Image processing menu

GAUSSIAN FILTER

Applies a gaussian filter (low-pass).The width of the gaussian used for the convolution is set by the Sigma parameter value. The convolution of an image by a Gaussian with a small sigma (less than 1) can be used to attenuate the noise.  A high value for sigma produces a fuzzy effect. A Sigma value less than 0.5 has no effect.

ADAPTATIVE FILTER

Adaptively  filters noise by using the "Minimum Mean-Square Error" method. The Noise parameter contains the typical value of noise in the sky background. This value may be obtained after drawing a rectangle about 30 to 40 pixels in width, avoiding the galaxy and stars, then clicking on the right button of the mouse and choosing the Statistics command. The noise is given in the standard deviation value Sigma. You can use also the the in-line command BGNOISE. The Adaptative filter command allows reduction of noise in the image while preserving fine detail.

UNSHARP MASKING

The unsharp masking is a high-pass filter.The Sigma parameter defines the strength of the filter that is used to generate the unsharp mask (same as for the gaussian filter). The Coefficient parameter defines the contrast of the final image. Typical values of Sigma=1 and Coef between 1 and 2. If you don't validate the Planetary option, a special version of the unsharp masking will be used: it is dedicated to deep-sky objects and suppresses the negative values and thus reduces the artifacts around bright objects (black voided rings around stars).

OFFSET

Add a constant value (positive or negative) to the current image in memory.

MULT

Multiply the current image by a constant value (integer or not).

CLIPMAX

All the pixels with an intensity greater than Old are assigned the value New. The Clipmax command allows you to control the maximum intensity of the pixels in an image.  It can be used, for example, when pixels with a high intensity may cause a calculation error in certain processes, or when you wish to reduce a 16 bit image to an 8 bit image.

CLIPMIN

All the pixels with an intensity less than Old are assigned the value New. The Clipmin command is practical for making the contents of an image strictly positive (Old=0 & New=0).

Remark: you may cancel any processing you have just done by clicking on the undo button  in the toolbar.