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.
- Take plenty of flats. I usually take 15
10-sec. flats and the same number of flat darks.
- 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
- 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.
- 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.
- Click on | Calibrate | Setup..| and choose
Advanced from the dropdown menu.
- 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.
- 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.
- 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.
- 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.
- 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.
- Click on | Multi-Image | Auto-Process | Deep
- 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.
- 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
of tracking or combining images from different nights, you'll need to use
- 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.
- 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.
- Primary Processing
- If there are any bad edges, use | Transform | Crop...
| to cut them off.
- 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.
- 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.
- 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.
- 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.
- Next use the Black/White arrow buttons in the Image
Display Control window to find the low and high points of the histogram.
- 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.
- 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
- 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.
- Final Processing
- 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
- 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.
- The final step is to use Save As to save the image as
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
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!