magpar -- Pset with parameters that define the magnitude scale.
For the sake of convenience, the ellipse task output includes intensity-related quantities such as isophotal, ellipse and circle-enclosed intensities, expressed in a magnitude scale. This scale is derived from task parameters mag0, refer, and zerolevel, and from the corresponding intensity output parameter, according to the following relation:
m = mag0 - 2.5 * log10 ((intens - zerolevel) / refer)where mag0 and refer are used to set the scale's zero-point, and zerolevel can be used to subtract any bias-type offset that applies to all pixels in the image.
Notice that this magnitude output is provided basically for convenience in visualization (e.g. by tasks isoplot and isopall), and not intended for subsequent detailed analysis. In particular, negative intensities (from, say, a sky-subtracted image) result in magnitudes being computed by:
m = mag0 - 2.5 * log10 (1. / refer)
Any type of pixel data can be input to the fitting algorithm in ellipse: linear intensity, magnitude, photographic density, etc. However, correct intensity and magnitude scales, and harmonic amplitudes, will result only in the case of linear intensity pixels.
- (mag= 0.0) [real]
- Magnitude of the reference source.
- (refer = 1.0) [real, min=1.E-5]
- Intensity of the reference source.
- (zerolevel = 0.0) [real]
- Intensity of the zero (bias) signal.
An image has an average sky level of 120 ADU/pixel, and it is know that the sky brightness is 22.5 mag/arcsec^2. The detector adds a 100 ADU bias level at every pixel. A magnitude scale roughly calibrated in magnitudes/arcsec^2 can be defined by setting
magpar.mag0 = 22.5 magpar.refer = 120.0 magpar.zerolevel = 100.0