Image Processing in MaxIm DL

For this image, MaxIm DL was used to subtract dark frames, remove blooming spikes, combine images, and create an RGB composite.  It was also used to enhance the stacked Luminance image using a process called deconvolution.

Above:  Raw luminance frame (single 10-minute exposure) compared to the final result

Standard Workflow

There are a number of workflows you can use in your imaging.  This is what I do, and it works well for me.  While there is no need to process in the exact same sequence, I recommend coming up with some sequence.  This allows you to do the same procedure for each new object imaged and helps you keep track of where you should be.  After processing a bunch of images you will be able to go through the processing sequence with no effort.

I process images in batches.  I do all the basic processing steps (calibrating, deblooming, stacking) for one set of images (Luminance), then save the final file.  Then I open the next set (Red) and do the same thing.  Only after the basic processing has been done do I continue on to other processing.  This means I always have stacked but otherwise unprocessed files to work from without having to go back and recreate the stacked images.

• Open Luminance files

• Calibrate all images (dark subtract)

• Combine images

• Remove blooming spikes

• Apply log stretch

• Save stacked Luminance image
   

• Open Red files

• Calibrate all images (dark subtract)

• Combine images

• Remove blooming spikes

• Save final Red file
   

• Repeat for Green files
   

• Repeat for Blue files
   

• Open stacked Luminance image

• Run deconvolution

• Save final Luminance image
   

• Open final Red, Green, and Blue files

• Color combine

• Apply log stretch

• Resize to match Luminance

• Align with Luminance image

• Save final RGB file
   

Initial Luminance Image Processing

Dark Subtraction

The first step with any set of images is to calibrate the files.  This involves at least dark subtracting the images, and also using flat fields if you have them.  For this image only dark frames were taken, no flats.  In MaxIm DL 4, you only need to specify the folder where the files are located; MaxIm will automatically identify the correct dark frames based on temperature, exposure time, and binning.  It will then automatically combine the darks to create a master dark frame.  This is pure genius.  It's worth the asking price just for this feature.

As indicated in the workflow above, all the luminance images were opened and then calibrated (dark subtracted).

Above:  Raw luminance image before dark subtracting

Above:  Luminance image after dark subtracting

Remember that you must dark subtract before any other processing.  You cannot dark subtract after stacking the images.

Combine Images

The Imaging Theory section on Ideal Exposure Times discusses various methods of combining images.  It can be shown that for more than 6 images, the best combine method is the Sigma Clip method.  For 6 images or less, Median combine works best.  So for this image, all the images were combined using the Median combine method.

The Auto Star-Matching method was used for automatic alignment of the images.  This method almost always work well.  There are certain situations where it may not work (such as when aligning significantly resized images), but normally it is no problem.

Remove Blooming Spikes

Technically, it is probably better to remove blooming spikes from individual images before combining, but I have never had a problem removing them after combining and it saves a lot of trouble.  Note that there is one rare situation where combining first would be a problem.  If you image an object on two consecutive nights and have to set up the telescope each night, the orientation may be slightly different and the blooming spikes from one set of images may not line up with the spikes from the other set.  Unless you are taking exceptionally long exposures and do not have a permanent observatory, this is unusual.  But I've had it happen (of course, I'm unusual).

By far the best way to remove blooming spikes in MaxIm DL is using the Manual selection method.  The Automatic method tends to make the selection boundaries too narrow and often cuts off the right and left edges of large stars.

Above:  Automatic (left) versus Manual (right) blooming selection

Above:  Stacked luminance image after blooming spike removal

Apply Log Stretch

Most images, especially galaxies and bright nebulae, will benefit from having a Log Stretch applied.  This compresses the dynamic range (by stretching the histogram logarithmically, as opposed to linearly) allowing both bright and faint detail to be displayed simultaneously.  For more details, see the MaxIm DL Software Instructions section on Permanent Stretching for more details.

Above:  Log stretch settings in MaxIm DL

Above:  Before and after log stretching the image

Saving Image

For now it is best to save the image as a FITS file.  Later, when exporting to Photoshop, the final file will be saved as a TIFF, which is a bit more involved.  For now, since there will be more processing of the image later in MaxIm DL, it is best to use the FITS format.

Initial Color Image Processing

The procedure from above--with the exception of log stretching--is repeated for each set of color images, red, green, and blue.  First the 3 red images are opened then calibrated.  Images are then Median combined using the Auto Star-Matching alignment mode.  Blooming spikes are removed, again using the Manual selection method.  The stacked red image is then saved as a master Red file.

The process is then repeated for the green and blue images to create master Green and Blue files.  All images are saved as FITS.

Note:  Log stretching is not applied to these images.  If it is, the color will be incorrect when the combined RGB image is created.  Instead the log stretch is applied after the RGB image is created, as described later.

Above:  Red, green, and blue images after calibrating, stacking, and deblooming

Final Luminance Processing

To create the final Luminance image, a deconvolution filter is applied.  This sharpens the image and enhances detail.  Since all the detail of the final LRGB composite will come from the Luminance file, deconvolution only needs to be applied to this image and not to the red, green, and blue files.  For more details on using the Deconvolve filter, see the Deconvolution Processing page of the Software Instructions section.

The deconvolution method used was the Lucy-Richardson method.  The Noise Model was set to Auto-Extract and PSF was set to Gaussian with a Radius of 2.  Four iterations of the deconvolution filter were run.  You will definitely want to run a test on a smaller subframe of the image (which MaxIm easily allows) since the full process can take several minutes, even on a relatively small 3-megapixel ST-10XME image.  (God help you if you have to deconvolve an STL-11000 image.)

Above:  Deconvolve settings

Above:  Before and after deconvolution

Use some caution.  It's easy to overdo the deconvolution filter and end up with an oversharpened image with hideous black halos around bright stars (the surest sign of poor image processing technique).  Subtle effects go a long way, especially after the other enhancements that will be done later in Photoshop.

Save Final Luminance File

This step is a bit more involved than saving the previous initial luminance file.  The reason is that we now want to save the file as a TIFF format image in order to open it in Photoshop.  (There is a FITS plugin for Photoshop that will read FITS files, but this process works fine for me and lets me open the image in Photoshop at any time without having to make any initial histogram adjustments).

To save a TIFF it is critical to set the Stretch.  If you don't, the image will not display correctly in Photoshop.  It is best to save as a 16-bit TIFF file, in order to save as much original data as possible.  Newer versions of Photoshop (such as CS and CS2) allow almost all operations to be performed on a 16-bit file.  When saving the file, click the Stretch button and be sure to set Linear Only, Screen Stretch, and 16 Bit.  If you log stretched the image previously, the previous Stretch settings will have been retained.  You do not want to use these settings to save the file.

An important step is to make sure the Screen Stretch is set such that the image shows all the detail you want to keep in the TIFF file.  Importantly, do not cut off the black end of the histogram as this will cause you to lose data.  Keep the sky background dark grey rather than black.  You can always darken it later in Photoshop, but you don't want to throw out the faint data that you worked so hard to capture.  Likewise, make sure the brightest parts of the image are not blown out.  Again, you can always brighten the image later in Photoshop, but you can't get back any washed out highlights.

Above:  Correct stretch settings for saving TIFF file

Final Color Image Processing

The last steps in MaxIm DL are to create the RGB composite image, log stretch it, resize it, then align it with the Luminance image for combining in Photoshop (since PS does not have an automatic alignment routine).  After opening the master Red, Green, and Blue files, the Combine Color feature of MaxIm DL was used to create the RGB composite.

The Red, Green, and Blue weight factors (the bottom left settings in the Color Combine window) were adjusted to produce the correct color balance.  The weights were selected according to the methods detailed on the True Color Imaging page of the Imaging Theory section.  Both the spectral sensitivity of the CCD chip and the atmospheric extinction were taken into account due to the low altitude of this object in the sky (30°).

Above:  Combine Color settings.  Note the RGB ratio of 1.2:1.0:1.9.

The new RGB image was then log stretched.  Log stretching the individual red, green, and blue images before color combining produces strange color balance effects in the RGB composite.  It is best to log stretch the final RGB image after combining.

Now the image needs to be resized to match the scale of the Luminance image.  Since the color images were shot binned 2x2, this means the size needs to be doubled.  This is a common procedure, so MaxIm DL has a function just for doubling size.  It's even called Double Size, and is located under the Process menu.  Note that doubling the size in MaxIm actually takes longer than saving the file, opening Photoshop, opening the file, doubling the size, saving the file again, and reopening in MaxIm DL.  It's a pretty inefficient algorithm.  But it's a lot easier just to do it in MaxIm and wait.

The last step before saving is to align this RGB file with the Luminance image so they line up perfectly when opened in Photoshop.  Sometimes the Auto Star-Matching feature will not work with resized images, but in this case it worked fine.  If it fails, select the Manual - Two Star method.

Now that the color file has been aligned with the Luminance, it is saved as a 16-bit TIFF so that it can be opened in Photoshop.

Above:  The final RGB composite from MaxIm DL

There are now two images--one the black and white Luminance file, the other the color RGB file--which can now be opened in Photoshop for the rest of the image processing procedure.  See the next page for the steps used to finalize the image in Photoshop.

Processing in Photoshop

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