The next eshine telescope

The next eshine telescope

Design ideas and tests for a new generation of automatic earthshine telescope

What did we learn from the first try?

More on testing DSLR RGB halos

CCDPosted by Peter Thejll Sat, October 19, 2013 12:22:24
We test Hans' 450D optics by imaging the Full Moon (t_exp = 0.02 s, f=5.6, focal length=300 mm (i.e. tele-lens)). Using IRIS to extract the R G and B fields we align, normalize and average these and plot the profile from Moon disc centre and out into the halo. We get:


The red, green and blue curves represent the halo in R G and B, respectively. The black line is a 1/r³ curve. R, G and B images normalized by their total flux before plotting.

It would seem that, as in the case of the image found on the internet, the profiles of R G and B are very similar, and also of 'high quality' - i.e. they are almost at the limit of what is possible in terms of dropoff slope.

This has bearing on choice of future cameras - CMOS or filtered CCD. I think the similarity of the profiles is due to the light having traversed the same optics - which is not the case with the filter-wheel.

I am not sure why the profiles are so close to 1/r³. It seems our optics in the MLO telescope cannot match this.

It should be said that the DSLR optics show ghosting - this can interfere with the DS photometry if unluckily placed - but apparently does not gives us shallow halo profiles.

The ghost image (same size as Moon, positioned at 11 o'clock) is due to internal reflections in the compact tele-lens used. Perhaps an image of the Moon is reflected from the CMOS surface, reflects off the back of the optics and is recorded. The above ghost would be very intense compared to the DS. Similar ghosts are seen in the wide-angle lens image taken with the Sigma camera, but as magnification is less the ghost is easily placed away from the Moon.

One thing to notice is the 2-orders of magnitude of drop in intensity from the disc brightness to the start of the halo - is this what we get with our MLO telescope? Perhaps the 1/r³ drop is an artifact of a high starting level? Need to check that.











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Posted by Peter Thejll Sun, October 20, 2013 09:42:49

OK! So the drop from lunar disc to start of halo is also 2-orders of magn. for CCD images. The plot I show here, for DSLR image, is not on the same scale as your profile plots. I will work on a more detailed view of the 'inner part' of the DSLR-PSF and see if we have the same differences there between G and B, say.

Posted by Chris Flynn Sun, October 20, 2013 04:46:16

In this post:

http://earthshine.thejll.com/#post317

I did something similar for the DSLR here.

The approx 2 order of magnitude drop off is the same as you are getting.

The problem of not really knowing our PSF well near the disc edge is coming to haunt us again here... hopefully progress on that soon!

- Chris