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Old 18-04-2018, 07:30 AM
LewisM
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Another FLATS discussion

Is ran a serioies of flats last night for my latest image. I use one of Peter's lightboxes, and I was experimenting to get around 16000-20000 peak in the histogram (my STF-8300C seems to prefer a max value of no more than 17000).

I experimented by adjusting the rheostat to give a dimmer or brighter white light, and with longer and shorter subs respectively - enough to get the target values.

Is there any science behind duller/longer vs shorter/brighter being better or worse, given the same final value?

Or just so long as the correct value is reached it does matter which way you get there?
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Old 18-04-2018, 07:46 AM
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RickS (Rick)
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If you have a mechanical shutter going too short will give you shadows. If your camera has high dark current then you don't want to go for very long flats. Otherwise, I can't think of any reason to prefer duller/longer or brighter/shorter.

To minimise the noise added by flat calibration you probably want to collect a total of a couple of hundred electrons in total. I usually target 500Ke- but that's easy to do with deep wells - only a dozen flats.
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Old 18-04-2018, 07:50 AM
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Shiraz (Ray)
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doesn't matter how you get there, but if you make them short enough, you can do a simple bias subtraction as the flat calibration.
If you make them too long, dark current will become more significant and you may need to use flat darks to calibrate the flats properly.
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Old 18-04-2018, 10:56 AM
LewisM
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Thanks Rick and Ray.

Well, after trialling ALL my flats, it seems the most consistently well performing were indeed my 17000 ADU ones, and more so, the ones I shot with the light box VERY dim (in the old standard, this would have been sky flats taken pre-dawn) and 40 seconds (). If I did my "old standard" of medium brightness and around 3 seconds, it seemed to in fact introduce some shutter shadow issue and noise, even with the shutter pause set in MaxIM.
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