In considering B-V images, as here we are have to know how far the BS halo reaches onto the DS. We have looked at that before, here. We now revisit this issue. Here is a contour plot of the DS and the halo, with colours so that we can see how far the halo is likely to reach – we see it reach into the sky on the BS – how far do we think it reaches onto the DS?

Consider the yellow end of the greens, for instance – on the sky that contour lies 40-50 pixels from the BS rim – on the DS it lies adjacent to the red area which is the BS. The yellow-green contour therefore does not interfere much with most of the DS. The blue contours on the DS, howvere, are represented on the sky far away from the BS – so the blue areas on the DS may be interfered with by that part of the halo.

The above is V-band image. In B-V images we rely on much of the halo being similar in B and V and thus cancelling. The above noticeable gradient in DS brightness is absent in the B-V image.