A strange structure has been found in some reduced images. To the right we show a dark frame taken seconds before the image on the left. The image on the left is an EFM image – i.e. the halo from the BS has been subtracted after fitting to the sky areas of the image. We clearly see the structure in the left image – a ‘band’ stretching to the left of the DS. This structure is not present in the dark image – so it is not an artefact of bias subtraction [Of course – the bias subtraction is not performed with adjacent dark frames – noise would be added that way – rather, a scaled almost noise-free superbias is subtracted. It is like a lower-noise replica of the image on the right – i.e. no structure, just mild level of noise.]. Below the two we show a plot along a vertical axis in the two images: columns 50 to 150 were averaged and shown in the plot as the black and the red lines.
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There is a very clear structure in the sky of the Moon image. Given that it is not induced by bias-subtraction it must be due to the presence of the Moon itself. We speculate that:
a) it is some optical effect – reflection – from the inside of the camera or telescope. Halo-subtraction has reduced the sky level to almost zero but a little too much has been subtracted in the ‘dark band’ and a little too little has been removed outside the band. The fitting of a rather smooth ‘halo’ from the observed image could give this effect, if the structure itself is present in the image.
b) some electronic effect is causing the rows with the very bright BS in to somehow ‘jump low’ due to some effect we do not understand. NB: The readout direction is ‘down’ – not to the left!
Note the presence of what looks like a truncated halo to the left in the image, at the frame edge: this could be an internal reflection showing the right hand side of the halo being reflected inside the camera. We saw this same effect in the animated sequence of the tau Tauri occultation – as the Moon migrated to the RHS of the image frame an ‘echo’ appeared on the left.
This therefore seems to support idea a) above. If this is the case we may have an effect on solution-quality from position of Moon in the image. We should investigate if there is a position nearer the centre that eliminates the effect, and then omit images that are too close to the edge.
My first thought was a reflection — just as you speculated (or “re-speculated”) – I have seen similar effects in images I have reduced as well. If we are getting internal reflections within the telescope and causing this, then the centering of the telescope is going to be rather important I suspect — we might want to analyse some images and figure out how badly we do the further off center the moon is. A good project to include for a student as part of the next round of grant applications (I am starting to put together a list of student type projects for that).