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Old 04-01-2010, 11:10 PM
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batema (Mark)
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Are my flats OK?

Hi,

I made a light box and have been using it with the camera stting on AV but have read in these post about people still working on improving the quality of there flats. I am wodering if the flats are OK ( what ever that means ) and if not what should I be doing.

Thanks
Mark
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Old 04-01-2010, 11:21 PM
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Thats... pretty red... I'm not sure that the colour of the flat matters, as when its stacked and a master flat is created, its a greyscale image...

What type of light box did you make? and what light source did you use?

Other than that, the flat looks ok..

Essentially you want to aim at having the histogram peaking at between 1/3 and 1/2 of the way from the left, and make sure neither the black or white end is clipped...

I would be addressing the colour of the flat though...
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Old 04-01-2010, 11:32 PM
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batema (Mark)
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Levels box to go with flat picture

I will try again to attach what should have been with the above flat. Maybe my battery is flat as previous, initial flats are more brown in colour ( light )

Mark
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Old 04-01-2010, 11:56 PM
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Flats generally are not "flat" You normally see dust doughnuts, vignetting and a few hot and cold pixels.

Unfortunately, your flat field does not look anything like that.

How did you get this image?
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Old 05-01-2010, 12:22 AM
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I really only know enough to be dangerous Mark, but here's what a typical single flat frame looks like for me and my light box. I mucked up wiring in a step down resistor and LED layout the Jarcar guy so nicely sketched out for me, so now just run it off a number of 12v bulbs - thus the yellow/orange cast. Box made up from white card as described by Barry from ASIGN observatory.

They end up as monochrome masters of course and seem to work fine. This was taken by the 450D through the 8" Newt.

Was yours through an ED80? Doesn't seem to show any vignetting out at the corners?
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Old 05-01-2010, 12:32 AM
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I dont get vignetting in the corners of my 80mm F/6, but yes, the flat should show some artefacts, such as dust motes etc..

Looks to me like its been over exposed, but I've never taken flats with a DSLR before...
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Old 05-01-2010, 12:40 AM
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The flats I take with my DSLR are very similar to the one you have posted Mark.

It seems that once you have processed the several flat frames the monochrome version your image stacking program creates comes out perfectly fine.

Here is an example of my master flat frame out of DSS.
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Old 05-01-2010, 02:18 AM
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As mentioned above, ideally your flat frame should be processable (and processed with) in raw form, before de-Bayering has taken place. This preserves as much raw data as possible, as the de-Bayering process averages out adjacent pixels to artificially create the colour data. Maximum resolution and colour saturation is preserved by ensuring that any manipulation to the data such as darks, bias, flats, etc, is done prior to interpolating the colour data.

On the question of colour tint of a flat frame, given the average DSLR uses an RGGB colour matrix, you want to make certain that your green component is not being ignored as it is 50% of your data. If the flat has to be skewed towards one part of the spectrum, I'd suggest green over red.

As Alex mentioned, you want the histogram peaks to be somewhere around 50%, with no clipping at either end. I would suggest that the histogram of the flat presented is likely clipped in either the green or blue data, or both. Being clipped or skewed significantly towards the end of the histogram will result in artefacts such as increased noise in those colour channels due to a lack of dynamic range to work with. Yes, a flat frame does add noise to your image, so ideally you shoot a bunch and median combine them to minimise the impact.

Regards,
Eric
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Old 05-01-2010, 08:05 AM
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Hi All,

Box was made up from white card as described by Barry from ASIGN observatory. I have 8 x 9V bulbs all conected in parallel. The are taken through a W/O Flt 110 and a canon 400d unmod. It has a diffuser perspex disc and shows the colour as shown in the photo below. The levels graph shows the three colours all in different places. I suppose the big difference I noticed when starting to use flats was when applying a threshold to assign my colour points the whole image changes across the board at the same time.

Mark
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Old 05-01-2010, 09:33 AM
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mill (Martin)
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Your green is over half way the histogram and the blue is half way.
The red channel is stuck all the way against the right side of the histogram.
This flat is no good
i would suggest getting the high brightness leds and start over again.

Martin.
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Old 05-01-2010, 08:38 PM
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Could someone please explain the physics of how a flat with a colour cast, subsequently converted to mono, is going to affect the processing of light frames?

The flats are designed for proportional correction of dust and other optical abberations. I can't see how vignetting correction could be affected (albeit at extremely low levels in relation to refraction/diffraction differences for visible wavelengths). I guess if you had coloured dust bunnies you might get into strife? (I haven't seen a good dust bunny yet on the Newt though).

I knew I should have posted a resized master flat in monochrome......
(that Matty knows what he's doing!)
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