Layer Stacking is used to combine bands from different files into a single file containing all the input bands.  It works for datasets that are in different spatial resolutions and projections.  What do you mean by 'wtihout success'.  What were the results of layer stacking your two files? If you only want to visualize the dataset over another dataset, you do not need to Layer Stack the data.  You might try ENVI Zoom which allows you to open the datasets into one display and use transparency to 'see through' one image of a specific transparency to another.

OK - sorry for not being clearer.  When I tried layer staking it didn't work because the image that I want in the background is 3 bands (RGB) and the data is one band. I therefore am unable to display all three bands on the underlayer as well as the data layer with its pseudo color applied.  If I save the data first (with the color table applied) it then also becomes three bands, so I have the same problem with layer stacking as before. Perhaps I am misusing layerstaking?   I have never used ENVI zoom, but initial tests appear that the colors become distorted . I shall continue to investigate this route.  

Hi hethomas, It sounds like you're expecting layer stacking to do something other than what it was designed for.  As Mari indicated, the purpose of layer stacking is to line up multiple images so that they have the same orientation and pixel resolution.  The purpose is not to put two images into the same display.  But there are some tools in ENVI that you can probably use to view your images on top of one another.  As Mari said, ENVI Zoom is set up to have multiple display layers (and each can have multiple bands loaded into the different color guns).  In ENVI Zoom, you can set up the top layer to have transparency, so you can see the bottom layer below it.  There's a transparency slider bar at the top of the interface for this.  It sounds like that may be what you want?  If not, can you give us more details about what you want to see? If you don't want to use ENVI Zoom, then you might want to check out ENVI's Dynamic Overlay capability.  To get this going, you load your two images into two separate ENVI displays and adjust the displays however you would like them (adjusting the contrast stretches, color table, etc.).  Then from one of the display menus, choose Tools > Link > Link Displays.  In the resulting dialog, make sure your two displays both say "Yes", and that Dynamic Overlay is On, then click OK.  Now if you click your left mouse button in the Zoom or Image window of one display, you'll see the contents of the other display from the same location.  You can use the middle mouse button to draw a box  in the display that will contain the dynamic overlay.  If you right-click, then you can choose Adjust Transparency and set the transparency of the dynamic overlay. I hope this helps.  Let me know if I've misunderstood what you're trying to do. Peg

Thanks for your ideas Peg - I have tried them both out but not yet figured out a solution! The problem with using the link displays is that the data sets are at different spatial resoltuions (even though they are the same image) as one set comes from ASTER TIR (90m), the other from ASTER VNIR (15m).  Therefore ENVI won't allow me to link them.  I had a look at ENVI zoom - the problem here being twofold - the first is that having applied an ENVI rainbow colour scheme to my data, ENVI zoom appears to change the colours. Also, I only want to overlay the actual data, and not the black background where there are no data.  There doesn't seem to be anyway to get rid of this in ENVI (even with the ROI tool, there are still black spaces to make the image up to a square/rectangle).  Perhaps I am expecting too much of ENVI!? Helen

Oh, I see.  Well, you can always try georeferencing your ASTER data first, using the tool found under Map > Georeference ASTER > Georeference Data.  Then once you have the TIR and VIS georeferenced, you could use the Geographic Link tool (from the display menu under Tools > Link > Geographic Link).  That might give you more of what you are looking for.  In terms of ENVI Zoom, it will display the data according to its default rules, but you can always change that.  I wasn't sure, though, whether you had a rainbow color table applied to your display, or maybe just a three-band color composite with a particular stretch that gave the colors you wanted to see.  If it's the former, I think you're out of luck in ENVI Zoom.  I don't think it has a way to apply a color table to a layer at this point.  If it's the former, you can choose the Data Manager icon to specify your own RGB composite, and the Interactive Stretch icon to customize the contrast stretch for each of the three displayed bands. In terms of not seeing the border, I couldn't find a way to give Zoom a value to ignore in the display, but you can set up portals, which are pretty handy.  Try loading your data, then clicking the Portal icon (near the upper left - it looks like a black and white box inside a black and white box with a diagonal line through it).  The Portal essentially punches a hole through the top layer so you can see what's under it.  You can grab it and pan and zoom it, etc. I hope this helps. Peg