Monochrome flats - how are they processed?

[andriy_melnykov]

Hi, may I ask, how monochrome flats are exactly processed? After debayering, r, g and b values of each pixel simply processed to one gray value? With wich factors?
Background of the question: I am calibrating a big flat panel for a 20 inch telescope. It consists of many single white LEDs, individually controlled. But color consistency of LEDs is not ideal, and I want to use similar rgb factors for measuring/calibrating as later for flat capturing. I am using the same ASI533mc for measuring individual points of the panel and later for captures at telescope.
Thanks!

[admin]

Hi,

good question

Normally to get a mono image from a RAW colour image, you debayer to RGB and then convert the RGB to monochrome using some weighting of the three channels. That is however not very good for flats as the debayer process naturally introduces some blurring, which then means that the flat correction around any hard edged dust shadows turns out badly (the flat is slightly blurred with a more gentle brightness profile at the edge than the light frames).

So, instead, SharpCap takes the RAW images and re-balances the colours in them to all have the same mean brightness - that is, the average brightnesses are calculated for all the Red pixels, all the Blue and all the Green (split into two parts - G1 and G2 - one the greens on the RG row, the other the greens on the GB row). Those averages are used to calculate correction factors that are then applied to the coloured pixels so that post correction the averages are all the same. This effectively wipes out the bayer pattern in the raw flat frames, leaving a monochrome flat without the blurring.

Hope this helps,

Robin

[andriy_melnykov]

Hi Robin, thanks! This is sufficient to set the factors in my measurement system precise enough.