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Earthshine blog

"Earthshine blog"

A blog about a telescopic system at the Mauna Loa Observatory on Hawaii to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Lamps on the Antenna

Exploring the PSF Posted on Dec 29, 2011 15:19

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.

Laser PSF in Canon 35 mm

Exploring the PSF Posted on Dec 29, 2011 12:54

As a follow up to the knife edge imaging of the moon with the Canon 35
mm camera, I took images of a laserpointer shining into the lens.
Laserpointer was set on with tape and shone into the camera lens from a
distance of about 5 meters. Camera was on a good tripod. The laser
pointer was aligned to be pointing into the lens by watching to see it
emerge from the back of the camera out the viewfinder and onto a wall
behind the camera. Eyes were kept well away from everything!
Exposure times of a few hundredths to a few thousandths of a second gave good halos.

Laser was pretty well centered. There is a weak secondary image off to lower right. The main image is saturated in the core out to about 10 pixels radius.

The laser has a strong core surrounded by a quite uniform intensity “platform”, after which the light falls away like a power law.

The radial profile of the laser (blue) compared to the moon (green) is shown below.

The laser pointer’s profile falls off faster than the moon — but not very much faster.

If we assume that the laser is a good point source, then there is scattering in the lens/CCD combination in the camera which is seen as a power law at large R.

Slope of the lunar halo is about -2.0, whereas the slope of the laser pointer halo is about -2.6. Getting close to the diffraction limit of -3!

The lunar profile is then interpreted as a combination of both scattering in the camera and scattering in the atmosphere. Its slope is shallower — so atmospheric scattering totally dominates at large R (scale both powerlaw falloffs to the same flux at log(R)=2 or 100 pixels to see this — atmospheric scatter would then dominate internal lens scatter by about an order of magnitude out at R=1000). Not sure what’s happening close in, as the laser pointer has a strong uniform intensity “platform” of light around the core — easily seen by eye just by pointing it at a piece of paper. A point like source of light, like a street lamp seen at a large distance might be a way around this problem.

I also imaged the laser pointer projected onto a white wall — long exposure times of many seconds — and got the same result — similar core and platform and a halo with the same slope.

All very interesting !


Observing log Posted on Dec 29, 2011 09:23

At sundown T=8.8 wind dropping, 10-14 m/s, RH about 20%. 2 hours later outside T was 5 degeres C while dome T was still 7.

Shutter failing often – and ‘open while reading out’ occurring. Again we see sets of images failing. Of course not related to the physical filter itself – but then why e.g. all VE1 images failing while adjacent images OK?

No clouds. halo starting to reach DS edge towards sky.

Noted that an antenna gets in the way of the Moon during some Moonsets.