For this tutorial, you will need a folder of raw images to stack. We will demonstrate stacking LRGB images; however, a similar process works for monochrome, one-shot color, and RGB image sets.
For best results, capture the images using MaxIm DL, so that the FITS headers include important information including the filter selection and object name.
Select the Process
menu Stack command.
On the Select tab, turn on Classify by OBJECT and FILTER.
Click the FILTER
button to set up the filter mapping. This will determine what filters
are mapped into which LRGB color channels. To change a row, click once on the row,
then click once on the Filter Color item, and enter a new
value. This will make sure that your filters are automatically assigned
to the correct group.
If your images have not yet been calibrated, turn on Mark added images as Auto Calibrate. Each image will automatically be calibrated as it is needed, using the settings from the Set Calibration command.
Click the drop list on the multi-function button and select Add Folder. Browse to a folder on disk where your images are located. This folder may contain images of several different targets, as long as the OBJECT FITS keyword is set in each image (during imaging, this is automatically obtained from the Observatory Control window if a telescope is connected).
The Tree View on the left side of the Stack dialog will now show all the image files you have selected, arranged into groups according to their FITS header. You can open the groups to view the files by clicking on the + sign. If you would like to view an individual file, right-click it and select Display Image. Alternatively, using the right-click menu select Auto Display, and any image you click on will be automatically displayed.
If necessary, you can move images or entire groups by dragging them. You can set properties for groups or individual files using the right-click menu.
Select the Quality
tab. If you would like to eliminate images with out-of-round stars
caused by bad tracking, turn on the Roundness
check box. The Threshold should
be set to limit how far out of round the star can be; for example,
0.1 will throw out any image with stars that are more than 10% out
of round. Click the Measure All
button, and any bad images will automatically receive an X in the
enable box. You can manually override this decision if you like, by
changing the enable box.
Select the Align
tab. If your images have been solved using PinPoint
Astrometry you may wish to use the Astrometric
align mode, which is extremely accurate. Alternatively, the Auto Star Matching mode is fully
automatic and quite accurate. There are a variety of other alignment
modes available, including manual alignment.
You can select one of the images to be the Reference Image, using the right-click menu. This image will not be shifted during alignment; all other images will be aligned to it. If you do not select the reference image, the first image in the list will be used.
Select the Color
tab. If you wish to customize the color balance settings you may do
so; otherwise click the Defaults
button, which assumes 1:1:1 color balance. Also turn on Auto
equalize background to remove any background color cast due
to sky glow.
Select the Combine
tab and set the Combine Method.
For large numbers of images we recommend using Sigma
Clip; a Sigma Factor
of 3 is a good starting point.
If you have a modest number of images the SD Mask mode will produce
a better result (but will take more time); a good starting point for
the settings is a Sigma Factor
of 0.5 and a Number
of Passes of 3.
Be sure to turn on Ignore Black Pixels. If there are any areas where the images do not completely overlap, this will ignore the contribution from the missing data. Otherwise you may get strips across the edges of the image that look dimmer.
Set Normalization to Linear and set Area to 50%. This setting assumes that any changes in intensity between the images are due to extinction rather than changes in background level. If you have strong background level changes you may want to use Delta-Level instead.
Select whether you want the results created as new images or written to a disk folder using the commands in the Options menu.
Click Go to start the stacking process. This may take some time; you can interrupt and restart the process if needed. If you have multiple image groups, they will be stacked separately. Color sets will automatically be color combined. To view results written to a folder, you must close the Stack command before opening them with the Open command. Alternatively, you can simply drag and drop them from Windows Explorer onto the MaxIm DL window.
Tip: Once you have all the settings the way you like them, you can simply dump in the images and click the Go button.
Tip: You can also drag-and-drop images onto the Tree View from the Windows Explorer, or select files individually.
Tip: When you use the Add Folder option calibration files are automatically ignored; the other methods for adding files will include them. Usually you don't want to stack the calibration files automatically, since the Set Calibration command does that for you with proper normalization, etc.
Tip: This tutorial has barely scratched the surface; this command has many more capabilities. For more information, please refer to the Process menu Stack section of the Command Reference.