How to grab the backbuffer *with* alpha

Thanks skidracer, but that doesn't seem to work.
In all my tests the alpha byte is always $FF.
Changing the last argument of SetClsColor2 seems to have no effect on the backbuffer.
Here is a small example:

Const Image$ = "good1.tga"

Graphics 800,600,32,0

Function SetClsColor2( red,green,blue,alpha=255 )
	glClearColor red/255.0,green/255.0,blue/255.0,alpha/255.0
End Function

Global TestImage:tImage = LoadImage(Image$)

SetClsColor2(0,0,0,0)
SetBlend(ALPHABLEND)

While Not KeyHit(KEY_ESCAPE)
	Cls
	SetAlpha(0.8)
	SetScale 0.5,0.5
	DrawImage(Testimage,MouseX(),MouseY())
	
	If KeyHit(KEY_BACKSPACE) Then
		saveTGA("Testsave.tga",GrabPixmap(0,0,800,600),800,600)
	End If

	Flip
Wend
Function SaveTGA(Name$,PM:tPixmap,W,H)
	Local f,width%,height%,x%,y%
	Local pix,IR,IG,IB,alpha
	f=WriteFile(name$)
	WriteByte(f,0) 'idlength
	WriteByte(f,0) 'colormaptype
	WriteByte(f,2) 'imagetype 2=rgb
	WriteShort(f,0) 'colormapindex
	WriteShort(f,0) 'colormapnumentries
	WriteByte(f,0) 'colormapsize 
	WriteShort(f,0) 'xorigin
	WriteShort(f,0) 'yorigin
	WriteShort(f,w) 'width
	WriteShort(f,h) 'height
	WriteByte(f,32) 'pixsize
	WriteByte(f,8) 'attributes
	For y=H-1 To 0 Step -1
        	For x=0 To w-1
			pix = ReadPixel(PM,X,Y)
			WriteInt(f,pix)
		Next
	Next
	CloseFile(f)
End Function 

loading the saved image in photoshop or a hex editor shows that no alpha was preserved from the back buffer.
Is there any way at all to get alpha from the back buffer?????