Without seeing your image, it is hard to tell you what exactly to do. 
Can you provide a link to your image (you cannot upload the image 
here, but you can host it elsewhere and provide a link to it).

Nevertheless, there are three approaches.

1) threshold the image to change the background to 0 (or to create  a 
binary mask and use the mask to composite wih the image to isolate 
the ellipse)

see

http://www.imagemagick.org/script/command-line-options.php#threshold

http://www.imagemagick.org/script/command-line-options.php#black-threshold

http://www.imagemagick.org/script/command-line-options.php#white-threshold

2) draw an ellipse using -draw to create a mask

see http://www.imagemagick.org/Usage/draw/

3) using some other application, interactively outline the ellipse to 
create a mask.



see http://www.imagemagick.org/Usage/compose/
and http://www.imagemagick.org/Usage/channels/

for compositing



>Hi,
>I am very new to imagemagick
>
>i have a png image that i can load in imagemagick
>
>now that png image contains a elliptical object and i need to extract that
>shape only by removing all the other intensity values to 0 or 1.
>
>how can i do this?
>
>thank you
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