Processing

All of my primary processing was done with Astronomical Image Processing for Windows (AIP4Win), Version 2, with some final steps in PaintshopPro.  A very few of the images were cleaned up a little with NeatImage.  I'll provide a "cookbook" version of my normal processing and a few comments on special situations and things to try on your own.  I'm not providing a complete tutorial for AIP4Win.  I've assumed that you have done the tutorials, set your Defaults and Imager in the Image Display Control window, and have processed at least a few image sets on your own. 

You will probably notice that my backgrounds are typically not as black and clean as you are used to seeing.  That is an almost essential tradeoff when trying to show extremely faint, low surface brightness features.  The alternative is to accumulate many hours of exposure on each object and I would still only be one-third of the way through the list if I had done that.

  1. Acquisition
    1. Take plenty of flats.  I usually take 15 10-sec. flats and the same number of flat darks.
    2. Take plenty of darks throughout the entire imaging session, for a total time at least as long as the longest image set.  I shoot darks while I'm repositioning to the next target.
    3. Take biases.  I normally take 25 1-sec. bias frames.  I know they don't need to be taken every night but it is so easy that I do it anyway.
  2. Stacking
    1. Go through your flats, flat darks, darks, and biases.  Delete or fix any bad ones -- for example, cosmic ray hits -- unless you are going to use median or k-sigma stacking.
    2. Click on | Calibrate | Setup..| and choose Advanced from the dropdown menu.
    3. Under the Bias tab, choose Use Bias Frame, Select Bias Frame(s), Median Combine, and Process Bias Frame(s).  I save the Master Bias and then delete the individual frames.
    4. Under the Dark tab, choose Select Dark Frame(s), Median Combine, Automatic Dark Matching, and Process Dark Frame(s).  Save the Master Dark.  You can delete the individual darks after all processing is complete but I keep them a month or so just in case.
    5. Under the Flat tab, choose Select Flat Frame(s).  If you use a flat box, select Average or Median Combine.  I use twilight flats so I need to use Normalize Median.  Then choose Use Flat Darks, Select Flat-Darks, Average or Median Combine, and Process Flat Frame(s). Save the Master Flat.  Like the darks, I keep the individual frames at least a month before deleting them.
    6. Very few of these images used a Defect Map, but now I wish I had made one long ago.  They are easy to make and work great for removing hot and cold pixels.  Just use the Help file instructions for Defect Map in AIP4Win.
    7. Review your images.  If you have any major cosmic ray hits, either fix them, don't use them, or use median or k-sigma stacking.  Meteor and satellite trails are a personal choice.  I leave meteors in unless they go right across the key galaxy.  I get rid of satellites by just leaving them out of the stack or using median or k-sigma stacking.  Reject any images with serious tracking errors. 
    8. Click on | Multi-Image | Auto-Process | Deep Sky... |.
    9. Under the Pre-Process tab, use Select Files to load your image set.  Until recently, I used the default Average Stack.  But now I have converted almost completely to Median Stack.  I only use Average when I have a very small number of individual exposures.  Turn on Calibrate Image and Square Up Pixels, Prescale, and Noise Filter.  I use approximately the square root of the number of images as the Prescale value (say 5.5 for 30 images) and a Deviation of 2 for the noise filter.
    10. Under the Alignment tab, pick the best of your first several images in Select Master Frame.  Turn on 2 Star Alignment and increase the Track Radius to between 12 and 16.  Select two widely spaced alignment stars on the Master Frame.  Sometimes I use Tracking Error Rejection at an Elongation Limit of 1.125, but normally I reject any images with tracking errors in Step 2.7. above.  Whenever possible, I use Automatic alignment.  But if there are any large image-to-image shifts in the set, like a temporary loss of tracking or combining images from different nights, you'll need to use Manual.
    11. Ignore the Enhancement tab.  Click OK.  If you are using Automatic, just sit back and watch.  In Manual, you will have to confirm that the alignment stars are still inside the Track Radius circle, or click on the stars to move the circles if they aren't, for each image.
    12. Save and rename the Track and Stack image.  Use a name that identifies the object, identifies the image as a Raw, and will still be meaningful months later.
  3. Primary Processing
    1. If there are any bad edges, use | Transform | Crop... | to cut them off.
    2. The next step is to do any necessary editing.  Click on | Edit |.  You can use | Patch Tool | to fix any remaining cosmic ray hits, hot or cold pixels, or blooms.  A good clean image will probably need a Patch Noise setting of 1 to 4, but a very noisy image might need as much as 18 -- you will need to experiment with this setting for your images.  Use Undo if your first try doesn't blend well with the background. 
    3. Most images will benefit from gradient correction, even those without obvious gradients.  First go to | Measure | Pixel Tool... |, select Annulus with preset radii, set the Inner radius at 0 and the Outer at 10, and measure the Mean brightness at roughly the center of each of the four sides.  Then, in | Edit | Gradient Correction |, under the Plane tab, enter these four values, but reversed.  The measurement from the top of the image gets entered at Bottom, the left side measurement get entered at Right, etc.  Click on Preview to make sure it will do what you want and then click Apply   For a hot spot from vignetting or amplifier glow, use the Hot Spot tab.  You can use Measure | Pixel Tool | again to measure the center of your bright area and a value for the background.  The difference gives you a starting point for the Correction value, but it's better to do about half the correction at first, then a quarter, etc.  Remember that in AIP4Win, the upper-left corner is the 0,0 point and y increases as you move down.  Hot spot correction often takes quite a bit of experimenting.  I generally stay away from Rubber Sheet unless the gradient is really weird.
    4. Finally, I almost always apply a couple of iterations of Sky Background Fixer.  Use Auto-Spot and make sure you delete any circles that have stars, galaxies, or nebulosity in them.  You can replace at least some of the ones deleted by clicking in a clean background area.  After using Sky Background fixer, I always save the image again with a file name ending in int (for intermediate).  If I want to reprocess an image later, I don't need to repeat the editing steps.
    5. The first step of the real processing is to use | Enhance | Digital Development.. | with the Radius set to the minimum (0.1) and the other values at their defaults.
    6. Next use the Black/White arrow buttons in the Image Display Control window to find the low and high points of the histogram.
    7. Go to | Enhance | Brightness Scaling... | and enter the Low and High pixel values from Step 3.6. above.  The program-calculated values are usually close but I prefer to make sure I'm not cutting anything off too soon.  Click the Transfer tab, click on Gamma and set the Scaling Parameter to a value between 1.3 and 2.5.  A value around 1.5 or 1.6 usually works well.  See the Comments section below for more suggestions about selecting this parameter.
    8. Return to the Image Display Control window and use the Black/White arrow buttons to create the most pleasing image.  Then go to | Enhance | Brightness Scaling... |, enter these values in the Low/High boxes and click Apply.  This does a linear scaling between these values.
    9. Save the image with an appropriate name.  If you are going to do the final processing in another program, like PaintshopPro or Photoshop, also export the image as a 16-bit TIFF.
  4. Final Processing
    1. I use PaintshopPro for these steps.  The image is Flipped, Mirrored, or Rotated as necessary to put North up and East left.  If some sharpening is desired, I use one or two iterations of a mild Unsharp Mask (Radius = 2.0, Strength = 25, and Clipping = 5).  If the background needs some darkening, the Highlight/Midtone/Shadow function with Shadow between -4 and -11, and the other values left at 0, does a good job.  Several iterations at -4 or -7 usually work better than a single large correction.
    2. I also use the Resize function, from the Image menu, if the image needs to be resized.  All of  the images on this site were resized to smaller dimensions to fit easily on the pages.
    3. The final step is to use Save As to save the image as a JPEG.

Comments

I did not mention deconvolution in the cookbook processing because the need, amount, and approach to it are so variable.  In AIP4Win, use | Measure | Star Image Tool... | to measure the Sigma for 6-8 of the medium brightness stars.  Do a mental average of the results.  If it is less than 1.3, you probably don't need any deconvolution but can certainly do a little just to tighten up the stars.  Between 1.4 and 1.8 is where you will get the most obvious improvement.  At a Sigma of 1.9 and above, deconvolution will help things but the stars will still look "fat".  When I apply deconvolution (go to | Enhance | Deconvolution... |), I normally select Gaussian PSF and set the Sigma at 0.05 to 0.1 less than my mental average.  Then click the Deconvolution tab and select Lucy-Richardson, either Fast or Slow.  I prefer Slow but some of my friends prefer Fast, so try them both.  For the first set of iterations, leave the Number of Iterations at 16 and the Relaxation Parameter at 0.05.  Click on Execute Deconvolution, and just sit back and watch.  Sometimes the processing does funny things to the brightness scaling but just click on Auto in the Image Display Control window to see it the way it should look. At this point, it's really a matter of judgment.  Sometimes I do a second or even third set of iterations, dropping the Sigma setting by 0.1 each time.  If the galaxy itself has some fine structure you want to enhance, you can try 5-10 iterations with 'Process high-frequency components only' turned off at the Settings tab.  This will, however, increase the background noise and can easily be overdone, so approach it cautiously.

If you have an image with very high Signal/Noise, such as a long exposure of a Messier object in dark skies, a second Digital Development can produce very good results.  In Step 3.7 above, enter the Low/High values but do a Linear Scale instead of a Gamma.  Then go back to Step 3.5 and proceed with the rest of the processing.  After two Digital Developments, you will probably have to use a very low Gamma scale, or none at all.

Experiment with different Gamma Scaling Parameters in Step 3.7.  Don't judge the results until you have adjusted the Black/White values, because the higher parameters result in a very washed out image until the scaling range is reduced.  Also try Gammalog with a variety of Scaling Parameters.  Gamma generally gives better results but occasionally Gammalog does better.   In Version 2.1.10 of AIP4Win, Jim Burnell introduced Sigmoid Scaling, and I've just started experimenting with it.  In some cases, it can replace both the DDP and Gamma scale steps; in others, it replaces Gamma and allows a wider range of brightness in the image.  Sometimes the default scaling works but I often have to move the 50% point up or down.

While all of the above applies to images that will be displayed either positive or negative, I often stop with a DDP and Linear Scale for the negative images.  Using Gamma, Gammalog, or Sigmoid processing can produce a grainy background unless the image was done in very clear dark skies.  Again, there is no pat answer to this.  You just need to play with your images a bit.

All of the Final Processing steps can be done in AIP4Win instead of PaintshopPro, except there is nothing that exactly replaces the Highlight/Midtone/Shadow function.  You would need to use Linear Scaling with careful adjustment of the Low/High values, and it doesn't permit quite the same degree of fine control.  All of this, and more, can also be done in Photoshop but I don't have it, so can't offer any guidance.

There are a number of image processing programs on the market and, as far as I know, all give excellent results.  I've limited this outline to AIP4Win because it is the one I use.  And other imagers have great success with vastly different approaches than what I have described.  View my approach as a starting point from which to develop your own, or to learn from others.  Good luck!