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Old 02-12-2013, 09:01 PM
bloodhound31
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Problem with flats, Orion Starshoot and MaximDL5

My instruction manual tells me to take flats at 0.1 second and look for a FWHM of around 20000.

At 0.1 I'm getting FWHM values of 35000 to 40000
At 0.5 I'm getting FWHM values of 50000 and up
at 0.01 I'm getting FWHM values of 28000.

0.01 is a small fraction of 0.1 so is this right? will it work? I've tried reducing the exposure times even further but that's getting into the realm of bias frames is it not?

Oh, and I;m using a light box.

Correction: everyhwere I have said FWHM, I meant MAX pixel value.... oops.
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Last edited by bloodhound31; 02-12-2013 at 09:31 PM.
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Old 02-12-2013, 09:22 PM
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Instruction manual for a camera can't tell you what exposure to take flats with a light box, as it depends on how bright the light box is. What you're after is a histogram that isn't clipped at either end, and a peak in the histogram around 1/3 to 1/2 way along from the dark end. Depends who you talk to. I don't think the correct term is FWHM either, that's usually referring to stars. Maybe ADU count? Any way, if you use the histogram it's irrelevant as the peak will do the job for you.

My flat exposures are very short, especially for Lum filters. 3nm SII is much longer obviously. Don't worry about how short the exposure is, as long as it's not too fast for your shutter - ie you don't get half the image dark. If you're still getting even illumination, that's all that's important.
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Old 02-12-2013, 09:32 PM
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Thanks Troy.
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Old 03-12-2013, 11:40 PM
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I like to aim for around 28k to 30k for flats. That usually gives a good illumination
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Old 04-12-2013, 02:25 PM
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But those values would be camera-specific (or more correctly, sensor-specific) wouldn't they? They're a function of the well depth?
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Old 05-12-2013, 11:48 AM
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Quote:
Originally Posted by troypiggo View Post
But those values would be camera-specific (or more correctly, sensor-specific) wouldn't they? They're a function of the well depth?
They would be sensor specific if the units were electrons, but we're talking ADU. The gain is usually set so that full well gives an ADU count around 2^16. Around 30,000 ADU means half of full well, whatever that happens to be for the specific sensor.
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Old 05-12-2013, 01:07 PM
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Gotcha. Thanks mate.
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Old 07-12-2013, 12:26 AM
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2stroke (Jay)
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Dim your light box bud, its to bright i just call it the count or value lol no need to say analog digital unit we all know that already haha, and paul is right on the dollar.
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