                      (Tycho 'Sky' = annulus width beyond Middle)
  Tycho sky check   : stars/tail in annulus raise a plain-mean sky by +0.10 ADU → Tycho mag +0.011  (expect ≈ 12.88 in Tycho)
                      (not applicable if Tycho's star removal is used)
  m_inst            : -10.844 ± 0.003
  m (ZP 23.711)     : 12.867   (header ZP, a check only; m1 comes from Tycho)
  Coma in sky       : ≈4.6% of aperture flux → m too faint by ≈0.051 (1/ρ-coma estimate)

SENSITIVITY  (how m1 depends on the aperture definition; the coma has no sharp edge)
  Aperture × coma D : 1.0× 12.87   1.2× 12.57   1.5× 12.22   2.0× 11.78
  Coma edge level   : 5σ: D 1.29', m 12.87   3σ: D 2.58', m 11.77   2σ: D 3.84', m 11.16
  Use one definition for every night; report it with the observation (e.g. coma at 5σ isophote, aperture = coma).

FIXED ρ = 10,000 km  (light curve / Afρ; not m1)
  Aperture          : D 3.6 px  (r 1.8 px)   D 5.1"
  Sky annulus       : r 47 – 67 px   (least error for Tycho (stars +0.002, coma-in-sky +0.020 mag))
  Tycho, Diameter   : Inner 3.6 · Middle 94 · Sky 40
  m (ZP)            : 17.025   Tycho sky check +0.003 mag
  In ICQ, report this coma diameter, not the aperture: Coma Dia ≠ Aperture Dia for fixed ρ.
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Details
  Nucleus x, y      : 761.03, 759.57
  Pixel scale       : 1.430 "/px
  Δ                 : 5.43372 AU   [IMCCE Miriade (MPC O58)]
  Sky (plane)       : 440.00 ± 1.42 ADU/px  (85417 px used)
  Stars masked      : 1363 px
  Apertures compared:
    coma (m1)       : D =    54.0 px (77.2")     m_inst = -10.844
    fixed ρ         : D =     3.6 px (5.1")      m_inst = -6.686

Notes:
  • Sky gradient 0.633 ADU per 100 px removed by the plane fit (= 0.7σ across r_max). Check the result visually; non-linear gradients/vignetting are not removed.