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Walking on the Moon

Funny Flats


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I have just come back from the SGL11 star party and have been looking at my images. They all have a number of faint doughnut shaped blemishes on them, luckily they are on the flats too, so they should come out in the processing.

I was wondering if any one has seen these before (see attached flat frame).

Does anyone know what they may be caused by?

I think they may have been caused by dew evaporating from the IDAS LP filter (I had a problem with this until I started using a dew band for the camera recently). If this is the case, are there any tips for cleaning the surface of the LP filter?


Cheers in advance,




Rings 1.tif

Edited by reddoss
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This would be, I believe, classical case of dust on some surface in optical train. A bunch of little particles out of focus. One of the reason you are taking flats is to calibrate these out.

You can actually calculate where in optical train this is happening (filters, camera protection window, etc):



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