We introduced in a previous tutorial the Aladin stack, which is the pile of individual planes that have been loaded during your Aladin Desktop session. You can click and drag the plane icons in the stack to change their order.
This can be useful, because the view in the main panel is looking through the stack, from the top.
There are sliders at the bottom of the stack for adjusting respectively the opacity level, the size of objects, or the zoom factor.
If you want to view the 2MASS image with some transparency on top of the DSS2 image, make sure the 2MASS plane is on top of the DSS2 one, select the 2MASS plane, and use the opacity slider. You can also use the mini slider on the trapezium icon.
If you want to view the DSS2 image on top of 2MASS, drag one of the planes to change their order, set 2MASS as a reference plane, and change the opacity of the DSS2 image. You can also use sliders to change the size of catalogue planes symbols.
And if there are proper motions measurements in the plane, use the epoch slider to view the positions at different epoch. If you use the multi-view mode, you can click and drag planes from the stack to assign them to one specific panel.
You can organize your stack in folders, via the popup menu. Right-click and create a stack folder. You can then move stack planes to a folder, and possibly restrict the scope of some catalogue planes to images in that folder. The 2MASS catalogue is now only visible on top of the 2MASS image.
Leaving the mouse pointer over a plane's icon/name, you display additional plane information on the top of the stack panel.
There is an option to backup the stack, which should save most of the current state in a local file, and allow you to reload it later in Aladin. Feel free to leave your own tips on using the Aladin stack as a comment below.
Download Aladin desktop : http://aladin.u-strasbg.fr/java/nph-aladin.pl?frame=downloading
SIMBAD : http://simbad.u-strasbg.fr/simbad/
VizieR photometry : http://vizier.u-strasbg.fr/vizier/sed/