Earthshine blog

We now have good radial profiles of Altair and Menkab. Altair was observed in very good conditions, and its profile is close to the expectation of r^-3 for the diffraction limit — in three of the 5 bands. In B and VE1 funny things are happening which we don’t yet understand. The Menkab profiles have halos like r^-1.5 — much broader. The plots below compare both stars:

Altair:

Menkab:

We think that Menkab might have been taken on a not-quite-perfect night photometrically. We’re looking into this, so watch this space!

Notes on the photometry for ALTAIR on JD2455845 (about October 11, 2001)
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Altair was observed with the B,V,VE1,VE2, and IRCUT filters. Dark frames were obtained before and after. Images had the interpolated dark field subtracted (i.e. the average of the closet DF before and after was formed and subtracted).

The IDL routine BASPHOTE.pro was used with a 9/9/12 annulus aperture to find the counts in each stellar image. The exposure time was read from the FITS header and was the actually measure exp time, not the requested one.

Fluxes were calculated for each filter and the standard deviation of the fluxes expressed as a percentage of the mean flux.

Results:

Band SD
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B 8.5%
V 8.4%
VE1 9.9%
VE2 4.5%
IRCUT 7.6%

Exposure times were in the range from 0.17s to 1.1 s for the various bands.

Some frame-dropout was noted – about 5 frames in 100 failed to open at all.

The variability in the flux is consistent with previous considerations – i.e. there is scintillation. The increase in variability with blueness may be because there is more scintillation in the blue or because exposure times are shorter in the blue, or both.