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Hi Greg,

> Can you tell us more about sub pixel processing? I'm not so sure sub
> pixel processing is a very good way to add precision to angular
> measurements on a digital image. Better to stick with whole pixels.

The detailed pattern of	the star image does add	information, though,
and allows one to deduce position well into subpixel territory.  It's
a reliable technique.  For example, suppose we have a one-dimensional
CCD and the raw analog-to-digital converter values for a stretch of
pixels read 0, 0, 10, 20, 10, 0, 0 (after we've subtracted the sky
background, and suppose there is no noise).  Now we can conclude that
the star falls pretty much right in the center of the middle pixel.
If the values instead read 0, 0, 7, 20, 13, 1, 0, though, we can
conclude that the star is situated about one-fifth of a pixel to the
right of the middle pixel.  A star detector subroutine would have an
arithmetic technique built in to do this estimation, say by fitting a
Gaussian or a known point-spread function (PSF).

> Experiment with partial field neutral density filtering of the Moon
> when bracketing exposures.

Right, this could work, but it would require a custom attachment to
the camera.  Given the moon's small size it might have to sit
fairly far out in front of the lens, and the filter would have
to be accurately plane-parallel so as not to deviate the moon's
position, just like a sextant shade.  But this could be a good way
to go.  Would apply to the sun, too.

> Give black and white a shot to improve sharpness.

I guess in some sense I already do this, since my habit is to use
only the green pixels (1/2 the total pixel count).  This wastes all
the photons in the red and blue channels and undersamples the image
even worse than normal, but it does remove any chromatic aberration
issues and avoids the interpolation/demosaicing that could confuse the
analysis.  "Real" CCD cameras for astronomy have filter	wheels and
non-mosaiced CCDs, so they're already black-and-white, with your
choice of color	filter on each separate exposure.

Cheers,
Peter

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