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Old 15-05-2009, 06:27 AM
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pmrid (Peter)
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Bloats and Halos

Hi folks, I'm on the steep learning curve in astrophotography so do not offer the attached image as being anything more than a beginner's stumbling step. OK. That said, I need some advice on how to deal with the bloated star images I'm getting. Here is a fairly typical shot - it's unguided and with a rough alignment only - the Lagoon throuhg my home-made 5" refractor and a 1000D (unmodd'd) from about 10 x 30sec shots.
Advice appreciated.
Peter
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  #2  
Old 15-05-2009, 07:32 AM
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troypiggo (Troy)
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Not sure of what optics you've used in your home made refractor. Are they ED or apochromatic glass? I would have thought purple fringing/halos are a sign of chromatic aberration?
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Old 15-05-2009, 07:54 AM
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allan gould
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Peter
The 'star bloat' that you are seeing is due to chromatic abberation (CA) and is due to the inability of the lenses in your scope to bring all coours to the same focus. You may only see a small amount of blue by eye but when you do astrophotography and CA will become more prominent. The rest of your frame looks quite good, ie sharp stars except for the lower left hand side. This could be due to a slight mis-collimation issue ie the camera is not parallel to the optical axis, a fault due often to a misalignment caused by the focuser. Check that the camera is firmly held in the focuser and parallel to the face of the focuser.
The excessive CA you see in the shot MAY be tamed somewhat by 1) a fringe killer filter or semi-apo filter (expensive and mask your problem or 2) Noel Carnoni's actions for photoshop will reduce the (cheap $19.95).
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Old 15-05-2009, 08:08 AM
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jjjnettie (Jeanette)
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Peter, you can borrow my fringe killer.
PM sent with details.
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Old 18-05-2009, 10:29 PM
Prickly
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Looks like a nice refractor Peter. The CA is not too bad really and stars look pretty sharp across the frame. A UHCS filter (good for light pollution too) or a contrast booster might be even better than a fringe killer (more cutoff in the blue) - but the fringe killer might eliminate most of that.

You can do halo reduction also in post processing with some software like images plus.

Good luck and look forward to seeing more pikkies.

Regards
David
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Old 19-05-2009, 01:34 PM
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pmrid (Peter)
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I'm grateful for the replies and offers of help. Thanks you all.
Peter
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