Now that you've captured some images with the HyperStar lens and Starlight Xpress SXV-H9C camera, you can begin to process the images to get the most detail possible out of them.  While each object is different and may require some fine tuning in Photoshop, the basic steps used in MaxIm DL will almost always be the same.  You can use the procedure described below for almost any object.  For fine tuning in Photoshop, see the Software Instructions section on Image Processing with Photoshop.

Once you've read through the detailed description below, print out the HyperStar Image Processing Quick Reference to have handy as you work on your images.

Note:  This tutorial uses a handy color conversion script in MaxIm DL to convert all your images at once, saving a tremendous amount of time.  Much thanks for Klaus Lux for writing this script and allowing us to share it.  Download the appropriate version for your camera below.  Note that some SXVF cameras may still use the SXV conversion, so if you get purple nebulas, try using the other script.

• Color Conversion Script for SXV-H9C

• Color Conversion Script for SXVF-H9C

Place the script file in a convenient folder, such as Program Files > Diffraction Limited, or wherever you'll be able to easily find it.  Running the script is described below.

Outline of Procedure

• Open & Select Images

• Color Convert Images

• Align & Combine Images

• Color Balance

• Log Stretch

• Digital Development Processing

• Save File

Open & Select Images

Begin by opening all images of a particular object.  For this tutorial, we'll use images of the Horsehead Nebula taken by Dail Terry with a HyperStar C11.  A total of 30 images at 30 seconds each were obtained in this sequence, but the procedure is the same for any number of exposures.

Above:  A single raw frame from the sequence and the final processed image

Once the images are open, click through and glance at each file.  You want to eliminate any bad images before stacking the exposures.  Look for elongated star images from wind or tracking errors.  With an unguided telescope, expect to toss out a couple images for every 20 to 30 frames.  One of the great things about HyperStar is that if you lose an exposure, you're only out 30 seconds!  Close any bad images, leaving the good ones open.

Color Convert Images

This is where the color conversion script is invaluable.  Normally, converting each image requires selecting the file, clicking on RGB Convert, and clicking OK.  Three steps isn't bad for one or two images, but pretty tedious for thirty shots.  Why do ninety steps when you could do one!

To run the script, go to File > Run Script.  Navigate to the folder where you placed the script file and select it.  Click OK.  A black Microsoft Script Host window appears while the images are being converted.  Allow a few seconds for the conversion to run.  When finished, the files have all been converted to RGB images.  The script also adjusts the red, green, and blue color weights to compensate for the spectral sensitivity of the camera.  This eliminates the need to color balance the images, although there is a section below on fine tuning the color balance.

Align & Combine Images

Above:  Close-up of a single exposure and of 30 frames stacked.  Note the significant noise reduction in the stacked frame.

Now for the real magic of MaxIm DL.  Select Process > Combine from the main menu.  All the open images will be listed under Available Images.  Click Add All to select all the open images for stacking.  (This is why it is easiest to close the bad images before combining, in addition to speeding up the computer processing time.)

In the Align window, select Auto - Star Matching under Align Mode and Median for Output.  These are almost always the settings you will use, and they will stay selected when you open this window in the future, so you only need to select them once.  Click OK and wait for the images to combine.  Stacking may take 1-2 minutes depending on the number of exposures and speed of the computer.  When finished the final stacked image will appear.

Tip:  If your computer chokes and appears ready to light itself on fire when trying to stack, say, 30 images, there is a workaround (other than buying more RAM).  Simply open only 10 images at a time and combine these.  Save the file, close the first set of raw frames and repeat the procedure with the next set of ten and so on.  Then, open the 3 combined images and stack those.  The end result will be almost identical, plus your computer won't hate you.

Once the combined image is created, close all the other raw frames.  If MaxIm DL asks, choose No for saving changes.  You want to leave the raw files unchanged for future use.

Save Combined Image

It's a very good idea to save your stacked--but otherwise unprocessed--image at this point.  That way you can start again in the event that you do something you really regret later on in the processing of the image.  Go to File > Save.  The File Format should default to FITS, but if not, select this file type.  Give the file a name you can remember.  For example, I would name the current file Horsehead_med30.  That way I know later that this file is a picture of the Horsehead Nebula and is a median-combined stack of 30 exposures.  Use whatever scheme works for you, but I recommend coming up with a consistent method so you know what you did at a later date.

Color Balance

Note:  The Color Conversion Script used above automatically color balances the images.  However, if you manually color convert your images, this section describes the procedure.  It also shows you what the script is doing.  There is also an optional procedure described to color compensate objects imaged low in the sky.

This should be the first step after combining an image, before doing any other processing.  Color balance is necessary for two reasons.  Primarily, it is required because the camera does not have equal sensitivity in each color.  The SXV-H9C is most sensitive in red, slightly less sensitive in green, and least sensitive in blue.  To compensate for this, the Color Balance window allows you to increase the amount of green and blue to compensate for their lower sensitivity.  For the H9C, the correct balance uses 100% red, 110% green, and 140% blue.  Go to Color > Color Balance on the main menu.  Enter 100, 110, 140 into the red, green, and blue scaling, respectively.  Click on the Auto button under Background Value.  This is an important step!

Note:  The following section is optional, but it is not done by the script, so if you choose to do this process, it must be done manually.

A less critical adjustment, but recommended in certain situations, is to compensate for atmospheric extinction.  The lower an object is in the sky, the less blue and green light will reach the telescope.  This is why sunsets are red.  For objects above 45° elevation, atmospheric extinction is negligible.  Ideally you would image all objects when they are high in the sky, but some targets never get that high above the horizon.  For these objects, color balancing for the atmospheric effect can help the image considerably.

Compensating for this effect is easy.  Below is a table that shows the recommended color balance values for different elevations.  The table takes into account the necessary balance for the H9C itself already (since the color conversion script already does that step for you).  If you know an object's elevation, just enter the values below for the red, green, and blue scaling as described above.

Example:  The images below show Omega Centauri (photographed by Larry Moore) when the object was only 10° above the horizon (as high as it gets from southern Arizona).  The first image is the unbalanced color image, the middle image is after balancing for the camera's color sensitivity, and the right image is after balancing for both the camera and the atmospheric extinction.

Adjusting the Screen Stretch

You may not be thrilled with how the image looks at this point.  In fact, look at the default display settings for our Horsehead Nebula shot.

Not exactly what you might hope for.  But MaxIm DL is not displaying all the data contained within the image.  This is where the screen stretch comes into play.  The screen stretch window shows the histogram of the image, basically a scale from the darkest to the brightest parts of the image.  There are two triangles, one red and one green, under the histogram.  Learn to love these little triangles; they are your friends, and they control how much detail your image will show.  You will use them a lot.

The concept is easy.  The red triangle is the black point slider.  Everything to the left of this point is displayed as pure black.  The green triangle is the white point slider.  Everything to the right of this point is displayed as pure white.  In other words, only the brightness levels in the histogram between the two sliders will be shown in the image.  This is of critical importance.  If any of the faint detail is to the left of the red slider, it will not appear in the image.  Doing this defeats the purpose of taking and stacking a bunch of exposures to try to get faint detail in the first place!

The most important thing when setting the screen stretch is to be sure that no important data is cut out of the picture--what is known as black point clipping.  This is a common beginner's mistake.  In an attempt to make the background appear black, the red slider is moved to the right, darkening the background but cutting off some of the faint data.  It is much better to leave the faint details intact and to darken the background later using a more versatile technique such as Curves in Photoshop.

To prevent clipping the black point, run the red slider almost up to--but not past--the left toe of the histogram curve, as shown below.

Above:  Clipping data, and not clipping data.  Always use the setting on the right.

As for the white point slider, the green triangle, this is more subjective.  Ideally you want this to be as far left as possible, without washing out too much of the bright areas.  This will bring out the most faint detail without losing the highlights.  For some objects this may prove impossible, and for that there is a solution, discussed in the following section.  For now, we just want the image to look halfway decent.  The screen stretch will become more important when we save the file after finishing the processing.

Above:  The Horsehead Nebula after setting the screen stretch

Log Stretch

This step may not be necessary for all images, but most objects benefit from this procedure.  For example, our Horsehead Nebula image does not gain much from this process, but many bright nebulas as well as most galaxies and globular star clusters can be greatly enhanced by log stretching.

This function compresses the brightness of an object into a range that can be displayed more easily by a computer monitor.  Objects with both very bright and very dim components (such as the core and spiral arms of a galaxy, respectively) often cannot be displayed in all their detail without stretching.  Adjusting the screen stretch to show highlight details renders the faint detail invisible, while enhancing the faint detail results in the bight areas being washed out.

Above:  The Whirlpool Galaxy screen stretched to show bright detail, faint detail, and then displayed after log stretching

Log stretch compresses the histogram logarithmically to fit a larger range of brightness within the range that can be displayed.  To run this process, go to Process > Stretch.  Under Permanent Stretch Type, select Log.  For Input Range use Max Pixel.  Select 16-bit under Output Range.

After running the log stretch you will have to readjust the screen stretch.

Above:  Omega Centauri before and after log stretching

Digital Development Processing

Digital development processing (DDP) is a more advanced processing step, but for galaxy images especially, it can make a huge difference.

Above:  The Whirlpool Galaxy before and after DDP

DDP works by taking the idea of histogram compression from log stretching even further, while giving you more control over how the stretch operates.  It also sharpens the image.  On galaxies, this filter is like magic, but it is also useful on nebulas.

To use DDP, go to Filter > Digital Development.  Under Filter Type choose Low Pass.  FFT Hardness should usually be set to Mild to begin, but you may change this later depending on the initial results.  Under DDP Parameters, select Auto under Background.  For Mid-Level, check the Mouse box, then move the cursor over the image and select a moderately bright, but not too bright, part of the image.  For a galaxy picture, the brightest part of a spiral arm is perfect.  Don't worry too much about it because you may end up changing this value after the initial DDP run.

Click OK and get ready for an ugly picture.  You will definitely have to adjust the screen stretch after running DDP.  Odds are things will still not be ideal, even after screen stretching.  You will most likely end up clicking Undo and running DDP again with new settings until the picture looks right.  You have to experiment each time as every image is different.

Here are some things to look for after a DDP run and how to correct them:

• If the screen stretch histogram is clipped at the black (left) end, lower the Background value in the DDP window
   

• If the bright parts of the image are not compressed as much as you would like, decrease the Mid-Level value.  (This value must be greater than the Background value.)
   

• If there are hideous black halos around the stars, select Custom under FFT Hardness and increase the Cutoff % value.

Saving Images

There is more to this than you think.  But it is very easy once you know what to do.  You just have to follow the steps below and everything will save perfectly.

First, you need to adjust the screen stretch to make the image appear as you want it saved.  Typically after saving a file in MaxIm you will be opening it in Photoshop for further processing.  You will normally want to save your file as a 16-bit TIFF.  Newer versions of Photoshop can operate on 16-bit images.  If you have Photoshop Elements, however, you will have to save the file in 8-bit format.  TIFF is the best file format choice because it preserves all the data--it is not "lossy" like JPEG format.  If you want to save an image as a JPEG for posting online or emailing to a friend, save it as a TIFF first in MaxIm then use Photoshop to save as a JPEG--you will have more control.

You must adjust the screen stretch to avoid clipping and faint you want to retain.  For the black point slider (red triangle), this means keeping the slider to the left of the toe of the histogram.  In fact, it is best to go far enough left that the background is dark grey rather than black.  You can darken it later in Photoshop with more control, and you want to avoid losing and faint detail at this stage.

For the white point slider, there is a tendency to bring it left far enough to make the majority of the detail in the image appear reasonably bright.  But it is important that the bright spots in the image not be washed out.  Adjust the white point slider only as far left as necessary to keep the bright parts from looking overexposed.  If the middle-brightness portions of the image are still a bit dim, this can be adjusted in Photoshop, but you can't get back overexposed parts of the picture.

Once the screen stretch is set, you can save the file.  Go to File > Save As.  Be sure not to select Save since this will overwrite your previously saved FITS file.  In the window that appears, write in the desired filename, and select TIFF Images under File Filter.  Under Size Format select 16-bit Int (or 8-bit Int if using Photoshop Elements).

Finally, click Stretch and in the window that comes up, select Linear Only, Screen Stretch, and 16-bit (or 8-bit for Elements).

This will keep the TIFF image looking like (and containing all the data displayed on) the screen-stretched image in MaxIm DL.

More

Be sure to check out the Software Instructions section on Image Processing with Photoshop for tips on further image enhancement.

Print out the HyperStar Image Processing Quick Reference as a handy guide to the steps on this page.

Return to HyperStar Imaging Page