Processing B&W RAW files from a debayered DSLR

[Fred_76]

Hi!

Just having my Canon 500D debayered, I wonder how to process the black & white images.

I was used to process the images of my coloured Canon 1000D with Christian Buil's Iris software, but the files of the 500D are too big and my computer too slow : it is a Core i7, so as Iris just uses 1 thread over the 8 available, Iris only uses 12.5% of the computer ressources !

That is why I have to use DeepStackStacker but there is no tutorial on how to process B&W RAW images... When I use DSS, it assumes the images are in colour and tries to develop them (debayerisation) and I end up with a RGB image.

The aim of this thread is therefore to collect all your return of experience on processing B&W RAW images from a CFA removed DSLR, with DSS, or any other software.

Clear sky to all

Fred

[frugal]

PixInsight will certainly do it, because it loads RAW files in as greyscale in order to calibrate them with bias, dark and flats, (then it debayers them to RGB to register and stack them).

PI will also quite happily manage the 18MPixel files from my 60d

[lensman57]

Drop Luis Campos a line, he is a member of SGL and at the moment he has a couple of images in the Deep Sky Imaging section taken with a Mono 550d so he should know what to advise you.

A.G

[frugal]

Most applications used DCRAW to convert the image from RAW to TIFF/PGM. DCRAW has a series of command line options, one of which is to do no interpolation and produce a greyscale image. I think it is the -d or -D option.

[Fred_76]

Thank, I had a reply of Luis : he is using Iris from Christian Buil.

DSS definitely doesn't like B&W images. It is only made to process colour images.

Even if you import pure B&W FITS images, it will try to develop them and make a 3 chanels RGB picture... there is no option to make a single chanel B&W picture. It is just missing from the source code.

[lensman57]

Thank, I had a reply of Luis : he is using Iris from Christian Buil.

DSS definitely doesn't like B&W images. It is only made to process colour images.

Even if you import pure B&W FITS images, it will try to develop them and make a 3 chanels RGB picture... there is no option to make a single chanel B&W picture. It is just missing from the source code.

Hmmm, in DSS go to Options ( at the bottom left pane ) and click on Raw, Fits, DPP settings. In the dialog box click the second tab on the top, Fits Files, and then Uncheck the box , labelled monochrome 16 bit fits files etc. All the debayering options will be grayed out. Give this a try and it might work.

A.G

PS: I am assuming that you can capture in fits format or use a batch conversion to change raw to fits. I also had a look at the tab RAW, there are two options that may work the Bayer Drizzle option as it does not interpolate or debayer and The Super Pixel option.

[Fred_76]

Thanks

I will give a try. I will first convert the CR2 (RAW) files to FITS with Iris, then use the unticked mark in the FITS tab.

In the RAW tab, the superpix is doing a kind of binning. One of the four pixels will generate the blue channel, another one the red and the two others the green. There is no interpolation but the picture size will be reduced by a factor of 2 in each dimension. When the image is desaturated to get a B&W picture, there will be a loss in resolution.

Using the Bayer Drizzle is also loosing some resolution. The algorithm is analyzing when a pixel of each color chanel of each image of your stack is going over a certain location. Then it can reconstruct a full colour image. There is therefore again a loss in resolution when you convert this 3 chanels colour image to a B&W image.

The only solution is therefore the trick you give : untick the mark in the FITS tab and use FITs images instead of RAW images.

Having the B&W tick in the RAW tab would be great in a future version so that we don't need to go through the RAW to FITS conversion before...

Thanks

Fred

[lensman57]

Thanks

I will give a try. I will first convert the CR2 (RAW) files to FITS with Iris, then use the unticked mark in the FITS tab.

In the RAW tab, the superpix is doing a kind of binning. One of the four pixels will generate the blue channel, another one the red and the two others the green. There is no interpolation but the picture size will be reduced by a factor of 2 in each dimension. When the image is desaturated to get a B&W picture, there will be a loss in resolution.

Using the Bayer Drizzle is also loosing some resolution. The algorithm is analyzing when a pixel of each color chanel of each image of your stack is going over a certain location. Then it can reconstruct a full colour image. There is therefore again a loss in resolution when you convert this 3 chanels colour image to a B&W image.

The only solution is therefore the trick you give : untick the mark in the FITS tab and use FITs images instead of RAW images.

Having the B&W tick in the RAW tab would be great in a future version so that we don't need to go through the RAW to FITS conversion before...

Thanks

Fred

The fits trick should do the job. I use Neb 3 for capture at times with the modded 1100d and it is set to save the files as fits. Keep us informed please.

Regards,

A.G

[Fred_76]

Hi!

I tested yesterday night and it works.

1) capture your images in RAW with what you want (BYE, APT, EOS Utilities)

2) convert the CR2 files with Iris into FITS files (all files, including the DOF)

3) in the Import Parameters of DSS, under the FITS tab, uncheck the "Monochrome" mark

4) in the luminosity text box, enter 16 if your DSLR is in 12bits, or 4 if it is in 14 bits so that the FITS will be better viewed (it will not change the dynamic)

5) load your FITs images and DOF

6) process as usual

7) post process with PS or Gimp or whatever as usual

Hopefully a next version of DSS will import the CR2 in B&W directly... who knows ?

Fred

[frostynixon]

Hello, 

thanks for sharing your workflow. I haven`t tried the monomod of my camera yet, but i will try it in the next couple of weeks. I tried to follow your guide for the raw file conversion however i don`t know how to use IRIS to batch convert my files. Is this functionality existing or do i have to convert every file one by one?

What i tried so far is using DCRAW to batch convert my raw files into pgm files and then use XNCONVERT to convert pgm to fts. It works also this way, but its a two step process.

greetings

[Fred_76]

You can process a batch of files directly with Iris. It is a bit complicated to explain here, but there is a good tutorial here :

To download the file : http://www.astrosurf.com/buil/us/iris/iris.htm

Tutorial : http://www.astrosurf.com/buil/iris/tutorial3/doc13_us.htm (and stop just before the "Create the master offset image")

Fred

[Fred_76]

I just forgot to say that in the first step of the tutorial above, you shall obvioulsy select "FIT" instead of "PIC" file format...

[frostynixon]

Thanks!

[Fred_76]

Here is a quick comparison of two processings of IC434 taken with my debayered 500D.

On the left side, the RAW CR2 files have been directly imported into DSS for stacking, then the stacked image has been desaturated and the levels stretched into Photoshop.

On the right side, the RAW CR2 files havre been converted into FITs by Iris, then the FITs have been imported into DSS, the stacked image has just been levels stretched with PS.

One can see that the two steps CR2>FIT>DSS give a better resolution image, less noisy.

Fred

[frostynixon]

Definitely a difference!