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Struggling with flat fields


LunarLight

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Hey guys

I have been and always have struggled with flat fields mainly because I don't have a mobile even light source large enough to fit over the end of my scope.

For that reason I have decided to construct a light box and have found some decent instructions for a easy to build box.

I have a few questions that I would really appreciate some advice on from an experienced imager

1. I use a nikon d300 dslr what exposure time would be a good starting point to experiment with? I know it depends on set up etc but just a ball park figure would be awesomely helpful.

2. I have no idea what a flat should look like, whether it should all be perfectly and evenly illuminated or whether it should show vignetting etc. I searched on line and found some examples of master flats that had been heavily stretched but would it possible if someone could post an example of what a single unstretched or edited flat field image looks like? Just so I can see one to get an idea of what I should be aiming for?

Thanks

Mark

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Oh they can look worse than that!! They can look HORRIBLE. I guess you can get the histogram to come up on your screen? The peak should be a third to two thirds of the way up the brightness range. I go for a third but I don't think it matters, for all the fuss.

I do get bad flats sometimes for no good reason that I can see. What makes me suspicious of a bad one is a gradient in one half or a hot corner. A credible flat shows some vignetting (dark corners) and big soft darkish dust bunnies. That I like!

I am awaiting a new EL panel but in the meantime have a light box with a bad spectrum as it turns out. Blue flats just don't work and Ha are poor but red and green are fine. It must be down to the bulbs and the light abosorbing properties of the inside of the box. Its days are numbered...

Olly

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Photosbykev - excellent! thank you very much for that I now know the sort of look I should be going for. I constructed my light box last night and had a quick play with it. Your sensor is extremely clean ha! I have a beast of a bunnie in the bottom left corner ha! I will have to clean my sensor I think.

ollypenrice - great advice I will try to stick between a third and 2 thirds. my light source has a red/yellow sort of colour so hopefully now I will be able to get some decent flats eventually.

hope the new EL panel works out for you :)

thanks for the advice guys

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