Author Topic: flat Frames  (Read 1111 times)

Offline tgervais

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flat Frames
« on: 2017 January 16 08:29:07 »

I need some direction on how to properly make flat frames.

I have a commercially made box to use to make the frames so that is not the problem. What the problem is,  is what is the exposure for making these flat frames.

I am being told that the exposures will be the same as the light frames.  Along with same temperature and ISO..
If so,   that is easy to do and straight forward.

Others have said  NO..  You have to make your exposures such that you can see the vignetting produced my your camera in your image.   In other words,  you have to take a whole series of pictures until you are satisfied that the exposure will show clearly  the vignetting in your image.

I have also read other statements such as - you let the camera decide what the exposure will be??  I am confused with that statement.

So - this is where I am at.  I have been making up the flats simply by using the same exposure  as my lights.  But I am willing to throw  that out if that is wrong.

Any help would be appreciated.



Offline aworonow

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Re: flat Frames
« Reply #1 on: 2017 January 16 09:45:57 »
See Starizona's help pages: https://starizona.com/acb/ccd/advimcal.aspx, among many other authoritative descriptions of flat-fielding. Note too, a light box can have its own vignetting and hue issues.

Alex

Offline Duncan

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Re: flat Frames
« Reply #2 on: 2017 January 16 11:32:12 »

You definitely don't want flats to be the same exposure time as the lights; you would end up with something completely saturated. As the Starizona article referenced above says, you want the average value in the center of your flat frame to be somewhere around 1/3 to 1/2 of the camera's saturation value. Personally I go for something near to 1/3.

Also be aware that, depending on your camera, if flat frames are very short, they can be compromised by effects from the shutter opening and closing. If your flat frames are at least a second, this shouldn't be a problem. This may, in turn, mean that you have to set the brightness of the flat box quite low in order to get an exposure time that's long enough without the level being too high.

The result should show up any vignetting in the optics, as well as any dust in the optical path. Dust motes will show up as donut shaped shadows on the flat frame, with a size and darkness that depends on how close to the optical sensor the dust is. You want to capture all of that because the flat calibration process will compensate for the effects of dust. For that reason, it's very important to not rotate the camera or filters between taking lights and taking the corresponding flats.

Once you have figured out an appropriate configuration of flat box brightness and exposure time, you want to take a sizeable set of flats. I take about 30 for each filter.

One more gotcha that I've run into sometimes: if the flat box uses incandescent bulbs, when they're turned down to get exposure time into a reasonable range, the spectrum shifts to the red end. What this means is that if you have incandescent bulbs rather than LEDs illuminating your flat box you need to check the levels independently for each color channel. I switched to LEDs a couple of years ago, but when I was using incandescent bulbs I would always have to turn up the brightness in order to take blue flats...

Anyway, hope that helps, and best of luck

Duncan

Offline tgervais

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Re: flat Frames
« Reply #3 on: 2017 January 17 06:26:27 »
Thanks Duncan and others for you responses.

I have it figured out now.  It is a bit different than how I was doing it before. Now I will first determine the maximum exposure level of my setup making sure it is just at the maximum level and not more. And than from there chose the median figure or a figure just  over half of the total saturated value.. Something like 51%.

That certainly is different than how I was doing it before.  But it seems this is more logical.  I know that once I get into discussions on problem areas that it eventually will help me fully resolve my issues.

Thanks again guys for the input.