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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.

Ghost and Dragging

Optical design Posted on May 21, 2013 07:58AM

Here are example sof a ‘ghost’ and ‘dragging’:

The dragging is due to no shutter being used – i.e. readout was only way to terminate exposure and hence frame was illuminated as the image was shifted to the ‘hidden register’ We hav a frame-transfer camera. The image is from the test phase in Lund and no shutter was installed then.

The BS ghost is faintly vissible: It is a copy of the BS and the lower cusp is seen poking out slightly down and to the left of the real BS. This was due to the CCD camera being aligned almost perfectly along the optical axis of the system – the ghost arises as the shiny CCD surface reflects light back into the optical system and reflections are produced at all optical surfaces – the back of the second secondary lens, the front of the second secondary lens, the back of the first secondary lens, the front of the first secondary lens and the back of the primary objective and its various surfaces. At MLO the camera was tilted at an angle so that the reflection from the CCD did not go back up the system. This tilting did not appear to have any adverse effects on focus etc.

Images from lab hard disc recovered from MLO.

More on SKEs

Optical design Posted on Jan 26, 2013 10:06AM

In this post:
the importance of the SKE for scattered light was discussed. The images shown, though, were not shown fairly – with intensities scaled to comparable levels. I therefore extracted a line across the BBSO image and a line across our image, at right angles to the SKEs, rescaled the intensities, aligned the plots and get this:
The black curve is from our image (whichis a sum of 10 well-exposed images). The red curve shows the cut across a single BBSO image.The BBSO image has only the DS peeking our behind the SKE, while our image has the BS in full view.

We see entirely comparable ‘halos’! The BBSO image ha a more pronounced sloping ‘tail’ onto the black side of the SKE than we do, and more noise. If that sloping tail is ‘halo’ we had a better system than the BBSO!

What does the above mean? It does NOT mean that we have less ‘halo problems’ than BBSO does – because the BBSO expose their DS so that the halo from the BS is not allowed to be formed. Yes there is a similar halo from the DS on their images as there is from the BS on our images – but the halo from our BS is very much stronger than their DS halo.

When the BS halo is small – i.e. near New Moon – we have minimal effect of the BS halo.

Telescope spectral report

Optical design Posted on Dec 30, 2011 09:38AM

This is Ahmad and Rodrigo’s report on the spectral properties of the telescope:

Colour filters

Optical design Posted on Dec 27, 2011 02:52PM

The filter transmission as measured by Rodrigo for the B, V, VE1 and VE2 filter up to 800 nm and the IRCUT filter for a much wider range as given by the manufacturer.

ND0.9 filters do nothing

Optical design Posted on Dec 21, 2011 08:43PM

On the lamp in the dome we can test the densities of the various ND filters by calculating and comparing the observed fluxes with and without the filters. So far I have tested most of the ND0.9 filters. They appear to be blanks – or have not rotated into position.

Filter Without ND With ND0.9
B 3511 3512
V 26500 26500
VE1 210700 209700
VE2 268200 273000
IRCUT 196050 no data

The numbers are in counts/second and are based on the measured mean counts in the various frames and the exposure time MEASURED and reported in the FITS header.

The Spur

Optical design Posted on Oct 18, 2011 10:21PM

Ahmad pointed out that ‘the spur’ seen in coadded images of stars, and interpreted as an effect of the shutter staying open while readout is performed, is due to something else – not readout; because the readout direction is at right angles to the spur.

Diffraction-limited optics

Optical design Posted on Oct 03, 2011 02:03PM

According to theory, the aberration for a lens in a circular aperture, like ours, is given by the Airy function which is proportional to (BESELJ(r,1)/r)^2.

For a single wavelength the function looks like this:

Theory also predicts that assymptotically the envelope of the Airy function should go as 1/r^3, which is also plotted on the graph above.

For mixed-wavelengths as in our case the curve becomes smooth(er) and we should expect to see our PSF as a 1/r^3 envelope. Steeper envelopes than this should not be observed in correctly analysed optical systems. This has bearing on the 1/r^2 ‘King profile’.

Scattered light analysis

Optical design Posted on Mar 18, 2008 10:34AM

Here is Mette’s analysis of the BBSO telescope vs. the Lund design, in terms of Ghost intensities and scattered light from front lens surface.