Preventing -extent from converting to RGB

Hi,

I have a grey scale source image and I am applying a -rotate and an 
-extent operation. I'm finding that the output is RGB, presumably 
because of the background colour? Is there any way of getting the 
equivalent of -extent without the RGB conversion? (I know I can add a 
-channel to go back to greyscale, but it seems superfluous.)

Thanks
--Toby

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