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Flats causing image to be red/magenta, borders resemble a pile of termites

graemlourens

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graemlourens

graemlourens

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Hi all.

Really need your help!

(My setup is in the signature.)

As the weather is horrible currently i decided to do a few dry-runs for processing flats, darks.

I focused a transport container that was standing around, and made few lights, darks, bias....

For the flats i took my 15" macbook, set it to the highest luminance on a white screen, and shot a few flats, with AV-Mode on the DSLR.

I ran the lights, darks, and flats through PIPP, and the outcome has got 2 problems.

1) The overall color is suddently strongly magenta, i suspect its got smth to do with the coloring of the flats, but after reasearching the color tint is not really a problem (at least not for DSS based on their FAQ)

2) The edges (that are very dark in the original, as outside of the fov of the OTA) instead of staying dark, are now an ant-colony probably termites! 

Pictures are attached.

I would appreciate to know what i did wrong, so when i really go and shoot, i don't make the same mistake again.

Regards, Graem

post-39779-0-42252500-1418381610_thumb.j

post-39779-0-88673600-1418381615_thumb.p

post-39779-0-67594700-1418381844_thumb.j

post-39779-0-01130700-1418381846_thumb.j

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NickK

NickK

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I would use a thin white t-shirt - use both front and back layers between the scope and the screen. It appears you're picking up the pixels so you need to provide a mechanism to blur those lines.

True flat panels don't have pixels hence the lack of lines - it's also possible to use a white piece of paper/translucent acetate.

Flats are usually divided as the idea is to "flatten" by weighting the individual pixels. So each pixel of the flat is used to weight the corresponding pixel of the input image. If there's a pattern on the flat - it will make a pattern on the output image.

Also I can see a bending of the horizontal lines showing field curvature.

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graemlourens

graemlourens

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Hi Nick.

I just tested the white double folded shirt in front of the lcd, but exactly the same is happening after running it through PIPP.

any other ideas why this could be happinng?

I quickly ran the same new flats over a group of lights in DSS, and the magenta problem is not there, but the edges are still the same static kind of fuziness as in the pictures above.

I will continue trying different things, as i can't imagine its the problem of the laptop screen, now with the t-shirt certainly no structure (pixels) are visible.

Regards, Graem

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cgarry

cgarry

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Hi Graem,

The calibration frames support for PIPP is very new and to be honest I added it without having a lot of test data to play with.  The magenta cast is down to the colour of your flats but it should not happen I need to correct this in PIPP.

The fuzzy edges are caused because the flats are trying to correct these areas with a lot of gain but there is no real data in these areas in the actual image so the result is amplified noise.  I think this is just the way flats work though I am happy to be corrected on this!

Would it be possible to get hold of your test images to help as I update PIPP's flat frame calibration code?

Thanks,

Chris

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NickK

NickK

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The fuzzy edges are caused because the flats are trying to correct these areas with a lot of gain but there is no real data in these areas in the actual image so the result is amplified noise.  I think this is just the way flats work though I am happy to be corrected on this!

The edge is going to have less and less as it approaches the limit of vignetting. As the colour resolution (depth of image pixel values) is not infinite you're going to find less and less recoverable information. The result is that you'll start very few values appearing.. hence they start appearing as one or two colours rather than a dimmer version of the picture. Also with less light.. it means less signal and the noise starts affecting more.

A 16 bit mono astro camera gives 16bits for each image - with LRGB filtering that means 4 channels where each has 16  bits of values (i.e. 65535 reds, 65535 greens, 65535 blues). However they still have the same issue - everything is an approximation.

However a DLSR are normally 12 bits with the colour matrix causing more reduction as image values are interpolated and often you need to calibrate the colour processing against a known star colour (look for a class of star that has a known colour then calibrate against it) as RGB are not equal. The same calibration has to be done for CCD LRGB. 

So as you can see there's lots of approximation going on.. 

Now rendering the image on the LCD display of the monitor is another approximation.

So the short answer is - as you get vignetting, the signal size drops as the light signal arriving at the sensor drops. Add the noise and the CCD is now left with a very small range of values to represent.. hence you start seeing lots of flat colour that starts fluctuating randomly by a small amount (but enough when stretched to be visible).

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graemlourens

graemlourens

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Hi Cgarry, Hey Nick

@Cgarry. Of course, I will put everything together and send you PM with the location.

FYI, congratulations on the tool, very intuitive and fast! As a developer myself, i know how much work you put in it, thx for sharing for free with all of us, its not taken for granted.

@Nick. Thank you for the explanation. I was just wondering if this is 'normal'.

As i discard of the edges in any case I'm not bothered too much with that effect, its more the overall color change that is obviously a problem.

Aside from maybe an issue in PIPP, whats the alternative?

Registax or AS2! do not take RAW (or at least not mine), is there another way of combining lights, darks & flats to get TIFF's to further process (other than PIPP?)

regards, Graem

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cgarry

cgarry

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@Cgarry. Of course, I will put everything together and send you PM with the location.

FYI, congratulations on the tool, very intuitive and fast! As a developer myself, i know how much work you put in it, thx for sharing for free with all of us, its not taken for granted.

Thanks Graem, that would be most useful.

Cheers,

Chris

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graemlourens

graemlourens

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Hi Peter.

Thx for the hint, I will definitively also try that, even if that i think with having a t-shirt infront of the tube i doubt any pixels or structure is visible to the sensor that could cause this problem.

But its certainly worth a try!

Regards, Graem

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