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Thanks all.
Andrew, I set the exposure to "auto" when taking the flats, that way correct exposure will happen, and I guess the brighter the screen the faster the shutter speed for the flats, as long as screen is evenly illuminated it should be okay. The actual flats look greenish, thats because of the UHCS filter.
Terry, I have two 2 inch filters, an IDAS uv/ir filter, and a Baader UHCS filter
screwed together. They sit inside the canon mount that in turn screws onto the back of the lens. There is a slight amount of play, I can hear the filters rattle slightly when I pick up the lens, Im thinking that this may cause them to not be completely flat, causing the abberations around the bright stars. If I use no filters at all I get star bloat in bright stars but nice even round ones, as my 350D has no internal filter at all, just clear AR coated glass, because I wanted to experiment with IR photography.
The above is what I think is happening anyway.
Scott
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