anchor.diff:              //let stars = ursaMajorStars
               let dirs = stars |> Array.map (fun s -> V3f(Conversion.CartesianFromSpherical(s.RArad, s.DErad)))
       
  +            let intScalePerMag = float32 (Fun.Pow(100.0, 1.0/5.0)) // intensity scale per magnitude ~2.511
  +            let i0 = 0.0325f // luminance of star with mag 0.0 / ~luminance of vega
  +
               let colors = 
                   stars 
                   |> Array.map (fun s -> 
  -                    let mag = s.Hpmag
  -                    let intScalePerMag = float32 (Fun.Pow(100.0, 1.0/5.0)) // ~2.511
  -                    let i0 = 0.0325f // luminance of star with mag 0.0 / ~luminance of vega
  -                    let l = i0 / (pow intScalePerMag mag) // mag -26.7 -> 1.6e9 (per solar disc size), mag -12.7 average full moon -> 2.5e3
  +                    let l = i0 / (pow intScalePerMag s.Hpmag) // mag -26.7 -> 1.6e9 (per solar disc size), mag -12.7 average full moon -> 2.5e3
                       // luminance the star would have if sun was 1px -> scale with viewportSize and fov (currently in shader)
                       C4f(l, l, l, 1.0f)
                   )
  
positive.code:     
                  let lum = geoInfo.JulianDayUTC |> AVal.map (fun jd -> 
                      let rc = Astronomy.RectangularHeliocentricEclipticCoordinates(p, jd)
                      let distAu = rc.Length
                      let distKm = 149597870.7 * distAu // km
                      let sunRadiusKm = 695700.0 // km
                      let sunLuminance = 1.6e9
                      let a = atan (sunRadiusKm / distKm)
  
negative.code:                     let s = starDict.[hip]
                      if s.Hpmag < float32 t then 
                          // distance = size
                          // TODO/ISSUE: angle offset would get rotated by star trafo
                          let t = Trafo3d.Scale(0.15) * Trafo3d.Translation(10.0 * Conversion.CartesianFromSpherical(s.RArad, s.DErad))
                          Some (AVal.constant(t), AVal.constant(str))
                      else 
  