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"Earthshine blog"

A blog about a system to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Lamps on the Antenna

Exploring the PSFPosted by Peter Thejll Dec 29, 2011 03:19PM
As the night was fairly clear, I put the telescope on the lamps on the Antenna (Alt/Az: 21*34'/256*25').

As the several lamps on the Antenna seem distinct it is at least not an 'extremely foggy night'. I took V-band exposures at about 1 second.

Then I extracted the profile from the 25 coadded images. Only the quadrant below and to the right of the lower of the two sources above was used:

To about 20 pixels we see the actual lamp (i.e. the glass enclosure and filament). From about 30 pixels to short of 100 pixels we see the halo dropoff. The red line is a 1/r^(2.8) PSF.

This is contrary to what Chris found using the Moon and the occulting balcony! Unless the halo we see above is built up in the few hundred feet between the lamp and the telescope it must be due to the optics in the telescope. We cannot rule out that there was some fog, but the size of the exponent (2.8) indicates a 'clear night', I think - or is that circular thinking?

Anyway - it is not impossible that both optics and 'air' scatter in the same way.

Wonder if we can detect any examples where there is one halo from the optics and another from the atmosphere?

The above is not an occulting experiment.

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Posted by Chris Flynn Dec 30, 2011 01:10AM

Really interesting!

This suggests that the halo we see in the telescope is due to the optics and not atmosphere.

All my moon results with the 35 mm camera only pertain at large angular distances from the source -- from about 3 degrees out to about 15 degrees.

I have been wondering what happens closer in -- hence the laser experiment.
I get a quite steep slope for the halo with the laser -- powerlaw index of -2.6!

There is a kind of flat pedestal around the point source which means I can't tracer the power law in very close. The inner limit is about 20 arcmin from the source. Actually, that's not bad at all, since the moon is 30 arcmin.

In other words, the laser shows that the halo in the 35 mm camera can be traced with a power law of -2.6 -- that's not far from the ES telescope and the lamps (-2.8) the diffraction limit (-3).

This seems to be telling us some very interesting things!!

On large scales, the atmosphere will eventually dominate. On small scales, the optics dominate.

I think we should try to measure the sky brightness falloff from the full moon using the ES telescope by stepping away from it half a degree at a time and simply measuring the sky flux. We could then follow both scattering laws... :) hopefully!

Posted by Chris Dec 29, 2011 03:45PM

excellent! looking forward to seeing this! Will try this with the Canon camera too...