Thread: Flats again!
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Old 22-04-2011, 01:02 PM
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pvelez (Pete)
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Join Date: Aug 2006
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Quote:
Originally Posted by richardo View Post
Hi Pete, yes finicky things flats for sure and I've had similar dramas of over correction when I started doing them as well so your with some company here!

*First off, take your flats in the same binning as the images you are correcting.. also, you need to take an individual set of flats for each of your filters.. ie.. R,G,B, L, Ha.. etc
*Do flats at 2/3 of your full well capacity... for the Kodak8300 chip with capacity of 25500, look to go around 17000.
*Make sure you are doing the flats at the same temperature of what you've been taking your light images with.
The best way is to set up a 'flat' run in maxim, and allowing enough time in between exposures so your chip can cool back down to the temp it needs to be.
*I use about ten flats.. it works, but more is probably better.
*Remove a bias master from all your individual flats.
*When you do your masters, make sure you set up your 'pedestal' in MaximDL.. this I found did really help.
You'll find this in the 'set calibration' window.... hit the 'advanced' button, should be set 3xs the 'standard deviation' of your background... setting this to around 100 is in the ball park.
*Once you have all your flats and they are bias corrected, make your masters of each individual channel, 'median combine' is best.
*When calibrating your images, make sure you bias correct the lights first and then flat correct them.

I don't do any dark subtraction to the flats as flat exposures are so short, bias removal is far better... dark removal is really unwarranted (could induce more noise to the flat). I don't dark correct my lights either. When combining the subs, I 'Sigma combine' them. Using Sigma combine gets rid of the majority of your shot noise. Those more recalcitrant hot pixels you can make up a hot pixel map and use that to the subs.
I think if you are getting your cooling down to at least -15*C, you should be fine.

Hope this helps...
The above has been working for me.

Rich
Thanks Rich

those are all useful tips. I have most of these in hand - including the pedestal setting - even though I had no idea that I needed to set that, its been set already.

I'm intrigued by the no dark approach - I image at -10 here. I probably should go lower.

Perhaps I need more flats than 10?

Pete
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