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FLATS & BIAS


Mav359

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Hi guys

FLATS:

a) Does it make a difference if I leave the DEW sheild on?

:) Attached is a single shot of a FLAT taken against my LCD Conversion, does this look right, too bright etc

BIAS:

a) Do i need to take BIAS frames for each ISO setting?

:) Can i make a Master BIAS and just subtract that rather then re-stacking the BIAS frames after each shoot

post-22348-133877746371_thumb.jpg

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flats

a) if your dew shield is affecting the light you are collecting, you are not using a dew shield correctly. So, no, it shouldn't matter what you do with it.

:) to answer that you have to discover for yourself how to inspect the histogram of your images and learn to tell if your image has saturated at all. I recommend learning the software IRIS just for that alone. Iris software

BIAS

a) yes

:) yes, but why not let DeepSkyStacker do that for you?

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Cheers Themos,

No the dew shield isn't preventing any light being collected i just wasn't sure if the FLAT's source needed to be right against the scope or if having distance would make a difference.

Cheers for the IRIS tip i'll check that out

As for the last BIAS question it was just a time saver by giving the computer one less thing to do, subtracting one Master BIAS rather then multiples.

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I've loaded a .CR2 raw file in IRIS here. The command "stat" shows me the maximum value, 16383. So I adjust the histogram so that I can see what happens around there. And indeed, I see, to the right of the 16000 tick mark a little red spike. That tells me that I have saturated pixels. This is what must not happen in a FLAT frame.

themos-albums-misc-picture16216-iris-histogram-example.png

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suppose you have a camera with four pixels that can only take the value 0 or 1. If your image looks like 0100 then your histogram has the value 3 at position 0 and the value 1 at position 1. Now, go on to a camera with 10 pixels and values are allowed to be 0 to 100 but integers. Your image is 0 0 1 4 20 50 80 100 100 100. The histogram has a 2 at position 0, a 1 at position 1, a 0 at positions 2 and 3, a 1 at position 4, a 0 at positions 5-19, a 1 at 20, a 0 at all positions between 21 and 49, a 1 at position 5, a 0 at all positions between 51 and 79, a 1 at position 80, a 0 at all positions 81-99 and a 3 (a spike) at position 100. You have no way of knowing if the actual light intensity at those 3 positions was exactly 100 or a value higher than 100 as the "dial" gets stuck at the maximum it can ever reach (which is 100). That situation where you don't know is what you want to avoid with flats.

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