magpar: Set magnitude scale parameters (pset).

Package: isophote

Usage

magpar

Description

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.

(mag0 = 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.

Examples

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

Bugs

See also

ellipse, elcursor