adjusting alpha channel levels

[enlait]

I have an image I want to edit. It is a png tileset for a game, with opaque icons with semi-transparent background (yep the result in game is just as annoying as it sounds).

I'm not satisfied with background and want to make it more transparent (or maybe completely transparent). How do I do that?

There are adjustment tools in PDN that can modify separate channels; AFAIK alpha is technically identical to color channels but these tools only let me edit RGB.

I couldn't figure out how to specify a layer as alpha mask either.

[Ego Eram Reputo]

To make the background fully transparent, use the Magic Wand tool

Select the (partially) transparent background and press delete. NB you may need to play with the Tolerance to get the whole background selected and not the icon (Hint: Tolerance slider is in the Tool Bar when the Magic Wand is selected).

Resave or Save As.. with the PNG filetype (because PNG preserves transparency).