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Old 14-06-2014, 10:27 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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Quote:
Originally Posted by Octane View Post
I got my RoboFocus back from the US. It's all fixed. But, do you think there's been a break in the clouds and rain since it arrived?

I'm holed up inside going stir crazy; playing around with Skytools!

H
Quote:
Originally Posted by gregbradley View Post
Thanks for that Ray, very helpful. The numbers on those images - what do they represent?

Greg.
Quote:
Originally Posted by Octane View Post
The ADU of the flat vs the ADU in the signal of the light.

Hence, the 20,000-30,000 ADU value for the flat vs the 2,000-3,000 ADU for the light.

About 10x difference.

The 0.05 would represent 1,000-1,500 ADU flat, and, so on.

H
Quote:
Originally Posted by trent_julie View Post
Ray,
I got something out of this thanks for posting!

Trent
Thanks very much for the feedback guys. - H, thanks for the explanation ....clouds, arrgh.

Greg, the model shows that, for the flat noise to be under control, the sum of the ADU in the flats has to be >10x the sum of the sky ADU in the lights.

The four images show what I got from a single test set of lights (about 4 hours of subs), after I calibrated and stacked them four times using four different sets of flats. The sets of flats had total ADUs of 0.05x, 0.5x, 2.7x and 11x the total of the sky ADU in the lights - the number on an image shows which set of flats was used. These real life results suggest that the model is not complete crap, since the fixed pattern noise is visible in the images where the flat/sky ratio is 0.05x, 0.5x and 2.7x, but has effectively gone in the image where the flats sum is 11x the sky sum - just as the model says it should. The graph just summarises these results in numerical SNR form, rather than imagery.

To put some numbers on it, if you choose to take 50 light subs and the sky in each is 2,000 ADU, you will have a total of 100,000 sky ADU. You will need 10x that much (or 1,000,000) in the flats, so if your flats each have an ADU of 20,000, you will need at least 50 of them to get to the required 1,000,000 ADU total.

There is no fixed number for how many flats you should take - that will depend on how long you image for, how bright the sky is and how you expose the flats. But, provided you keep the total flat ADU to at least 10x the total sky ADU in the lights, the flat noise will always be negligible. Or, as a straightforward rule of thumb that will work well enough (for LRGB imaging), if you keep your lights around 2-3,000 ADU and your flats around 20-30,000 ADU, then you should:

******

Take at least one well exposed flat for every well exposed light sub that you take.

******



I know that I tend to add confusion when trying to explain things - be grateful if anyone would point out inconsistencies or lack of clarity in what has been said. Although the model itself is slightly arcane, the guidance that it provides is remarkably straightforward - I hope that is coming across.

regards ray

Last edited by Shiraz; 16-06-2014 at 01:03 PM.
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