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One group of calibration frames (the dark frames)

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Figure 12.3. Set Calibration menu specifies where your dark frame files are located and how to handle them. These settings are remembered from one session to the next.

Don't be alarmed that the exposure time is shown as 0.00 in the menu. If your image files were in FITS format rather than Canon or Nikon raw, MaxDSLR would be able to recognize the dark frames and read their parameters. Here, 0.00 simply means the exposure information is unavailable or unused. It actually exists in the raw files, but MaxDSLR does not decode it.

Performing the subtraction

It's finally time to open your image files and perform the dark-frame subtraction. The first step is to go to File, Open, and open the raw files, taking care not to decode the color matrix (Figure 12.4).

Figure 12.4. When opening files, do not de-Bayerize ("convert to color").

The images will be displayed at 100% magnification, which means the pixels on your screen correspond to the pixels in the image. Since your screen has only 1 or 1.5 megapixels, you are viewing only the upper left corner of what the DSLR captured. Figure 12.5 shows what to expect. Don't be alarmed if the star images at the corner of the picture are distorted by aberrations; you are viewing them highly magnified.

On the main menu, go to Process and choose Calibrate All. This actually performs the subtraction. After a moment, the images will be appreciably less speckled, as shown in Figure 12.6.

You can save the images at this or any other stage of processing. MaxDSLR will not save them as Canon or Nikon raw files, of course; instead, by default, it will create FITS files.

12.3.3 Converting to color (de-Bayerization, demosaicing)

So far, you've been looking at black-and-white raw images that have a strange checkerboard pattern superimposed on them. Now it's time to de-Bayer or demo-saic them - that is, interpret the pixels as red, green, or blue, and combine them according to the Bayer algorithm.

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Figure 12.5. Images before dark-frame subtraction. Each window shows the upper left corner of an image, greatly magnified, with aberrated star images.

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Figure 12.5. Images before dark-frame subtraction. Each window shows the upper left corner of an image, greatly magnified, with aberrated star images.

Unfortunately, MaxDSLR does not have a "Convert RGB all" command, so you are going to have to click on each open image and convert it individually. (A later version may add this feature, so check for it.) For each of the images, do the following:

• In the main menu, choose Color, Convert RGB and look carefully at the menu, making settings as shown in Figure 12.7. See the MaxDSLR help file for advice specific to your camera.

Then go to the next image and do the same thing. The settings won't need to be changed; just click OK each time. Although they may not look like much, you should now be able to tell that you have color images.

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Figure 12.6. Same as Figure 12.5, after dark-frame subtraction.

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Figure 12.6. Same as Figure 12.5, after dark-frame subtraction.

The crucial settings are X Offset and Y Offset. Recall that the canonical Bayer matrix is: R G R G ••• G B G B R G R G G B G B

Some cameras start the matrix with the R in the upper left, but others start this pattern with its second row or second column. That's what the X and Y offsets refer to. Other software packages automatically use the right offset for each type of camera, but for maximum versatility, MaxDSLR leaves it up to you.

To check the offset, I urge you to process a daytime picture or two (skipping the dark-frame step, of course). It's OK if your pictures have a color cast, but they should not swap colors, rendering yellow as blue or the like. Only a picture of a familiar, colorful daytime object can confirm this.

Figure 12.7. Settings to de-Bayerize (demosaic) the raw images and convert them to color.

12.3.4 Combining images How to align and stack

Now it's time to stack (combine) your color images. MaxDSLR can usually do this automatically. But you have some decisions to make:

• Whether the alignment will include rotation or just shifting (translation). In MaxDSLR, rotation is always included unless you say to use only one alignment star.

• How to line up the images. Auto-Star Matching is usually best for deep-sky images; MaxDSLR will automatically find star images that line up. Second choice is Auto-Correlation, for planet images and for star fields that do not initially match very closely. If neither of these works, you can pick the alignment stars manually (see MaxDSLR's help screens); this may be necessary if the images are not very similar. If you pick the stars manually, you should choose two stars near opposite corners.

You can also combine images without aligning. That's the right thing to do if the pixels are already in matching positions, such as when you're combining a set of flat fields or a set of dark frames. Just select None as the type of alignment.

Conversely, you can align without combining - that is, shift and rotate the images, but then save them separately with the pixels in the shifted positions, rather than making one image out of the set. To do this, choose Align rather than Combine on the Process menu. Perform the alignment and then save the images individually. This is what you'll want to do if you are going to combine the images some other way, such as with layer masks in Photoshop.

• How to combine the images. I prefer to average them if there are just two or three, take the median if there are at least four, or sum them if they are severely underexposed and dark all over.

The advantage of taking the median is that pixels are ignored if they differ wildly from the other images. Taking the median will actually remove an airplane trail or the like if it is present in only one image in the set. More advanced techniques, combinations of taking the median and averaging, are offered in MaxIm DL. The idea is to reject bad pixels no matter what the source, but average good ones.

Go to Process, Combine, and on the image selection menu, choose Add All (i.e., add to the combination set all the files that are open). Then click OK. That will bring up the menu shown in Figure 12.8.

After making appropriate selections, click OK. MaxDSLR will compute for a few minutes, and then a new image will appear with a name ending in X (Figure 12.9). Now is a good time to zoom out (reducing the magnification to 50% or 25%) and see what you've achieved. Our example is the Omega Nebula (M17).

Alternative: Combining from files

If you're combining more than three or four images, I strongly recommend combining from files rather than combining images that are open on the screen. That will keep MaxDSLR from running out of memory. Here's how it's done:

1. If your color images are open on the screen, choose File, Save All. They will be saved as FITS files, not DSLR raw files.

2. Choose File, Close All. You no longer need to keep the images open.

3. Choose File, Combine Files and choose the files to be combined.

4. Fill out the Combine Files menu (just like Figure 12.8), and click OK.

The files will be combined and the result will appear on your screen.

You can also use this method to combine JPEG or TIFF files. Don't use it on camera raw files, even though MaxDSLR is willing to try; it will disrupt the Bayer matrix.

12.3.5 Stretching and gamma correction

At this stage, particularly if you were to view it without screen stretch, two problems with the image would become obvious:

• It doesn't use the right range of pixel values, which should be 0 to 255 or 0 to 65 535 depending on whether you're aiming for an 8-bit or 16-bit file format.

Instead, the raw output of a DSLR usually ranges from 0 to 4095 (from 12-bit digitization). The maximum pixel values in your image may be higher due to

Figure 12.8. To stack images, you must specify how to align them and how to combine them.

scaling during de-Bayerization, but they still don't extend to 65535. The image needs stretching.

• Even taking this into account, the midtones are too dark relative to the highlights.

Besides stretching, the image needs gamma correction (p. 170). Briefly, the problem is that the numbers in a DSLR raw file are linearly proportional to the amount of light, but computer monitors and printers render brightness on a roughly logarithmic scale. To correct for this, the midtones need to be lightened substantially.

In MaxDSLR, you can take care of both problems in a single step. Making screen stretch permanent

The first thing you'll want to do is stretch the brightness range of the whole image, allowing the centers of the stars to be overexposed (maximum white). The easiest way to do this is to take the effect of screen stretch and turn it into a permanent change in the image.

To do this, set screen stretch so that the image looks good, though a bit dark in the midtones. Do not let anything important become too light, since the maximum whites are going to be chopped off.

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Figure 12.9. Result of combining images. Remember that this is viewed through screen stretch; the real image is much darker than this.

Figure 12.9. Result of combining images. Remember that this is viewed through screen stretch; the real image is much darker than this.

Then choose Process, Stretch, and you'll see the menu in Figure 12.10. Check the settings and click OK.

You may want to fine-tune the process by immediately doing another stretch of the same kind. Just remember that at every step, you are chopping off part of the brightness range. Everything outside the range selected by screen stretch will become minimum black or maximum white.

Gamma correction

MaxDSLR provides a particularly elegant way to lighten the midtones to fit a specific amount of gamma correction. Use the Process, Stretch operation again, but this time make the settings as shown in Figure 12.11.

You should be working with an image that has already been stretched to approximately the right brightness range. Now you'll be leaving the brightness range the same but altering the midtones relative to the highlights and shadows. The number that you specify, typically 0.5, is the reciprocal of the gamma of a typical computer screen (1.8 to 2.2). As Figure 12.12 shows, this brings out the fainter areas of the nebula.

Figure 12.10. Brightness range selected by screen stretch can be expanded to constitute pixel values 0 to 65 535. Everything outside the range becomes either minimum black or maximum white.

Curves

Another way to lighten the midtones is to use Process, Curves and raise the portions of the response curve that need to be brighter. Figure 12.13 shows a curve shape that often works well with nebulae. Note that the output range is set to 16-bit.

By unchecking "Luminance only," you can edit the red, green, and blue curves separately. For instance, if you have blue sky fog, you can make the blue curve sag while raising red and green. The same adjustment can also be done in Photoshop and works much the same way.

Levels

MaxDSLR also offers a Levels control that works much like its counterpart in Photoshop (Figure 12.14). You view a histogram of the image with three sliders under it for minimum black, maximum white, and midtones. As with Curves, you can work on the red, green, and blue pixel values separately.

12.3.6 Saving the result

I normally do my final processing with Photoshop, so my goal at this point is to save the picture onto a file that Photoshop can process. Accordingly, my last step is to go to File, Save, and make the choices shown in Figure 12.15.

Figure 12.11. Gamma stretching is an elegant, numerically defined way to lighten midtones.

Note carefully: 16-bit integer TIFF, Packbits compression, and Auto Stretch. (Here "Stretch" is something of a misnomer; with Auto Stretch checked, MaxDSLR will reduce the brightness range if necessary to fit in the file format, but will not expand it.) If using Photoshop Elements, you'll want to specify 8-bit TIFF instead.

When I get the image to Photoshop, I perform further level adjustment, unsharp masking, cropping, and color adjustment, and then I convert the image to 8-bit RGB data (instead of 16-bit) and save it as JPEG or as a properly compressed TIFF file. (LZW compression, which Photoshop supports but MaxDSLR does not, produces a much smaller file; the compression is lossless.) Figure 12.16 shows the finished product.

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