Nikon D5000 Subframe.

Hi Guys,

I use a Nikon D5000 over a network to take astro-images and I control the camera with gphoto2.  I'm in the process of implementing an auto-focus routine using HFD (half-flux diameter), which is the radius of a circle containing half the flux of a star.  In order to get best focus you need repeated exposures downloaded over the network, which is quite slow as each photo is about 12 MB.  Is it possible to take a partial frame by specifying the position (on the sensor) and size of the subframe? So instead of the frame being 4,288 x 2,848, it could be 400 x 400 around a star?  Much smaller and less network load.

Thanks,

Steve.