Earthshine blog

Sep 29 — Peter and Chris observed Markab — noticed much broader halo around the stars than usual. Image below shows that this can even be seen in the sky camera.

Left: clear night from 2011-09-27, Jupiter in center of frame. Right: not photometric conditions 2011-09-29, Jupiter has a slight halo. Note that the bright “star” right above Jupiter is possibly an internal reflection (of Jupiter?) in the sky camera.

We will observe Jupiter through whatever this stuff so we can compare directly to previous determinations of the PSF.

The plot shows astigmatism of the psf across the image plane. A short single exposure of the open cluster M7 was used, exposure time = 60 seconds. Ellipticity is shown by the length of the lines in the lower plot — and the lines are aligned with the position angle of the sources. The upper plot shows that ellipticity rises pretty sharply towards the CCD edges — from about 0.05 in the center to about 0.4 at the edge. n.b. ellipticity is defined as e = 1 – b/a, where a is the semimajor axis and b the semiminor axis. Ellipticity at the center of the field is about 0.05 – not 0.0. These results are for the ellipticity of the inner core of the PSF (it’s measured out to 3 pixels radius). Whether the power-law halo of the PSF is non-round on any scales is still under study.

On night of Septembter 10, 2011, 28 images on the almost full moon were obtained at 0.015 second exposures. We compute the mean level of these images. They are dominated by the full moon, the sky flux is negligible, and the moon is well centered.

The mean count rate for these frames is 5546.7 counts / 0.015 sec and the standard deviation of the mean counts over all 28 frames is 8.4 counts – if this standard deviation is all due to shutter timing error for these frames, the timing error is ~1.4%. On these short timescales the shutter timing is at least this accurate.

Longer sequences with bias frames in between the lunar exposures would be good. None of the frames dropped out, but we know that this can happen occasionally, with as much as 20% of the frames dropping out.