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OSC camera flats question


michaelmorris

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Late last year I bought a ZWO 294 MC Pro cooled camera.  It's a great camera.  However, I am a little concerned about the flats I am getting with my light box.  I'm aiming to get and ADU value that sits right in the middle of the histogram.  However, peaks on the histogram for R G and B are a long distance apart, meaning it is impossible to get all three colours near the centre of the histogram.

So here are my questions

1 - Does this matter?

2 - Should I be looking to introduce a coloured filter in to my light box to even out these differences a bit?

 

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In principle it matters, but in reality there is a way to minimize impact.

We put peak of the histogram to the right - meaning high values, because we want strong signal in flats to maximize signal to noise ratio (more signal - higher SNR).

There is another way to improve SNR - that is stacking (which is very much like taking very long exposure and bypasses full well limit of the sensor).

When doing OSC flats - aim for highest peak to be around 80-85%. This will put lowest peak at around 40ish % - or about half of highest one. That is difference of ~ x1.41 in SNR for high peak color vs low peak color. Doing twice as many flats improves SNR for same amount (stacking improves SNR by square root of number of frames - do twice as many and you will improve SNR by ~ x1.41).

Most sensors these days are linear enough so you don't have to worry about that aspect. If you feel that your flats are noisy, just take more of them, it's that simple (I always recommend more calibration frames - more is better, simple as that - same as with lights - more lights better image).

Don't be tempted to add additional filters when doing flats - it will mess up your flat calibration, and there certainly is no need to do it.

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1 hour ago, vlaiv said:

Don't be tempted to add additional filters when doing flats - it will mess up your flat calibration, and there certainly is no need to do it.

The idea was to put the filter on the light box to correct the colour imbalance of the light source.

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I've always used an ADU of 18000 in APT for my flats using a qhy183c/qhy183m. 

My last set of flats had a histogram peak near the right hand side, but they still worked perfectly fine. 

I've thought a out lowering the ADU but I'm not going to try and fix what isn't broken. 

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  • 2 weeks later...
On 09/09/2019 at 15:41, michaelmorris said:

The idea was to put the filter on the light box to correct the colour imbalance of the light source.

You can always scale your flats to 100% percent to avoid any color balance issues that your flat box produces. Best way to do it would be to measure top 5% pixels of each color - read of average of those and divide each color with that particular ADU value - that way flats will be scaled to 0-1 range (or 0-100% however you like to look a that). It does require treating each color separately.

Alternatively you can produce color transform matrix based on results you get from your light box. Shooting raw simply does not produce proper color balance and you need to color correct your image even if you have "color balanced" flat panel. Just scaling R and B compared to G will give you some correction but not full correction. Star color calibration can be one way to do it. Shooting color chart and then calculating transform can be another way to do it.

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