Combine Tab

The final step is to combine all of the images. At this stage, the image alignment is actually performed, and the resulting aligned images are stacked. Various methods are available for combining the pixels, ranging from a simple average to complex algorithms that reject outlier pixels caused by cosmic rays, passing airplanes, etc.

The following Combine Method options are available:

Some of these modes have additional options.

Normalization is available for Median, Sigma Clip, and SD Mask. Normalization is required because these three methods calculate statistics between the images; if there is any overall change in brightness or scaling it will bias the results. In Median mode normalization can simply be turned on or off; the normalization is performed by simple linear scaling. In Sigma Clip and SD Mask modes normalization can be Off, Linear, or Delta Level. Linear scaling is useful when the overall brightness level of the image is changing. Delta Level is useful when the image brightness is constant, but the background level is shifting.

The Ignore Black Pixels option causes any source image pixel that is zero to be excluded from the combine process. This is particularly useful if image alignment leaves areas where some of the images do not overlap. It is also very useful when dithering and using the Remove Bad Pixels command's Zero Pixels option.  The pixels in this area will be calculated from the remaining images, without being contaminated by zero values from the missing pixels. This option is selectable for all combine modes except Sum and Drizzle where it has no effect. It is recommended that this control be left checked.

Ignore Pixels Over is only available for SD Mask and Sigma Clip modes. This might be used if pixel values above a certain level are known to be "bad". A threshold level must be entered.

Number of Passes must be set for SD Mask. This determines how many cycles through the algorithm are taken. A higher number will produce a more accurate result but will take more time to calculate.

Sigma Factor must be set for Sigma Clip and SD Mask. A lower value rejects more pixels, whereas a higher value is more permissive. The value is typically determined by trial-and-error or user experience, further guided by monitoring the percentage of pixels at which Sigma Clip rejected an outlier or SD Mask used the median value. MaxIm DL reports this statistic in the Log window. A good starting point for Sigma Factor is 3.0 for Sigma Clip, and 0.5 for SD Mask.

For Drizzle, the last two controls described are replaced by two others. Drop Size specifies the reduction in size each input pixel undergoes before being "drizzled" into the output image. The smaller this value, the finer the detail that can be recovered, but the larger the number of input images required to avoid speckling artifacts in the result. Values of 50-75% are typical. Enlarge Image specifies the size of the output image as a percentage of the reference image and will usually be from 125% to 250%.

You can control how the results of combine operations are created using commands in the Options menu:

To start the stacking process, click the Go button on the Combine tab. If you have not manually performed any required Quality and Align functions, they will be run automatically at this time. This allows you to simply load images and click Go, if the settings are already configured.

NOTE: The FITS header for the result image will be a copy of that of the image that was selected as the reference, with the following exceptions: