Peter took data on Menkab in 5 bands (October 7th, 2011) – B, V, VE1, VE2 and IRCUT. Surprisingly, the halo around the star follows r^-1.5 in all bands. Was the night a bit hazy? On a clear night we have been expecting r^-3.
The central PSF, within ~5 pixels, is very regular, as the stars are nicely round (Altair, with the best inner PSF to date, was a very boxy image — see posting on this below).
The VE2 image has a significantly broader transition region to the outer halo. Quite strange!
The 5 profiles are here :
The are compared to the profile we got for Altair in V band a few nights ago and our best guess profile in V from the modeling the scattered light from the moon.
The halo slope is about r^-1.5 in all 5 bands for Menkab. They also wiggle a bit, unlike when we get r^-3, which is straight as best we can tell. The “shutter bounce” effect is also much harder to see because, presumably, the flatter halo is washing it out. It’s visible but weak in the 2-d images.
This is all a bit surprising and very potentially very important. We may be much more dependent on the atmospheric conditions on each night that we thought.
It would be very good to do aperture photometry of the star as a function of time in each band for each obtained image, to check if the night really was “photometric”.