Earthshine blog

For reducing data so that the terrestrial albedo can be determined we have forward and inverse methods.

Inverse methods:

1) The BBSO linear sky-fit method. Yields a cleaned-up DS so that DS/BS ratio can be found and used for the inversion. Probably works best near the edge of the DS as the scattered light is not a linear function of distance except far from the BS.

2) Andrew’s clean-up method based on fitting a model image convolved with a variable PSF until it can be subtracted perfectly from the sky-part of an observed image. As a model of the Moon an observation – minus the sky part and the DS – is used.

Forward method:

3) A synthetic model of the Moon is used within a convolution framework to produce an image that ‘looks just like the Moon and its halo’. Since a parameter in the method is the terrestrial SSA it is directly determined in this way, but does depend on the same assumptions about time, geometry, lunar reflectance and so on as does the inverting methods. Depending on how much of the lunar disc is included in the fit there may be a difference in how the knowledge of lunar albedo and lunar reflectance influences bias.

Chris made this plot from two observing sessions of Jupiter. The first involved approx. 100 imags while the second (green) involved approx 1700 images.

Comments from Chris: The plot shows the psf around Jupiter. About
200 x 0.1 second exposures were averaged (black dots) and about 1700 a
few weeks later (green dots). T

Black dots: the psf can be traced out to log(r) ~ 1.5, Green dots: the
psf is very similar to the first one and can be traced out to about
log(r) ~ 1.8.

Note well: the background level of the sky has been adjusted be a small
amount — by not more than the uncertainty in the sky itself — to allow
the points at large to fall off with the same power law out to 100
pixels.

The power law fall off of the PSF is ~ r^-3 in the outer regions.

Signs of the non-axisymmetric psf are seen as extra dots above the main
sequence of dots. This is mainly caused by shutter bounce, leaving a smear of photons to the right in the psf.

The clusters of points at particular radii and sticking up are the Galilean
satellites.

The black line shows our current (Sept 29, 2011) best estimate of the psf from forward scattering of ideal lunar images and fitting them to lunar observations. It’s very close to the Jupiter PSF at large radii. This is the reason we think the
psf is falling as a power law all the way to the edge of the frame.

The departure of the inner part of the PSF around Jupiter from the black line is caused by the finite size of Jupiter — about 6 pixels across.

The inner part of the PSF is still not measured anyway. We are working on this!