Processing from linear TI FFs

If your astronomical image processing software doesn't support your camera's raw files, you can work with linear TIFFs instead. A linear TIFF file is one that has not been gamma-corrected - the pixel values in it are linearly proportional to the numbers output by the camera.

The reason we want TIFFs is that (unlike JPEGs) they contain the full detail of the image. The reason they need to be linear is so that dark-frame subtraction f MaxQSLR-CflW_207X

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Figure 12.12. After gamma stretching, more of the nebula is visible.

Figure 12.13. Raising the middle of the curve is another way to lighten the midtones.
Figure 12.14. Another way to lighten midtones is to drag the middle slider to the left in the Levels menu.
Figure 12.15. Important settings for saving a file for subsequent editing with Photoshop.

will be possible. If the image has been gamma-corrected, then it is no longer the case that 2 + 2 = 4. That is, a pixel with dark current D and signal S will no longer have the value D + S and can no longer be corrected by subtracting D.

12.4.1 Making linear TIFFs

One way to convert raw files to linear TIFFs is to use the software that came with your camera. For example, in Canon's Digital Photo Professional, do the following:

• Choose Edit, Tools or press Ctrl-T to bring up the Tools palette.

• Check Linear as shown in Figure 12.17.

• Choose File, Convert and Save and save the file as 16-bit TIFF.

Figure 12.16. Finished product: the Omega Nebula (M17). Stack of five 3-minute exposures with a Canon Digital Rebel (300D) and an 8-inch (20-cm) telescope at f /6.3, processed as described in the text. This is an enlargement of the central part of the picture.

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Figure 12.17. To make linear TIFFs with Canon's Digital Photo Professional, just check Linear. The curve does not appear to be a straight line because the horizontal scale is logarithmic but the vertical scale is linear.

This procedure produces an uncompressed TIFF file, which is very large; resave it in compressed form at your first opportunity. Also, don't be put off by the fact that the "linear" curve on the screen is strongly bent; it's being plotted on a logarithmic scale that would make it straight if it had been gamma-corrected.

Other cameras generally include software with the same capability. You can also make linear TIFFs with BreezeBrowser (www.breezesys.com) and other imagemanagement programs.

A word of caution: Photoshop CS2 does not produce linear TIFFs, as far as I can determine (though this feature may be added in the future). When you open a raw image with the "linear" tone curve, what you get is a fully gamma-corrected image. To Photoshop, "linear" means "no curvature besides gamma correction." A sufficiently clever Photoshop user could, of course, experiment and create a curve that undoes the gamma correction, which could then be saved for future use.

Also, the "linear" version of Adobe's DNG file format has to do with the way the pixels are arranged in the file, not the relation between photon counts and pixel values. All DNG files contain raw, uncorrected data.

To test whether you are getting linear TIFFs, take two pictures, one with half as much exposure as the other. Its pixel values, displayed in the information window of Photoshop or MaxDSLR, should be exactly half as high.

12.4.2 Processing procedure

To process linear TIFFs with MaxDSLR, go through all the steps in Section 12.3 except that:

• All the files you open will be TIFF rather than raw.

• The convert-to-color step (de-Bayerization) is skipped.

That's all there is to it.

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Reasonable care has been taken to ensure that the information presented in this book is  accurate. However, the reader should understand that the information provided does not constitute legal, medical or professional advice of any kind.

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