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Flats; what brightness / exposure setting do I use?


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Hi All,

I have just made myself a light box for doing Flats which on my first play with it last night, seemed to work okay, but I am not sure what brightness / exposure level I should be aiming for?  Attached are a few screen shots taken at different exposures (0.2 seconds up to 1.0 second), all taken at the same brightness level.  Now, rather than changing the exposure, I can make the lightbox darker or brighter, but I will see similar images to those below.  So my questions is, which one of these exposure times is the 'best'?  I have included the histogram in each screen shot, as I understand that this is the important thing, or is it the number in the White / Black box which is important?

Hope someone can offer some advice here :):confused:

0.2 Second Exposure

Flat.2%20seconds_zpsbqxugyp6.jpg

0.4Second Exposure

Flat.4%20seconds_zpsq2awe2f1.jpg

0.6 Second Exposure

Flat.6%20seconds_zpscaarud8x.jpg

0.8 Second Exposure

Flat.8%20seconds_zpsmgihkvpf.jpg

1.0 Second Exposure

Flat1.0%20seconds_zpsuaxitf2n.jpg

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I use a light box on my 10"SCT and just adjust the brightness to give an average measurement of the image of around 25000 / 30000, I use Maxim to scan the information cursor around the image.

Different filters need different brightness to achieve the same average though I seem to remember Olly saying he used the same flat for all filters.

All yours look like they would work OK.

Dave

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Thanks for that.  Yes, different filters will definitely need different exposure from what I tried last night.  The images above were for the Lum. filter, for the Ha filter I needed ~4 second exposure to get the same level

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Different filters will indeed need different exposure lengths but the .4 seconds with whichever filter that was seems just right to me  - I aim for 22,000 and have a 'plug-in' that ensures this but anywhere between 20,000 and 30,000 is fine.

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I agree with Steve, too.

I have to admit that, having experimented with my own setups, I find no need for separate colour flats. I don't assert that this will generally be the case but it was the case when I did some fairly careful tests. I was quite surprized but the bunnies (which were plentiful!) were not filter-related at all and subtracting a red flat from a green or a blue from a red etc gave a flat flat. On the final stacks I found no difference. This may not remain true and may not be true for other setups but, boy, it saves me time every morning!

Olly

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I aim for 24000 but according to the qsi site i should be 35000. my latest Sadr image uses 24000 flats and im happy with it so leave it at that, although it gives me room to official play if that makes sense.

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